Electrocatalytic Hydrogen Production by a Nickel(II) Complex with a Phosphinopyridyl Ligand

Article abstract of DOI:10.1002/anie.201511621

A novel nickel(II) complex [Ni(L)₂Cl]Cl with a bidentate phosphinopyridyl ligand 6-((diphenylphosphino)methyl)pyridin-2-amine (L) was synthesized as a metal-complex catalyst for hydrogen production from protons. The ligand can stabilize a low Nioxidation state and has an amine base as a proton transfer site. The X-ray structure analysis revealed a distorted square-pyramidal Ni^IIcomplex with two bidentate L ligands in a transarrangement in the equatorial plane and a chloride anion at the apex. Electrochemical measurements with the Ni^II complex in MeCN indicate a higher rate of hydrogen production under weak acid conditions using acetic acid as the proton source. The catalytic current increases with the stepwise addition of protons, and the turnover frequency is 8400s^-1 in 0.1 m [NBu₄][ClO₄]/MeCN in the presence of acetic acid (290equiv) at an overpotential of circa 590mV. A nickel(II) complex [Ni(L)₂Cl]Cl (L= 6-((diphenylphosphino)methyl)pyridin-2-amine), having an amine base as a proton-transfer site, was synthesized and used as a hydrogen-production catalyst. Electrochemical measurements with the complex show that a higher rate of H₂ production (turnover frequency=8400s^-1) can be achieved under weak acid conditions using acetic acid as a proton source.

Full text of DOI:10.1002/anie.201511621

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International Edition: DOI: 10.1002/anie.201511621
German Edition: DOI: 10.1002/ange.201511621
Electrocatalysis
Electrocatalytic Hydrogen Production by a Nickel(II) Complex with
a Phosphinopyridyl Ligand
Ryo Tatematsu, Tomohiko Inomata, Tomohiro Ozawa, and Hideki Masuda*
Abstract: A novel nickel(II) complex [Ni(L)2Cl]Cl with
a bidentate phosphinopyridyl ligand 6-((diphenylphosphino)-
methyl)pyridin-2-amine (L) was synthesized as a metal-com-
plex catalyst for hydrogen production from protons. The ligand
can stabilize a low Ni oxidation state and has an amine base as
a proton transfer site. The X-ray structure analysis revealed
a distorted square-pyramidal NiII complex with two bidentate
L ligands in a trans arrangement in the equatorial plane and
a chloride anion at the apex. Electrochemical measurements
with the NiII complex in MeCN indicate a higher rate of
hydrogen production under weak acid conditions using acetic
acid as the proton source. The catalytic current increases with
the stepwise addition of protons, and the turnover frequency is
8400 sÀ1 in 0.1m [NBu4][ClO4]/MeCN in the presence of acetic
acid (290 equiv) at an overpotential of circa 590 mV.
600 mV. Use of a strong acid often induces a decomposition of
the molecular catalyst, which is disadvantageous to its
durability.
Jones and co-workers have succeeded in developing
a nickel complex with an S2P2-type ligand bearing a ferrocene
substituent and have achieved electrochemical hydrogen
production under weak acid conditions with acetic acid
(pKa = 22.48 in THF).[9a] Although its overpotential is only
265 mV, its TOF is low (1240 sÀ1) and thus further improve-
ment of this catalyst is required.
To prepare metal complexes that generate H2 with a high
TOF under weak acid conditions, we designed a new metal
complex according to the following design considerations
(Scheme 1): 1) a lower valent metal center stabilized using
H
ydrogen produces only water vapor during combustion. It
has thus attracted attention as a clean energy and renewable
fuel which will provide an important contribution to the
resolution of environmental problems.[1,2] Although it is well
known that platinum is an excellent catalyst for hydrogen
production, it is expensive and has low abundance.[3] In
nature, hydrogenase enzymes act as effective catalysts in
a reversible two-electron redox reaction of hydrogen at
ambient temperature and pressure.[4,5] In particular, [FeFe]-
hydrogenase, which includes two iron atoms as the central
metals, produces molecular hydrogen with a turnover fre-
quency (TOF) of 6000–9000 sÀ1.[6] Structural and functional
studies of the active sites of [FeFe]hydrogenase have led to
the understanding that the highly efficient catalytic activity is
provided by proton trapping and the provision of sites near
the active center, as well as a hydride-ion interaction site at
the iron center.[7] More recently, hydrogen generation by
hydrogenase model complexes using inexpensive metals such
as iron, cobalt, and nickel have also been investigated.[8,9a,b]
For example, DuBois and co-workers have reported
electrochemical hydrogen production using a mononuclear
nickel complex bearing a pendant amine as a proton capture
site, which has high catalytic activity over TOF =
100000 sÀ1.[8a,b] However, this reaction requires strong acid
conditions using [(DMF)H]+ (pKa = 6.1 in MeCN) as the
proton source and a much higher overpotential of circa
Scheme 1. Proposed mechanism for catalytic H2 generation in this
study.
a ligand with p-back-donating character, such as a phosphorus
atom, was employed; 2) to stabilize the complex under acidic
conditions, we employed a neutral pyridyl nitrogen derivative
(which is a borderline base according to the hard and soft
acids and bases (HSAB) principle)[17] because it should
stabilize the NiII complex (which is a borderline acid);
3) proton transfer (donor/acceptor) sites, such as an amine,
were introduced into the ligand (additionally, it is expected
that the hydride complex of the reaction intermediate is
stabilized by hydrogen bonding with the amine in the second
coordination sphere); and 4) a NiII ion was selected as the
central metal to provide an unsaturated complex with space
for the coordination of a hydride.
In this Communication, we report the synthesis and
characterization of a novel NiII complex [Ni(L)2Cl]Cl con-
taining a bidentate phosphinopyridyl ligand 6-((diphenyl-
phosphino)methyl)pyridin-2-amine (L) and investigate its
catalytic properties with respect to electrochemical hydrogen
production.
[*] R. Tatematsu, Prof. T. Inomata, Prof. T. Ozawa, Prof. H. Masuda
Department of Frontier Materials
Graduate School of Engineering, Nagoya Institute of Technology
Gokiso, Showa, Nagoya 466–8555 (Japan)
The synthesis of ligand L was performed according to the
method shown in Scheme 2. The amine group of 2-amino-6-
methylpyridine was protected by reacting with pivaloyl
E-mail: masuda.hideki@nitech.ac.jp
Supporting information for this article can be found under:
http://dx.doi.org/10.1002/anie.201511621.
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Scheme 2. Preparation of ligand L and NiII complex [Ni(L)2Cl]Cl.
Conditions: i) pivaloyl chloride, Et3N, CH2Cl2, 08C, 4 h; ii) NBS, AIBN,
CCl4, 708C, 6 h; iii) Ph2PH, t-BuOK, 258C, 24 h; iv) KOH, MeOH,
608C, 1 week. NBS=N-bromosuccinimide; AIBN=azobisisobutyro-
nitrile.
chloride and then the methyl group was brominated. The
resulting
2-pivaloylamino-6-bromomethylpyridine
was
Figure 1. ORTEP view of a) the cation part of [Ni(L)2Cl]Cl·2MeOH,
with thermal ellipsoids set at 50% probability. MeOH solvent mole-
cules, the other chloride ion, and H atoms are omitted for clarity. In
side view (b), the other phenyl group is omitted for clarity. Selected
treated with diphenylphosphine under basic conditions to
afford an amine-protected precursor. Ligand L was obtained
by hydrolyzing the pivalamide group under strongly basic
conditions. The details of the synthesis and assignment of its
1H NMR spectrum are described in the Supporting Informa-
tion. Addition of 2 equiv of L to a MeOH solution of
anhydrous NiCl2 changes the solution color from yellow to
purple/red. Fortunately, we obtained a purple/red single
crystal suitable for X-ray analysis by recrystallization of the
complex upon addition of Et2O to the MeOH solution. The
complex was characterized by elemental analysis, IR spec-
troscopy, and X-ray crystallography in the solid state and
UV/Vis spectroscopy in the solution state.
À
À
distances/bond lengths [Š] and angles [8]: Ni P1=2.2214(4), Ni
À
À
À
P2=2.2146(4), Ni N1=1.9424(13), Ni N2=1.9471(14), Ni Cl1=
2.737(6), Ni···N3=3.1330(14), Ni···N4=3.0978(16), N3···Cl1=
3.1991(16), N4···Cl1=3.1079(15); P1-Ni-N1=84.18(4), P1-Ni-N2=
91.81(4), P2-Ni-N1=92.63(4), P2-Ni-N2=82.39(4), P1-Ni-P2=
148.38(2), N1-Ni-N2=163.48(7).
148.388, N-Ni-N = 163.488) to form a structurally controlled
distorted square pyramidal structure (see Equation (S1) in
the Supporting Information; t5 = 0.25). Interestingly, one of
the phenyl rings of the diphenylphosphine interacts with the
coordinated pyridine ring in the p–p stacking mode with
a separation of about 3.5 Š. This interaction may contribute
to the structural stabilization of [Ni(L)2Cl]Cl.
X-ray structural analysis (Figure 1) revealed that the NiII
complex forms a slightly distorted square-pyramidal geome-
try with two bidentate phosphinopyridyl ligands L in the
equatorial plane and a chloride anion at the axial position.
À
À
À
The Ni P bond lengths are Ni P1 = 2.2214(4) and Ni P2 =
Electronic absorption spectra (see Figure S1 in the
Supporting Information) of the NiII complex in a range of
different solvents show an intense absorption band in the
range l = 488–508 nm (EtOH: l = 497 nm, molar extinction
À
À
2.2146(4) Š, and the Ni N bond lengths are Ni N1 =
À
1.9424(13) and Ni N2 = 1.9471(14) Š. These values are in
good agreement with those of a previously reported nickel(II)
complex (iodo-bis-[2-(diphenylphosphinomethyl)pyridine]-
nickel(II) iodide) with chelating diphenylphosphine (2.163,
2.169 Š) and pyridine ligands (1.956, 1.970 Š), respectively.[11]
However, the Ni Cl bond length (2.737(6) Š) is significantly
longer than the previously reported Ni Cl bond lengths
coefficient (e) = 1917 mÀ1 cmÀ1
; MeOH: l = 488 nm, e =
1643 mÀ1 cmÀ1; MeCN: l = 508 nm, e = 1700 mÀ1 cmÀ1; and
CH2Cl2: l = 501 nm; e = 1115 mÀ1 cmÀ1). These absorption
1
bands are assigned to d–d transitions (1A1! E) originating
À
from a low-spin square-pyramidal NiII complex.[13] In an
aprotic solvent, two additional weak absorption bands were
detected in the longer wavelength region, that is, at l = 903–
905 nm and 1025–1030 nm in MeCN and CH2Cl2, respectively.
À
(2.302–2.314 Š), indicating weak binding of ClÀ.[12] The
amines interact with the apical chloride atom at a hydrogen
bonding distance of Cl···N(H2) ꢀ 3.1 Š. The distance between
the amine and the nickel atom is about 3.1 Š, indicating that
both proton transfer sites are positioned near the active site of
the metal center. Additionally, the nickel complex can
convert into a flexible structure from the planar structure
formed by the coordinating atoms of ligand L (P-Ni-P =
3
These bands are assigned to d–d transitions (3A2g! T1g and
3
3A2g! T2g) originating from a six-coordinate octahedral NiII
complex.[14] These findings suggest that the complex is in
equilibrium between a pyramidal structure and an octahedral
structure with chloride ions weakly bound to the metal center.
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In a protic solvent, these absorption bands are not detected,
versus the square root of the amounts of added acetic acid
showing that an uncoordinated chloride ion has been
stabilized by solvation. The equilibrium behavior was also
evident in the 1H NMR spectra (Figures S11 and S12).
Electrochemical measurements (specifically cyclic vol-
tammetry (CV)) of [Ni(L)2Cl]Cl were carried out in 0.1m
[NBu4][ClO4] in MeCN as the supporting electrolyte at
a scan rate of 100 mVsÀ1 under an Ar atmosphere. The
redox waves attributed to nickel(II/I) and nickel(I/0) redox
couples were detected at E1/2 = À0.96 V (DEp = 70 mV) and
indicate a linear correlation until approximately 290 equiv
(circa 50 mL) of acid have been added (Figure S5). This
indicates that the reaction is first order for the acid concen-
tration [Eq. (S4)]. The icat/ip value for each aliquot of acid
added was measured at the potential where the current has its
maximum value. At high acid concentrations (that is, more
than 50 mL of acid added), the plots of the icat/ip value versus
the square root of the amounts of acetic acid added becomes
nonlinear (Figure S5), suggesting that the catalyst decom-
poses in this range.
E
1/2 = À1.48 V (DEp = 170 mV) versus Fc/Fc+, respectively, in
which the former and the latter waves are reversible and
irreversible, respectively. A new oxidation wave was gener-
ated at the potential of À1.19 V when the sweep rate was
increased from 100 to 500 mVsÀ1 (Figure S2). Plots of the
reduction current (ip) versus the square root of the scan rate
are linearly correlated (Figure S3), implying that the electro-
chemical event is diffusion controlled.
Figure 3 shows a plot of the amount of added acetic acid
versus the kobs values (TOF), calculated from the icat/ip ratio
using Equation (S6), which reaches a plateau around kobs
=
11000 sÀ1. In the range where the complex does not decom-
pose, the icat/ip value is estimated to be 120.2 when 50 mL
The electrocatalytic reduction of protons in the presence
of [Ni(L)2Cl]Cl (1 mm) was evaluated in 0.1m [NBu4][ClO4] in
MeCN as the supporting electrolyte under weak acid
conditions (Figure 2). The profile of the CV changes signifi-
cantly when acetic acid is sequentially added as the proton
Figure 3. Plots of the calculated kobs value versus the volume of added
HOAc for a solution of [Ni(L)2Cl]Cl (1 mm) in 0.1m [NBu4][ClO4] in
MeCN as the supporting electrolyte. Scan rate=0.3 VsÀ1
.
(290 equiv) of acetic acid is added, corresponding to TOF =
8400 sÀ1. Under acidic conditions, no increase in the icat value
was detected at scan rates faster than 300 mVsÀ1 (Figures S6
and S7). This suggests that in the range of the scan rates
applied, the catalytic process is not a diffusion-controlled
reaction. The overpotential value estimated using Equa-
tions (S7,S8) is circa 590 mV in the presence of 50 mL
(290 equiv) of acetic acid. Hydrogen production was con-
firmed by gas chromatography (GC) following the controlled
potential electrolysis, and its Faradaic efficiency was deter-
mined to be 89.8%. Furthermore, controlled potential
electrolysis of [Ni(L)2Cl]Cl (0.5 mm) at À2.5 V versus Ag/
Ag+ in the absence of acetic acid indicates a small amount of
hydrogen production (detected by GC), suggesting that
catalytic hydrogen production might proceed through the
basic sites of the amine acting as proton transfer sites.
Electrochemical hydrogen production under weak acid
conditions using acetic acid as the proton source has been
previously investigated,[9] but an abundant metal catalyst
which provides a TOF greater than 8000 sÀ1 is rare.[9a,b] We
believe that the electrocatalysis results obtained in this study
can be attributed to the effect of the basic sites of the amine
based on two key observations: 1) from X-ray structural
analysis, the hydride complex of the reaction intermediate is
stabilized by hydrogen bonding with the amine (NiH···HN) as
Figure 2. Cyclic voltammograms of [Ni(L)2Cl]Cl (1 mm) with various
amounts of acetic acid added. 0.1m [NBu4][ClO4] in MeCN was
employed as the supporting electrolyte; scan rate=0.3 VsÀ1
.
source to the MeCN solution (pKa = 22.3 in MeCN).[10] The
catalytic current (icat) increases with an increase in the amount
of acetic acid added (where the icat is derived from two-
electron reduction of protons, as described below). Addition-
ally, the potentials corresponding to the wave maximum value
shift to a more positive region in the presence of the acid, as
compared to in the absence of complex.[10] The NiII/I and NiI/0
reduction potentials do not change under the acidic con-
ditions, showing that catalysis proceeds through an EECC
mechanism (two electrons and two chemical steps).[15] These
findings suggest that [Ni(L)2Cl]Cl has reactivity as a catalyst
for hydrogen generation.
Next, the changes in the cyclic voltammogram of complex
[Ni(L)2Cl]Cl with larger increases in the acid concentration
added were studied by addition of 5 mL (29 equiv) aliquots of
acetic acid into the solution (Figure S4). The plots of icat/ip
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well as between the apical chloride ion and the amine
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electrolysis experiments in the absence of acid, catalytic
hydrogen production was found to be promoted by proton
transfer via the basic sites of the amine. Hydrogen bonding
and proton transfer between substrates and ligands in the
second coordination sphere have been reported by a number
of different research groups and it is clear that these
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reaction.[16] We have succeeded in developing a potent hydro-
gen-generating metal complex catalyst. However, from the
viewpoint of proton reduction, the overpotential is also a very
important factor. In the future, further improvement of our
catalyst will be necessary to generate hydrogen at a lower
overpotential. Additionally, as the contribution of the amine
as a proton donor/acceptor site to the catalytic reaction is not
fully clear, understanding the function of the amine is
currently under consideration.
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In summary, a new slightly distorted square-planar NiII
complex [Ni(L)2Cl]Cl with two bidentate phosphinopyridyl
ligands was synthesized and characterized by means of UV/
Vis absorption spectroscopy, X-ray structural analysis, and
electrochemistry. In the electrochemical reduction of protons,
the electrocatalytic currents indicating hydrogen generation
were found to increase with an increase in the amounts of
acetic acid added. The TOF value evaluated on the basis of
the electrochemical reduction is 8400 sÀ1 at an overpotential
of circa 590 mV, indicating that the NiII complex has a high
proton-reduction ability even under weak acid conditions.
These results will provide the foundation for the development
of similar complexes as future hydrogen-generation catalysts.
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Acknowledgements
This research was supported by a Grant-in-Aid for Scientific
Research from the Ministry of Education, Science, Sports and
Culture and from the Program for Advancing Strategic
International Network to Accelerate the Circulation of
Talented Researchers (JSPS).
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Keywords: electrocatalysis · electrochemistry · hydrogen ·
hydrogenases · nickel
Received: December 14, 2015
Revised: February 18, 2016
Published online: March 16, 2016
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