Synthesis, electrochemical properties and catalytic behavior for electrochemical hydrogen production of [Ni(1,3-bis(diphenylphosphino)propane)((2-mercaptopyridinate)-κN,S)]BF4

Article abstract of DOI:10.1016/j.poly.2017.12.006

A new nickel complex [Ni(1,3-bis(diphenylphosphino)propane)((2-mercaptopyridinate)-κN,S)]BF₄ was synthesized and characterized by NMR spectroscopy, an X-ray crystallographic study, elemental analysis and electrospray ionization mass spectroscopy. The complex was synthesized under ambient conditions and was air-stable. Catalytic behavior for electrochemical production of hydrogen was investigated by cyclic voltammetry. The cyclic voltammogram with acetic acid as a proton source was very similar to the voltammogram without the complex; however, significant catalytic cathodic current was observed when trifluoroacetic acid was used as the proton source. Differences between the behavior of the complex with different acids in cyclic voltammetry was also observed in UV–Vis spectrum. The possible catalytic mechanism for hydrogen production is discussed.

Full text of DOI:10.1016/j.poly.2017.12.006

Polyhedron 141 (2018) 267–270
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Polyhedron
journal homepage: www.elsevier.com/locate/poly
Synthesis, electrochemical properties and catalytic behavior for
electrochemical hydrogen production of [Ni(1,3-bis(diphenylphosphino)
propane)((2-mercaptopyridinate)-jN,S)]BF4
⇑
Tomohiko Hamaguchi , Keisuke Kai, Shohei Konishi, Isao Ando
Department of Chemistry, Faculty of Science, Fukuoka University, 8-19-1 Nanakuma, Jonan-ku, Fukuoka 814-0180, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
A new nickel complex [Ni(1,3-bis(diphenylphosphino)propane)((2-mercaptopyridinate)-jN,S)]BF4 was
Received 12 October 2017
Accepted 6 December 2017
Available online 9 December 2017
synthesized and characterized by NMR spectroscopy, an X-ray crystallographic study, elemental analysis
and electrospray ionization mass spectroscopy. The complex was synthesized under ambient conditions
and was air-stable. Catalytic behavior for electrochemical production of hydrogen was investigated by
cyclic voltammetry. The cyclic voltammogram with acetic acid as a proton source was very similar to
the voltammogram without the complex; however, significant catalytic cathodic current was observed
when trifluoroacetic acid was used as the proton source. Differences between the behavior of the
complex with different acids in cyclic voltammetry was also observed in UV–Vis spectrum. The possible
catalytic mechanism for hydrogen production is discussed.
Keywords:
Ni complex
Electrochemistry
Catalyst
Hydrogen
Proton reduction
Ó 2017 Elsevier Ltd. All rights reserved.
1. Introduction
We designed complex 1 shown in Scheme 1 as a potential cat-
alyst for electrochemical hydrogen evolution. This complex has one
One of the most serious problems the world faces is of the
steady decline in the amount of energy available from conven-
tional sources such as coal and petroleum [1]. Renewable energy
such as solar and wind power is a very attractive alternative source
of energy. The output of renewable sources is usually stored to a
battery as electric power or as various kinds of fuels to store chem-
ical energy because renewable sources of energy are not constant
due to weather conditions [2]. Hydrogen is one of the most popular
chemical energy storage fuels because it can make electric power
without harmful waste [3]. Considerable research has been carried
out world wide to make effective catalysis for electrochemical pro-
duction of H2 [4–9].
1,3-bis(diphenylphosphino)propane (dppp, hereafter) and two
2-mercaptopyridinate (2-pyS, hereafter). The two phosphorus
atoms of dppp can stabilize Ni in low oxidation states. The nitrogen
atoms of 2-pyS ligands do not coordinate to the Ni atom, and as
such have the potential to serve as a pendant base. We attempted
to obtain complex 1 as presented below, but an unexpected pro-
duct was obtained instead. Here, we report the synthesis, charac-
terization, and electrocatalytic activity for H2 production of the
[Ni(dppp)(2-pyS)]BF4 (2).
2. Experimental
DuBois et al. have reported many bis-(1,5-R0-3,7-R-1,5-diaza-
3,7-diphosphacyclooctane) Ni(II) complexes ([Ni(PR2N2R’)]n+, here-
after) as efficient catalysts for electrochemical hydrogen production
[10]. These complexes have nitrogen atoms in the ligand, and the
nitrogen atoms serve as ‘‘pendant bases” to capture protons and
All materials were purchased from commercial suppliers
(Wako Pure Chemical Industries, Ltd., Kanto Chemical Co., Inc.,
and Tokyo Chemical Industry Co., Ltd.) and were used without fur-
ther purification.
transfer them to the Ni center. In this manner, the ligands enhance
0
the catalytic behavior. Some drawbacks to the PR2N2R ligands is that
2.1. Measurements
they generally require anaerobic and anhydrous conditions.
1H NMR (400 MHz) spectra were measured in dimethyl sulfox-
ide-d6 and CD3CN with a Bruker Avance III HD spectrometer using
dimethyl sulfoxide-d6 (d = 2.50) and CD3CN (d = 1.94) as an internal
standard. 31P NMR (161 MHz) spectra were measured in CD3CN
with a Bruker Avance III HD spectrometer using 85% H3PO4 in
⇑
Corresponding author. Fax: +81 92 865 6030.
E-mail address: thama@fukuoka-u.ac.jp (T. Hamaguchi).
https://doi.org/10.1016/j.poly.2017.12.006
0277-5387/Ó 2017 Elsevier Ltd. All rights reserved.

268
T. Hamaguchi et al. / Polyhedron 141 (2018) 267–270
atoms were refined with anisotropic parameters. H atoms were
included in the calculated positions and were refined by a riding
model. Crystallographic diagrams were created with the ORTEP
program [14]. The selected crystallographic data were as follows:
formula
C ,
32H30B1F4N1Ni1P2S1, formula weight 668.09 g molꢁ1
crystal size 0.49 ꢀ 0.12 ꢀ 0.10 mm3, orthorhombic, Pca21, a =
18.7410(10) Å, b = 9.6065(5) Å, c = 16.7669(9) Å, V = 3018.6(3) Å3,
Z = 4, T = 120(2) K, Dcalc = 1.470 mg mꢁ3, 26799 reflections col-
lected, 6851 independent reflections, Rint = 0.0552, R1 = 0.0346 (I
> 2r(I)), wR2 = 0.0738 (all data), absolute structure parameter
ꢁ0.009(9), Goodness-of-fit on F2 1.030. CCDC 1578156.
Scheme 1. Structures of complexes 1 and 2.
3. Results and discussion
water (d = 0.00) as an external standard. Electrospray ionization
mass (ESI-mass) spectral data were obtained on a JEOL JMS-
T100CS spectrometer. The absorption spectrum was measured
using a SIMADZU UV-3600 UV–Vis-NIR spectrophotometer. The
optical path length was 1 cm. The concentration of the complex
was 4.57 ꢀ 10ꢁ5 mol dmꢁ3. Cyclic voltammetry was performed
on BAS BAS100B/W using three electrode cells (glassy carbon elec-
trode as working electrode, Pt coil electrode as counter electrode
and homemade Ag+/Ag electrode as reference electrode). The med-
ium was 0.1 mol dmꢁ3n-Bu4NPF6/CH3CN. The concentration of the
complex was 1 ꢀ 10ꢁ3 mol dmꢁ3 and the amount was 10 ml. The
stock solution of the acid were 4 mol dmꢁ3 acetic acid/0.1 mol
dmꢁ3n-Bu4NPF6/CH3CN and 2 mol dmꢁ3 trifluoroacetic acid/0.1
mol dmꢁ3n-Bu4NPF6/CH3CN. E1/2 of the ferrocenium/ferrocene cou-
ple was 0.11 V versus Ag+/Ag. The scan rate was 100 mV sꢁ1. C, H
and N analysis was furnished by Service center of the elementary
analysis of organic compound of Kyushu University.
3.1. Synthesis and characterization
Complex 2 was synthesized in two steps under ambient condi-
tions. The first step was the reaction of Ni2+ ion and dppp ligand,
that produced [Ni(dppp)(solvent)2]2+ as a precursor complex. The
second step was the reaction of the precursor complex and the
2-pySH ligand. The complex was purified by recrystallization.
The complex was air-stable in solid form and in solution, and
was characterized by 1H NMR, 31P NMR, elemental analysis and
ESI-mass (Figs. S1–4).
The crystal structure of 2 is shown in Fig. 1. A nickel(II) center is
coordinated by the dppp ligand and bidentate 2-pyS ligand. The
deviation of Ni from the least square mean plane of P1/P2/S1/N1
is 0.09 Å. The dihedral angle between the least square mean plane
of P1/P2/Ni1 and the least square mean plane of S1/N1/Ni1 is 9.57°.
These data indicate that the Ni(II) ion has a square planar
geometry.
In other bidentate-2-pyS octahedral Ni(II) complex [15–17], the
bond distances of Ni–S and Ni–N range in 2.475(1)–2.5517(7) Å
and 2.034(4)–2.083(5) Å, respectively. The bond angle of N–Ni–S
ranges in 67.27(4)–68.53(4)°. Therefore, compared with similar
reported species [15–17], complex 2 has shorter bond distances
of Ni–S and Ni–N and a wider bond angle of N–Ni–S. On the other
hand, the Ni–P distance and P–Ni–P angle are almost same values
2.2. Synthesis
2.2.1. [Ni(dppp)(2-pyS)]BF4 (2)
A mixture of [Ni(H2O)6](BF4)2 (100 mg, 2.94 ꢀ 10ꢁ4 mol) and
dppp (121 mg, 2.94 ꢀ 10ꢁ4 mol) in CH3CN (10 ml) was stirred.
The color of the solution turned to brown from blue. After 3 h, 2-
pySH (63.5 mg, 5.87 ꢀ 10ꢁ4 mol) was added and the solution was
stirred overnight. The resulting red-brown solution was evacuated
to dryness. The crude product was purified by recrystallization
from H2O/acetone. Orange plate crystals were recrystallized and
were collected by filtration with a yield of 112 mg (50.9%). Anal.
Calc. for C32H30NNiP2SBF4: C, 57.53; H, 4.53; N, 2.10. Found: C,
57.63; H, 4.47; N, 2.09%. 1H NMR ((CD3)2SO, 400 MHz) d 7.81 (d,
Ar-H, 8H); 7.56–7.49 (m, Ar-H, 13H); 6.77 (d, Ar-H, 1H); 6.54 (t,
Ar-H, 1H); 6.10 (d, Ar-H, 1H);2.67 (br, PCH2CH2CH2P, 4H); 1.85
(br, PCH2CH2CH2P, 2H). 1H NMR (CD3CN, 400 MHz) d 7.83–7.78
(m, Ar-H, 8H); 7.56–7.44 (m, Ar-H, 13H); 6.69 (d, Ar-H, 1H); 6.44
(t, Ar-H, 1H); 6.25 (d, Ar-H, 1H); 2.54 (m, PCH2CH2CH2P, 4H) (The
peak of PCH2CH2CH2P could not be identified due to residual sol-
vent or water.). 31P{1H} NMR(CD3CN, 161 MHz) d 13.04 (br); 3.59
(br). UV–Vis (CH3CN) kmax, nm (
e
, dm3 molꢁ1 cmꢁ1) 274 (2.38 ꢀ
104), 292sh (2.17 ꢀ 104), 326sh (9.13 ꢀ 103), 373 (3.2 ꢀ 103). ESI–
Mass: m/z 580 (calc. for M-BF4 = 580).
2.3. X-ray crystallography
Single crystals of 2 suitable for single-crystal X-ray analysis
were obtained by slow recrystallization from a H2O/acetone solu-
tion of complex 2 at room temperature. The data were collected
on a RIGAKU R-AXIS RAPID II IP diffractometer. A multi-scan
absorption correction was applied to the intensity data. The struc-
ture was solved by direct methods (SIR-2011) [11] and was refined
Fig. 1. Crystal structure of complex 2 showing the cation. The counter anion and
hydrogen atoms are omitted for clarity. The thermal ellipsoids are drawn at the 50%
probability level. Selected bond lengths (Å) and angles (°): Ni1–S1 = 2.2320(10),
Ni1–N1 = 1.966(3), Ni1–P1 = 2.1901(10), Ni1–P2 = 2.1663(9), N1–Ni1–S1 = 74.15
(9), P1–Ni1–P2 = 91.56(4).
by the full-matrix least-squares method on F2
using the Yadokari-XG software package [13]. All non-hydrogen
(SHELXL-2014/7) [12]

T. Hamaguchi et al. / Polyhedron 141 (2018) 267–270
269
as those of [NiII(dppp)(dithiocarbamate)]+ [18–21] (Ni–P distance:
2.1652(7)–2.192(2) Å; P–Ni–P angle: 93.75(8)–97(1) °).
3.2. Electrochemical behavior
Fig. 2 is the cyclic voltammogram of complex 2 in n-Bu4NPF6/
CH3CN. 2 exhibits two irreversible one-electron cathodic peaks at
E1/2 = ꢁ1.13 and ꢁ1.56 V (all potentials are shown vs. the Ag+/Ag
redox couple), and are assigned to Ni(II)/Ni(I), Ni(I)/Ni(0), respec-
tively. Two anodic peaks at Ep = ꢁ0.79 and ꢁ0.12 V are results of
the irreversibility of the Ni(II)/Ni(I) and/or Ni(I)/Ni(0) redox cou-
ples. The reversible couple (E1/2 = 0.11 V) is assigned to ferroce-
nium/ferrocene couple as an internal reference.
To investigate the catalytic activity for H2 production of the
complex 2, electrochemical measurements with acid were carried
out. Fig. 3(a) shows the electrochemical behavior of complex 2
with acetic acid as a proton source. A large cathodic peak is
observed at ꢂꢁ2.2 V; however, a very similar response is observed
without the complex (Fig. S5 and Ref. [22]). Any additional catho-
dic peaks are not observed, and the result reveals that complex 2
does not perform the electrochemical production of H2 in the pres-
ence of acetic acid. In the case of trifluoroacetic acid, the electro-
chemical behavior changes significantly as shown in Fig. 3(b).
Compared with no acid conditions, an enhancement of the catho-
dic current around ꢁ1.8 V is observed by the addition of trifluo-
roacetic acid. The cathodic current at the potential is also
observed under acid condition without complex 2; however, the
cathodic current with the complex is at least 1.6 times larger than
the cathodic current without the complex shown in Fig. S5. The
cathodic current increases with an increase in the amount of acid.
Therefore, complex 2 does act as a catalyst for electrochemical
hydrogen production in the presence of trifluoroacetic acid. Unfor-
tunately, a detection of hydrogen by a gas chromatography was not
carried out because the potential at which complex 2 showed cat-
alytic behavior was partly overlapped by the potential at which tri-
fluoroacetic acid was reduced without catalyst.
Fig. 3. Catalytic behavior of
2
for electrochemical hydrogen production with
varying concentrations of (a) acetic acid and (b) trifluoroacetic acid. Acid stock
solution were 4 mol dmꢁ3 (acetic acid) and 2 mol dmꢁ3 (trifluoroacetic acid). The
equivalents are denoted in parentheses.
3.3. Mechanism of H2 production
Complex 2 does not show electrochemical catalytic behavior
with acetic acid (pKaCH3CN = 22.3 [23]) but it does with trifluo-
roacetic acid (pKaCH3CN = 12.7 [24]) Dubois et al. have improved
catalysts for electrochemical hydrogen production by the introduc-
tion of pendant amines which act as proton relays. The 2-pyS
ligand of complex 2 could not act as a proton relay in the isolated
state due to the bidentate coordination of this ligand. However,
Henderson et al. reported that [Ni(2-pyS){PhP(CH2CH2PPh2)2}]+ is
protonated by 2,6-dimethylpyridinium tetraphenylborate followed
by dissociation of the nitrogen of 2-pyS [25]. Therefore, the differ-
ence of the electrochemical behavior could be affected by whether
the 2-pyS ligand was protonated by the acid. To check the assump-
tion, UV–Vis spectrum of complex 2 with acids was measured
Fig. 4. UV–Vis spectra of complex 2 with different concentrations of (a) acetic acid
and (b) trifluoroacetic acid.
(Fig. 4). The spectrum of a solution of complex 2 changes upon
addition trifluoroacetic acid, whereas adding acetic acid does not
affect the spectrum. The results show that complex 2 is stable in
acetic acid/CH3CN solution and that complex 2 does show struc-
tural change due to protonation in trifluoroacetic acid/CH3CN
solution.
Based on the above results and the literature of [NiII(P2RN2R’)2]n+
[10,26,27], we propose the mechanism of H2 production as in
Scheme 2. The first protonation makes the 2-pyS ligand monoden-
tate in its coordination to the S atom. In this intermediate species,
the N atom is protonated and the complex has additional solvent.
Fig. 2. Cyclic voltammogram of 2. The arrow indicates the direction of the scans.

270
T. Hamaguchi et al. / Polyhedron 141 (2018) 267–270
This research did not receive any specific grant from funding
agencies in the public, commercial, or not-for-profit sectors.
Appendix A. Supplementary data
CCDC 1578156 contains the supplementary crystallographic
data for complex 2. These data can be obtained free of charge via
http://www.ccdc.cam.ac.uk/conts/retrieving.html, or from the
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: (+44) 1223-336-033; or e-mail:
deposit@ccdc.cam.ac.uk. Supplementary data associated with this
article can be found, in the online version, at https://doi.org/10.
1016/j.poly.2017.12.006.
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ther protonation, then H2 and the initial complex are produced.
4. Conclusions
In this study, new nickel complex [Ni (1,3-bis(diphenylphos-
phino)propane)((2-mercaptopyridinate)-jN,S)]BF4 was prepared,
characterized and investigated electrochemically. The complex
does not act as a catalyst for electrochemical hydrogen production
with acetic acid; however, it does in the presence of trifluoroacetic
acid. Based on the UV–Vis spectroscopy and the literature, the
mechanism of H2 production is proposed.
Complex 2 shows catalytic behavior for strong acid, but does
not for weak acid. Furthermore, turnover frequency and current
efficiency could not be estimated. Our studies for improved cata-
lysts based on new proton relay ligands are currently under
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Acknowledgments
We would like to thank Prof. Dr. Clifford P. Kubiak and Dr.
Michael D. Doud for their advice and support.