Electrocatalytic H2 Generation from Water Relying on Cooperative Ligand Electron Transfer in “PN3P” Pincer-Supported NiII Complexes

Article abstract of DOI:10.1002/chem.202102031

Water is the most sustainable source for H₂ production, and the efficient electrocatalytic production of H₂ from mixed water/acetonitrile solutions by using two new air-stable nickel(II) pincer complexes, [Ni(κ³-2,6-{Ph₂PNR}₂(NC₅H₃)Br₂] (R=H I, Me II) is reported. Hydrogen generation from H₂O/CH₃CN solutions is initiated at ?2 V against Fc^+/0, and bulk electrocatalysis studies showed that the catalyst functions with an excellent Faradaic efficiency and a turnover frequency of 160 s^?1. A DFT computational investigation of the reduction behavior of I and II revealed a correlation of H₂ formation with charge donation from electrons originating in a reduced ligand-localized orbital. As a result, these catalysts are proposed to proceed by a novel mechanism involving electron/proton transfer between a Ni^0I species bonded to an anionic PN³P ligand (“L^?/Ni^0I”) and a Ni^I-hydride (“Ni?H”). Furthermore, these catalysts are able to reduce phenol and acetic acid, more active proton sources, at lower potentials that correlate with the substrate pK_a.

Full text of DOI:10.1002/chem.202102031

Communication
doi.org/10.1002/chem.202102031
Chemistry—A European Journal
Electrocatalytic H2 Generation from Water Relying on
Cooperative Ligand Electron Transfer in “PN3P” Pincer-
Supported NiII Complexes
Somayeh Norouziyanlakvan,[a] Gyandshwar Kumar Rao,[a] Jeffrey Ovens,[a] Bulat Gabidullin,[a]
and Darrin Richeson*[a]
hydrogen source even with electrocatalysis in aqueous solution.
Abstract: Water is the most sustainable source for H2
In general, complexes that are capable of catalyzing hydrogen
production, and the efficient electrocatalytic production of
evolution from simple water as a source are still rare.[2–5] While
H2 from mixed water/acetonitrile solutions by using two
many of these reports focus on the important performance
parameters of the acid-based catalysis, the lower environmental
and economic impacts as well as revealing fundamental
new air-stable nickel(II) pincer complexes, [Ni(k3-2,6-
{Ph2PNR}2(NC5H3)Br2] (R=H I, Me II) is reported. Hydrogen
generation from H2O/CH3CN solutions is initiated at À 2 V
features that would allow use of water as a substrate clearly
against Fc+/0, and bulk electrocatalysis studies showed that
inspires attention and motivates discovery of new earth-
the catalyst functions with an excellent Faradaic efficiency
abundant catalysts and mechanisms.
and a turnover frequency of 160 sÀ 1. A DFT computational
investigation of the reduction behavior of I and II revealed
a correlation of H2 formation with charge donation from
Nickel is an attractive metal center for designing new H2
generation catalysts due to its abundance and ability to form
complexes with a range of accessible oxidation states and
electrons originating in a reduced ligand-localized orbital.
flexible coordination behavior.[4] Added stimulus comes from
As a result, these catalysts are proposed to proceed by a
the appearance of Ni as
a component in hydrogenase
novel mechanism involving electron/proton transfer be-
tween a Ni0I species bonded to an anionic PN3P ligand (“LÀ /
Ni0I”) and a NiI-hydride (“NiÀ H”). Furthermore, these cata-
lysts are able to reduce phenol and acetic acid, more active
proton sources, at lower potentials that correlate with the
substrate pKa.
enzymes.[6] These features provide motivation to explore and
design ligand scaffolds that would enable Ni complexes as
hydrogen evolution catalysts. One particularly successful ap-
proach has been with ligands bearing pendant moieties (e.g.,
amines) as proton shuttles, which act cooperatively with the
nickel center, as a hydride donor, for electrocatalytic proton
reduction. For example, several well-known NiII phosphine
complexes bear pendant proximal amines that provide a crucial
role in electrocatalytic proton reduction.[4,7,8] Recently, redox
non-innocent ligands have been shown to participate in ligand-
based proton coupled electron transfer (PCET) for Ni-based
proton reduction.[5,9] Some examples include, NiII dithiolate and
nickel porphyrin electrocatalysts having ligand centered redox
and proton transfer capabilities.[10–12] Remarkably, the concept
of using redox active ligands for hydrogen evolution has been
expanded to include metal-free thiosemicarbazone and redox
inactive ZnII complexes of this redox active species.[13,14] Notably,
all of these examples employ acid as the proton/hydrogen
source. Recently, complexes of NiII and N-benzyl-N,N’,N’-tris(2-
pyridylmethyl)ethylenediamine ligands have been reported as
efficient catalysts for H2 evolution from water at À 2.09 V against
Dihydrogen, the simplest of molecules, represents an ideal,
environmentally innocent energy source that affords water as
the sole product of oxidation. In order to enable a sustainable
production of H2, it is crucial to discover and interrogate
electrocatalysts based on earth-abundant metals. Furthermore,
water represents the optimal substrate for the generation of
hydrogen and viable catalytic processes directly using water as
the hydrogen source are desirable.
The report that tetraazamacrocycle (cyclam) complexes of
Co and Ni were capable of generating H2 from neutral water
under electrochemical conditions was a starting point for
discovery of a wide range of metal complexes that can reduce
protons to hydrogen.[1–5] The majority of these species perform
in polar organic solvents and require the presence of acids as a
Fc+/0 [15,16] This reduction was assigned to being between a NiI/0
.
reduction couple and a ligand centered reduction at À 2.6 V.
Our efforts to reveal new Ni catalysts for the production of
hydrogen from water began with the choice of N,N’-bis
[a] S. Norouziyanlakvan, Dr. G. K. Rao, Dr. J. Ovens, Dr. B. Gabidullin,
Dr. D. Richeson
(diphenylphosphino)-2,6-diaminopyridine
species
(2,6-
{Ph2PNR}2(NC5H3; R=H, Me) as ligands. These “PN3P” neutral
pincer ligands offer tunability of sterics and electronic proper-
ties and are known to support catalysts for variety of
reactions.[17] In the case of R=H, the PN3P ligand has been
demonstrated to display non-innocence through deprotonation
and “dearomatization” of the backbone during catalysis.[18–20]
Department of Chemistry and Biomolecular Sciences
Centre for Catalysis Research and Innovation
University of Ottawa
10 Marie Curie, Ottawa, ON K1 N 6 N5
E-mail: darrin@uottawa.ca
Supporting information for this article is available on the WWW under
https://doi.org/10.1002/chem.202102031
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Furthermore, we have previously demonstrated PN3P-supported
Co complexes as efficient electrocatalysts for H2 formation.[21]
We report two air-stable NiII-PN3P pincer complexes that
perform electrocatalytic hydrogen evolution from mixed water/
acetonitrile solutions. We find, through a combination of
empirical and computational observations that the PN3P ligand
acts an electron reservoir for this transformation.
additional weak apical coordination of a bromide anion, Br(2),
to Ni.
Cyclic voltammetric scans of I and II in dry acetonitrile,
under cathodic potentials, are shown in Figure 2 (Figures S3–
S5). All of these potential measurements were referenced to the
reversible ferrocene couple (Fc+/0, ΔEp =200 mV). Both com-
plexes exhibited similar profiles with three reduction events.
The first was a reversible reduction at E1/2 =À 0.93 V (I, ΔEp =
160 mV) and E1/2 =À 0.89 V (II, ΔEp =160 mV), respectively. A
second reduction was irreversible at À 1.4 (I) and À 1.32 V (II).
Both complexes displayed a third irreversible reduction at
À 2.49 (I) and À 2.57 V (II). All of these reduction events
displayed a linear dependence of the current intensity on the
square root of the scan rate, consistent with freely diffusing
species (Figures S6 and S7). The similarity of the electrochemical
features of I and II, along with the structural features noted
above, encouraged our investigation of these species by
computation.
Ready access to the PN3P complexes of NiII was provided by
direct reaction of the appropriate ligand and a suspension of
anhydrous NiBr2 (Figure 1). Complexes I and II were stable
under ambient conditions and could be stored for months.
NMR spectroscopic analysis indicated that both species pos-
sessed symmetrically coordinated ligands as exemplified by a
single NMR resonance for the NÀ H groups, and symmetrical
pyridyl and phenyl substituents in the 1H NMR. Additionally, the
31P NMR displayed a single resonance. These features were
supported by single crystal X-ray analysis (Figures S1 and S2).
The structure of complex I consisted of the cationic complex
[Ni(2,6-{Ph2PNH}2(NC5H3)Br]+ (1+) with
a
charge balancing
Significant insights into the electronic structure and
proposed nature of the reduction behavior of I and II were
delivered through a DFT computational investigation using the
B3LYP functional and def2TZVP basis set and employing the
IEFPCM model for solvation with acetonitrile. This investigation
began with an optimization of the cation [Ni(k3-2,6-
{Ph2PNMe}2NC5H3)Br]+ (1+) of I. The computational results for
1+ produced an excellent match with the experimental
structure (Figures S8 and S9, Tables S5 and S6). Notably, the
analogous optimization process on II in acetonitrile led to
spontaneous dissociation of the apical BrÀ to produce the
analogue of 1+, [Ni(k3-2,6-{Ph2PNMe}2NC5H3)Br]+ (2+). The
cation 2+ was optimized to yield a comparable structure of 1+
(Figure S17, Tables S10 and S11) and this is consistent with the
observation of similar electrochemistry seen for the two Ni
species. These structures form a starting point for exploring the
reduction events for these two complexes and, as expected,
both displayed distorted square planar d8 NiII centers with filled
Ni-centered dxy, dxz, dyz and dz2 σ nonbonding MOs and an
bromide anion (Figure S1).The NiII center displayed a distorted
square planar coordination geometry described by a planar,
tridentate pincer PN3P ligand and completed by a bromo
ligand. The structure of 1+ compares favorably with the
reported structures of [Ni(2,6-{Ph2PNH}2(NC5H3)Cl]Cl,[22] [Ni(2,6-
{tBu2PNH}2(NC5H3)Br]Br[23] and Ni(2,6-{iPr2PNH}2(NC3N2H2(NMe2))
Cl]Cl.[24] In contrast complex II, [Ni(2,6-{Ph2PNMe}2(NC5H3)Br2,
presented an asymmetric unit with a monomeric five-coordi-
nate NiII with distorted square pyramidal geometry, an
unprecedented geometry with the PN3P ligand. The ligand was
coordinated to Ni in a planar tridentate fashion through the
nitrogen and two phosphorus centers, occupying three basal
positions of the square pyramid. One bromo ligand, Br1,
occupied the remaining basal position, trans to the pyridyl N
center, with Br2 filling an apical position, perpendicular to this
plane with the average angle between Br2 and the other four
°
coordinated centers of 97.4 . This apical Br(2)-Ni(1) distance was
much longer (2.7961(9) Å) than the basal Br(1)-Ni(1) length
(2.3100(6) Å). Interestingly, this Ni(1)-Br(1) distance is similar to
the analogous NiÀ Br distance in complex I (2.2741(4) Å).
Furthermore, the NiÀ P bond distances in II (P(1)À Ni(1)
2.1432(9) Å, P(2)À Ni(1) 2.1687(9) Å) are similar to those in I (i.e.,
2.1705(5) Å, 2.1686(5) Å) and the Ni(1)À N(2) distance 1.911(2) Å
is slightly longer than the corresponding NiÀ N distance in I at
1.8857(15) Å. These data suggest that II is related to I with an
Figure 2. CV of complexes I (red) and II (blue; 1.0 mM) under N2, in 0.1 M
tetrabutylammonium hexafluorophosphate (TBAHFP) solution in CH3CN and
scan rate was 100 mV/s. The arrow indicates initial scan direction. (open
circuit potential ca. À 0.6 V).
Figure 1. Reaction scheme and structures of complexes 1+ (complex I
bromide counterion not shown) and II.
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empty dx2 -centered σ* orbital as the LUMO (Figures S10 and
À y2
S18)
By employing the same computational optimization param-
eters, we were able to obtain a logical and consistent set of
structures for the sequential addition of three electrons to these
cations. Focusing on [Ni(k3-2,6-{Ph2PNMe}2NC5H3)Br]+ (1+),
addition of the first electron yielded a square planar neutral
species (1) (Figures S11 and S12) where this electron was added
to a dx2 -based orbital to give a SOMO which displayed similar
À y2
σ* character as seen in 1+. Mayer bond analysis indicated an
overall decrease in bond order compared with 1+ with the
largest effect on NiÀ Br. As a result, we interpret the first
reduction wave as a NiII/NiI couple. The addition of the next
electron converted 1 to 1À . This electron was again added to
the orbital with dx2 character (Figures S13 and S14) and thus
À y2
we assigned this irreversible wave to a NiI/Ni0I couple and
complex 1À as an 18 electron species. By filling the σ* MO, not
surprisingly, this reduction produced an optimized structure
with substantial changes that included deviation of the Ni0I
center from square planar geometry, an increase in NiÀ Br bond
length and decrease in bond order (Table S8). Finally, the
addition of an electron to 1À , led to a spontaneous and
anticipated loss of bromide anion to yield a planar T-shaped
complex [Ni(k3-2,6-{Ph2PNH}2NC5H3)]À (AÀ ; Figure S15). Examina-
tion of the electronic structure of this anion confirmed four Ni-
centered dxz, dz2 , dxz, dxy occupied orbitals that were essentially
nonbonding and an occupied σ* orbital that was about 70% Ni
dx2À y2 in character (Figure S16). The additional electron was
determined to reside in a ligand localized SOMO that was about
equally dispersed on the ligand P centers and py moiety with
less than 10% of the density on Ni as shown in Figure 3.
Following this process with the analogous complex 2+ led to a
parallel set of results at each reduction step (Figures S17–S22,
Tables S10–S13).
Figure 4. a) Cyclic voltammograms for [Ni(2,6-{Ph2PNH}2(NC5H3)Br]+BrÀ (I;
1.0 mM) under N2 (light blue) with various amounts of added water
(0.28 M=orange, 0.55 M=gray, 1.1 M=yellow) in CH3CN. Scan rate: 100 mV/
s with glassy carbon (GC) working electrode and 0.1 M TBAHFP. Reduction of
water in the absence of catalyst is shown in dark blue. Inset: the effect of
added water on the first two reductions. b) Plot of the ratio of the catalytic
current (icat) to the peak current of the reduction at À 2.55 V in the absence
of water (ip) as a function of the concentration of water.
positive potentials displayed a Nernstian behavior as shown by
a linear relationship of the log[H2O] against Eonset (Figure S23).
As we expected, CV measurements with added aqueous
phosphate buffer (pH 7) produced a slight increase in current at
a given potential (Figure S24). Complex II displayed parallel
behavior (Figures S23 and S24).
Linear sweep voltammetry (LSV) measurements substanti-
ated the operationally homogeneous catalytic behavior of I and
II (Figure S25). Repeated scans of a catalyst solution (1 mM Ni
complex, 0.5 M water in CH3CN) gave reproducible catalytic
scans. After electrolytic reaction, the electrode was removed,
rinsed, and immersed in fresh electrolyte/water solution con-
taining no Ni complex and no catalytic response was observed.
Furthermore, a CV of a sample of I, previously subjected to bulk
electrolysis at À 2.55 V against Fc+/0 for 1 h, displayed identical
reduction peaks of complex I. In addition, the difference in
catalytic rates of I and II is consistent with a homogeneous
system.
The first indications that these Ni complexes were active
electrocatalytically for water reduction were seen in the
voltammetric behavior of complex I in H2O/CH3CN mixtures.
Figure 4 displays the reduction behavior of I in the absence and
presence of water. In the absence of catalyst I (Figure 4a, dark
blue-dashed curve), there were no indications of enhanced
current with added water. The first two reductions of I were
unchanged with added water and
a catalytic current
enhancement appears that is associated with the third reduc-
tion of the complex (Figures S4 and S5). The shift toward more
The effect of varying the concentration of added water on
the catalytic current, ic, was also explored. Due to the reduction
of solvent, we were not able to achieve a cathodic potential
that allowed the observation a maximum catalytic peak current
in the data. However, we did measure the increase in ic relative
to the peak current in the absence of water (ip) at a fixed
voltage of À 2.55 V (Figure S26). The ratio of ic to ip, as a function
of [H2O] is shown in Figure 5. At low concentrations, a linear
Figure 3. Representation of the SOMO (viewed perpendicular to the
molecular plane) obtained from the computational optimization of [Ni(k3-
2,6-{Ph2PNH}2NC5H3)]À (AÀ ) using the B3LYP functional and def2TZVP basis
set and IEFPCM model for solvation in acetonitrile.
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overpotentials were 200 and 410 mV for phenol and acetic acid,
respectively.
þ
overpotential ¼ h ¼ Ecat=2À EH
(2)
(3)
EþH ¼ À 0:028 VÀ 0:05916 V � pKa
These reductions appear to correlate most closely with the
second reduction of the Ni complex and suggest that these
substrates follow a more conventional NiII/0 mechanism for H2
generation.
Experimental and computational data point to a key role for
the ligand in any proposed mechanism for the electrocatalytic
generation of H2 from water using either Ni complex I or II. The
redox activity of this class of pincer ligands and their ability to
undergo one electron transfer with first row metals is an open
question in the literature.[29,30] A CV measurement of the ligands
in CH3CN using a glassy carbon (GC) working electrode did
display an irreversible reductions at approximately À 2.55 V
against Fc+/0 (Figure S29). Importantly, bulk electrolysis meas-
urements with the ligands did not produce any measurable
hydrogen gas.
A proposed mechanism for the catalytic behavior of these
Ni complexes is summarized in Figure 6. For simplicity, the
scheme is presented and discussed based on I but is entirely
analogous for II. Hydrogen formation is correlated with the
reduction at À 2.55 V and computations have identified this
species as a d10 Ni0I species with an anionic ligand, AÀ . A first
step in hydrogen formation is likely a proton transfer from H2O
to AÀ . In order to explore the structure of this proposed species,
Figure 5. Cyclic voltammograms for [Ni(2,6-{Ph2PNH}2(NC5H3)Br]+BrÀ (I),
(1.0 mM) under N2 in presence of acetic acid in red, phenol in green and
water in blue. All substrates 0.55 M in CH3CN with 0.1 M NBu4PF6, at 100 mV/
s using a GC working electrode.
increase in the current ratio was observed with increasing water
concentration. When the [H2O] is sufficiently large that it
remains constant, the catalytic current changed from first to
zero order as observed in Figure 5. In this substrate concen-
tration-independent region the current ratio can be approxi-
mated by Equation (1) with R being the ideal gas constant, T
the absolute temperature, F the Faraday constant and ν the
scan rate used in the cyclic voltammetry measurement in units
of V/s.[7,25] As two electrons are used in the formation of H2, n
has a value of 2.
rffiffiffiffiffiffiffi
�
�
ic
ip
n
RTk
Fv
¼
(1)
0:4463
This analysis allows the calculation of k, a value which
represents the number of times a catalyst molecule produces a
molecule of H2 per second, or the turnover frequency (TOF) of
this catalyst under these conditions. With a scan rate of 0.1 V/s
the saturation current ratio was measured to be 29 giving a TOF
for catalyst I of 160 sÀ 1.
Quantitative confirmation of H2 generation was obtained
through bulk electrolysis experiments using either I or II as a
catalyst for generating H2 from H2O/CH3CN at À 2.55 V versus
Fc+/0 with a glassy carbon electrode having a surface area of
1.38 cm2. The reaction head-space was analyzed by GC and
confirmed dihydrogen evolution with 88% Faradaic efficiency
(Table S14, Figure S27).
As part of our continuing investigation of this catalyst
system we explored the effect of changing the substrate for
catalytic H2 generation. The ability to employ either phenol or
acetic acid as substrates was confirmed through appearance of
catalytic currents in the presence of catalysts I and II as shown
in Figure 5 (Figure S28). As expected, the catalytic onset
potential and current correlated with pKa values of phenol (I,
E
E
onset =À 1.7 V; Ecat/2 =À 1.95 V; pKa =29.14) and acetic acid (I,
onset =À 1.25 V; Ecat/2 =À 1.83 V; pKa =23.51) in acetonitrile.[26]
Figure 6. Proposed mechanism for the reduction of water and formation of
H2 from complex [Ni(k3-2,6-{Ph2PNH}2NC5H3)]À (AÀ ). Inset is the optimized
(DFT, B3LYP, def2TZVP) structure of NiÀ H.
The overpotential for reduction of these substrates can be
calculated using Equations (2) and (3).[27,28] With catalyst I, the
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Deposition Numbers 2098674 (I) and 2098675 (II) contain
the supplementary crystallographic data for this paper. These
data are provided free of charge by the joint Cambridge
Crystallographic Data Centre and Fachinformationszentrum
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Conflict of Interest
The authors declare no conflict of interest.
Keywords: Electrocatalysis · hydrogen production · nickel ·
Manuscript received: June 9, 2021
reduction of water
Version of record online: ■■■, ■■■■
Chem. Eur. J. 2021, 27, 1–6
www.chemeurj.org
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COMMUNICATION
Towards sustainable H2 production:
To bring electrocatalytic hydrogen
production into the sustainable realm,
earth-abundant metal catalysts should
function with water as the substrate.
These newly reported hydrogen-gen-
erating catalysts of NiII are robust,
stable in air and electrocatalytically
reduce water to hydrogen with
S. Norouziyanlakvan, Dr. G. K. Rao,
Dr. J. Ovens, Dr. B. Gabidullin, Dr. D.
Richeson*
1 – 6
Electrocatalytic H2 Generation from
Water Relying on Cooperative
Ligand Electron Transfer in “PN3P”
Pincer-Supported NiII Complexes
excellent Faradaic efficiency. Compu-
tations supplemented our under-
standing of a unique ligand-based
electron transfer with the cooperativ-
ity of the PN3P ligand as an electron
reservoir as a requirement for success-
ful catalysis.