Mononuclear nickel(II) dithiolate complexes with chelating diphosphines: Insight into protonation and electrochemical proton reduction

Article abstract of DOI:10.1016/j.jinorgbio.2021.111449

Inspired by the metal active sites of [FeFe]- and [NiFe]?hydrogenases, a series of mononuclear Ni(II) ethanedithiolate complexes [{(Ph₂PCH₂)₂×}Ni(SCH₂CH₂S)] (X = NCH₂C₅H₄N-p (2a), NCH₂C₆H₅ (2b), NCH₂CHMe₂ (2c), and CH₂ (2d)) with chelating diphosphines were readily synthesized through the room-temperature treatments of mononuclear Ni(II) dichlorides [{(Ph₂PCH₂)₂×}NiCl₂] (1a-1d) with ethanedithiol (HSCH₂CH₂SH) in the presence of triethylamine (Et₃N) as acid-binding agent. All the as-prepared complexes 1a-1d and 2a-2d are fully characterized through elemental analysis, nuclear magnetic resonance (NMR) spectrum, and by X-ray crystallography for 1b, 2a-2d. To further explore proton-trapping behaviors of this type of mononuclear Ni(II) complexes for catalytic hydrogen (H₂) evolution, the protonation and electrochemical proton reduction of 2a-2c with aminodiphosphines (labeled PCNCP = (Ph₂PCH₂)₂NR) and reference analogue 2d with nitrogen-free diphosphine (dppp = (Ph₂PCH₂)₂CH₂) are studied and compared under trifluoroacetic acid (TFA) as a proton source. Interestingly, the treatments of 2a-2d with excess TFA resulted in the unexpected formation of dinuclear Ni(II)-Ni(II) dication complexes [{(Ph₂PCH₂)₂×}₂Ni₂(μ-SCH₂CH₂S)](CF₃CO₂)₂ (3a-3d) and mononuclear Ni(II) N-protonated complexes [{(Ph₂PCH₂)₂N(H)R}Ni(SCH₂CH₂S)](CF₃CO₂) (4a-4c), which has been well supported by high-resolution electrospray ionization mass spectroscopy (HRESI-MS), NMR (³¹P, ¹H) as well as fourier transform infrared spectroscopy (FT-IR) techniques, and especially by X-ray crystallography for 3d. Additionally, the electrochemical properties of 2a-2d are investigated in the absence and presence of strong acid (TFA) by using cyclic voltammetry (CV), showing that the complete protonation of 2a-2d gave rise to dinuclear Ni₂S₂ species 3a-3d for electrocatalytic proton reduction to H₂.

Full text of DOI:10.1016/j.jinorgbio.2021.111449

Journal of Inorganic Biochemistry 219 (2021) 111449
Contents lists available at ScienceDirect
Journal of Inorganic Biochemistry
journal homepage: www.elsevier.com/locate/jinorgbio
Mononuclear nickel(II) dithiolate complexes with chelating diphosphines:
Insight into protonation and electrochemical proton reduction
,
*
,
Xiao-Li Gua 1, Jian-Rong Lia 1, Qian-Li Lib, Yang Guoa, Xing-Bin Jinga, Zi-Bing Chena, Pei-
,
Hua Zhaoa
a School of Materials Science and Engineering, North University of China, Taiyuan 030051, PR China
b School of Chemistry and Chemical Engineering, Liaocheng University, Liaocheng 252059, PR China
A R T I C L E I N F O
A B S T R A C T
Keywords:
Inspired by the metal active sites of [FeFe]- and [NiFe]‑hydrogenases, a series of mononuclear Ni(II) ethane-
dithiolate complexes [{(Ph2PCH2)2×}Ni(SCH2CH2S)] (X = NCH2C5H4N-p (2a), NCH2C6H5 (2b), NCH2CHMe2
(2c), and CH2 (2d)) with chelating diphosphines were readily synthesized through the room-temperature
treatments of mononuclear Ni(II) dichlorides [{(Ph2PCH2)2×}NiCl2] (1a-1d) with ethanedithiol
(HSCH2CH2SH) in the presence of triethylamine (Et3N) as acid-binding agent. All the as-prepared complexes 1a-
1d and 2a-2d are fully characterized through elemental analysis, nuclear magnetic resonance (NMR) spectrum,
and by X-ray crystallography for 1b, 2a-2d. To further explore proton-trapping behaviors of this type of
mononuclear Ni(II) complexes for catalytic hydrogen (H2) evolution, the protonation and electrochemical proton
reduction of 2a-2c with aminodiphosphines (labeled PCNCP = (Ph2PCH2)2NR) and reference analogue 2d with
nitrogen-free diphosphine (dppp = (Ph2PCH2)2CH2) are studied and compared under trifluoroacetic acid (TFA)
as a proton source. Interestingly, the treatments of 2a-2d with excess TFA resulted in the unexpected formation
[FeFe]- and [NiFe]-hydrogenases
Mononuclear Ni(II) complexes
Diphosphine ligands
Protonation
Electrochemical proton reduction
of dinuclear Ni(II)-Ni(II) dication complexes [{(Ph2PCH2)2×}2Ni2(μ-SCH2CH2S)](CF3CO2)2 (3a-3d) and mono-
nuclear Ni(II) N-protonated complexes [{(Ph2PCH2)2N(H)R}Ni(SCH2CH2S)](CF3CO2) (4a-4c), which has been
well supported by high-resolution electrospray ionization mass spectroscopy (HRESI-MS), NMR (31P, 1H) as well
as fourier transform infrared spectroscopy (FT-IR) techniques, and especially by X-ray crystallography for 3d.
Additionally, the electrochemical properties of 2a-2d are investigated in the absence and presence of strong acid
(TFA) by using cyclic voltammetry (CV), showing that the complete protonation of 2a-2d gave rise to dinuclear
Ni2S2 species 3a-3d for electrocatalytic proton reduction to H2.
1. Introduction
as catalytically-active enzymes for reversible HER between molecular
H2 and proton [6,7]. For [FeFe]-H2ases (left in Fig. 1), their active sites
Over the past decades, fossil fuel as limited natural resources is
overconsumed, which has led to serious environmental pollution and
global energy crisis [1]. Therefore, scientific researchers have generated
great interest in developing clean and sustainable alternative energy
sources [2,3]. Hydrogen (H2) has been considered as the most ideal
alternative energy carrier due to its high combustion heat and carbon-
free elements [4]. In reality, hydrogen evolution reaction (HER) from
proton reduction can be reversibly and efficiently catalyzed by a family
of metalloenzymes named as hydrogenases (H2ase) in nature [5]. Ac-
cording to the metal composition in their active sites, native H2ases are
usually classified into two major groups, i.e., [FeFe]- and [NiFe]-H2ases
contain a butterfly [Fe2S2] cluster (i.e., diiron subsite) that are bridged
with a cubic [Fe4S4] cluster via a cysteinyl sulfur atom [8,9]. Specially,
this diiron subsite, which is composed of an bridging azadithiolate units
and several terminal CO/CNꢀ ligands [10–14], plays the significant
roles in the efficient HER catalyzed by [FeFe]-H2ase. On the one hand, a
terminal open coordination site at the distal Fe core (i.e., Fed is desig-
nated due to its relative far way to the [Fe4S4] cluster) serves as
hydrogen binding or evolution [15–17]. On the other hand, a pendant
nitrogen base (i.e., NH) in the azadithiolate cofactor is thought to
mediate proton relay to and from the open coordination site at the Fed
core [18,19]. For [NiFe]-H2ases (right in Fig. 1), their active sites feature
* Corresponding author.
E-mail address: zph2004@nuc.edu.cn (P.-H. Zhao).
1
The two authors equally contributed to this work.
https://doi.org/10.1016/j.jinorgbio.2021.111449
Received 2 January 2021; Received in revised form 21 March 2021; Accepted 24 March 2021
Available online 27 March 2021
0162-0134/© 2021 Elsevier Inc. All rights reserved.

X.-L. Gu et al.
Journal of Inorganic Biochemistry 219 (2021) 111449
a heterodinuclear Ni/Fe cluster, in which the Ni core is ligated by two
CH2CHMe2 [29]) were prepared as seen in the Supporting Information
(Figs. S1-S4). Preparative thin layer chromatography (TLC) was per-
formed on glass plates (25 cm × 20 cm × 0.25 cm) coated with silica gel
G (10–40 mm). Elemental analyses were performed on a PerkinElmer
240C analyzer. HRESI-MS spectra were measured on a Thermo Scien-
tific™ Exactive plus spectrometer (Thermo Fisher Scientific Inc., USA).
NMR (1H, 31P{1H}) spectra were obtained on a Bruker Avance 600 MHz
spectrometer. FT-IR (KBr) spectra were carried out on a Nicolet iS 10 FT-
IR spectrometer.
–
terminal cysteine (Cys S) units and the Fe core is coordinated by three
terminal CO/CNꢀ ligands. Two metal cores of [NiFe]-H2ase active sites
are attached via two inherent bridging Cys-S ligands and the third var-
iable bridging ligands such as vacant site, oxygenous groups, hydride
[20–24]. It is worthy of noting that the Ni core shows an changeable
oxidation state from Ni(II) to either Ni(III) or Ni(I) whereas the Fe core
remains unchanged at a low-spin Fe(II) state in the catalytic HER by
[NiFe]-H2ase [25,26].
Inspired by these above insights and our current interest in devel-
oping mononuclear metal complexes as the structural precursors related
to the Fed core of [FeFe]-H2ases and the Ni core of [NiFe]-H2ases, we
recently prepared a new series of mononuclear Ni(II) ethanedithiolate
2.2. General procedure for preparation of mononuclear Ni(II) dichloride
salts {(Ph2PCH2)2×}NiCl2 (1a-1d)
(edt) complexes with aminodiphosphines (labeled PCNCP
=
A CH2Cl2 (2 mL) solution of diphosphines (Ph2PCH2)2× (X =
NCH2C5H4N-p, NCH2C6H5, NCH2CHMe2, and CH2, 0.84 mmol) was
added dropwise to an EtOH (7 mL) solution of NiCl2⋅6H2O (0.165 g,
0.70 mmol), in which the reaction solution became dark-red immedi-
ately. After the reaction mixture was stirred at room temperature for 5
min, the precipitate appeared and the suspension was filtered with a
steel pipe. The resulting solid was washed three times with ether to give
the target compound as black solid for 1a and golden-yellow solid for
1b-1d.
(Ph2PCH2)2NR, middle in Fig. 1) although a few mononuclear Ni(II)
analogues with PCNCP ligands were so far reported by DuBois and co-
workers [27]. This is because that to further explore the catalytic pro-
ton reduction steps for HER by [FeFe]- and [NiFe]-H2ases, these
mononuclear Ni(II) complexes have some unique structural features as
follows: (i) the pendant base (NR) of their PCNCP ligands is similar to
the NH unit of the azadithiolate cofactor in [FeFe]-H2ases; and (ii) the
ligand environment around their Ni cores is comparable to a free coor-
dination site at the Fed core of [FeFe]-H2ases and an almost square-
planar coordination geometry at the Ni core of [NiFe]-H2ases.
Complex 1a (X = NCH2C5H4N-p). Yield: 0.340 g (77%). Anal. Calcd.
for C32H30Cl2N2NiP2: C, 60.61; H, 4.77; N, 4.42%. Found: C, 60.43; H,
1
As a result, this work reports the synthesis and structural charac-
terization of three new PCNCP-chelate mononuclear Ni(II) complexes
[{(Ph2PCH2)2NR}Ni(SCH2CH2S)] (R = CH2C5H4N-p (2a), CH2C6H5
(2b), and CH2CHMe2 (2c)) and one known dppp-chelate reference
5.03; N, 4.21%. H NMR (600 MHz, d6-DMSO, TMS): δ 7.84–7.30 (m,
24H, 2 x P(C6H5)2 and NCH2C5H4N-p), 4.33 (s, 2H, NCH2C5H4N-p) and
3.37 (d, JHP = 67.2 Hz, 4H, 2 x PCH2N) ppm. 31P{1H} NMR (243 MHz,
d6-DMSO, 85% H3PO4): δ 28.0 (s) ppm.
analogue
[{(Ph2PCH2)2CH2}Ni(SCH2CH2S)]
(2d,
labeled
Complex 1b (X = NCH2C6H5). Yield: 0.356 g (80%). Anal. Calcd. for
(Ph2PCH2)2CH2 = dppp) [28] for comparison, which were readily ob-
tained from treatments of mononuclear Ni(II) dichloride precursors
[(diphosphine)NiCl2] (1a-1d) with ethanedithiol. Further protonation
reactions of complexes 2a-2d with excess trifluoroacetic acid (TFA)
resulted in the unexpected formation of dinuclear Ni(II)-Ni(II) dication
C33H31Cl2NNiP2⋅CH2Cl2: C, 56.87; H, 4.63; N, 1.95%. Found: C, 56.66;
H, 4.81; N, 2.21%. 1H NMR (600 MHz, d6-DMSO, TMS): δ 7.76–6.85 (m,
25H, 2 x P(C6H5)2 and NCH2C6H5) and 4.21–3.32 (m, 6H, 2 x PCH2N and
NCH2C6H5) ppm. 31P{1H} NMR (243 MHz, d6-DMSO, 85% H3PO4): δ
27.9 (s) ppm.
complexes [{(Ph2PCH2)2×}2Ni2(
μ
-SCH2CH2S)](CF3CO2)2 (3a-3d) and
Complex 1c (X = NCH2CHMe2). Yield: 0.554 g (62%). Anal. Calcd.
for C30H33Cl2NNiP2: C, 60.14; H, 5.55; N, 2.34%. Found: C, 59.91; H,
mononuclear Ni(II) N-protonated complexes [{(Ph2PCH2)2N(H)R}Ni
(SCH2CH2S)](CF3CO2) (4a-4c). In addition, the electrochemical prop-
erties of 2a-2d are studied and compared in the absence and presence of
TFA as a proton source by using cyclic voltammetry (CV).
1
5.34; N, 2.61%. H NMR (600 MHz, d6-DMSO, TMS): δ 7.96–7.31 (m,
20H, 2 x P(C6H5)2), 3.79–3.33 (m, 4H, 2 x PCH2N), 2.10 (d, J = 6.0 Hz,
2H, NCH2), 1.42 (t, J = 6.0 Hz, 1H, CH), and 0.34 (d, J = 6.0 Hz, 6H, 2 x
CH3) ppm. 31P{1H} NMR (243 MHz, d6-DMSO, 85% H3PO4): δ 27.2 (s)
ppm.
2. Experimental section
Complex 1d (X = CH2). Yield: 0.339 g (89%). Anal. Calcd. for
C
27H26Cl2NiP2: C, 59.83; H, 4.84%. Found: C, 59.54; H,5.21%. 1H NMR
2.1. Materials and methods
(600 MHz, d6-DMSO, TMS): δ 7.76–6.93 (m, 20H, 2 x P(C6H5)2) and
3.98–3.30 (m, 6H, P(CH2)3P) ppm. 31P{1H} NMR (243 MHz, d6-DMSO,
85% H3PO4): δ 27.9 (s) ppm.
All reactions and operations were carried out under a dry, oxygen-
free nitrogen atmosphere with standard Schlenkand vacuum-line tech-
niques. Dichloromethane (CH2Cl2) was distilled from CaH2 under N2.
Starting materials NiCl2⋅6H2O, HSCH2CH2SH, Et3N, (Ph2PCH2)2CH2
(dppp), and CF3CO2H (TFA) were commercially available and used as
received. Ligands (Ph2PCH2)2NR (R
= CH2C5H4N-p, CH2C6H5,
Fig. 1. Schematic structures of [FeFe]-H2ase active sites highlighting the Fed core in yellow (left), target mononuclear complexes highlighting the Ni core in pink
(middle), and [NiFe]-H2ase active sites highlighting the Ni core in blue (right). (For interpretation of the references to colour in this figure legend, the reader is
referred to the web version of this article.)
2

X.-L. Gu et al.
Journal of Inorganic Biochemistry 219 (2021) 111449
2.3. General procedure for preparation of mononuclear Ni(II) dithiolate
655.1065). 31P{1H} NMR (243 MHz, CDCl3, 85% H3PO4): δ 9.8 (s, 8%)
ppm for 2a, 8.1 (s, 8%) ppm for 4a, and 3.1 (s, 84%) ppm for 3a.
Complexes 3b and 4b (X = NCH2C6H5). HRESI-MS (MeOH, positive
complexes {(Ph2PCH2)2×}Ni(SCH2CH2S) (2a-2d)
To a CH2Cl2 (15 mL) solution of the as-prepared salts {(Ph2PCH2)2×}
NiCl2 (1a-1d, 0.29 mmol) and dithiol HSCH2CH2SH (0.02 mL, 0.24
mmol) was added Et3N (0.07 mL, 0.48 mmol). The reaction mixture was
stirred for 1 h at room temperature and then the solvent was removed
under reduced pressure. The crude product was purified by preparative
TLC separation using CH2Cl2/EtOAc (1:1, v/v for 2a), CH2Cl2/EtOAc
(15:1, v/v for 2b), and petroleum ether/EtOAc (2:3, v/v for 2c, 2d) as
eluent. The main red band afforded a red solid as the target complex.
Complex 2a (X = NCH2C5H4N-p). Yield: 0.136 g (86%). Anal. Calcd.
for C34H34N2NiP2S2⋅0.5CH2Cl2: C, 59.38; H, 5.06; N, 4.01%. Found: C,
59.56; H, 4.93; N, 4.22%. 1H NMR (600 MHz, CDCl3, TMS): δ 8.39 (d,
JHH = 4.2 Hz, 2H, 2 x C5H4N-o), 7.68 (dd, JPH = 12.0 Hz, JHH = 6.6 Hz,
8H, 2 x P(C6H5-ο)2), 7.41 (t, JHH = 7.8 Hz, 4H, 2 x P(C6H5-p)2), 7.31 (t,
JHH = 7.8 Hz, 8H, 2 x P(C6H5-m)2), 6.78 (d, JHH = 4.2 Hz, 2H, 2 x C5H4N-
m), 3.58 (s, 2H, NCH2C5H4N-p), 3.31 (s, 4H, 2 x PCH2N), and 2.73 (s, 4H,
2 x SCH2) ppm. 31P{1H} NMR (243 MHz, CDCl3, 85% H3PO4): δ 9.4 (s)
ppm.
mode) for [{(Ph2PCH2)2N(CH2C6H5)}2Ni2(μ-SCH2CH2S)](CF3CO2)2
(3b): m/z 607.1133 [(M-2CF3CO2)/2]+ (higher peak, Calcd. 607.1157)
and for [{(Ph2PCH2)2N(H)(CH2C6H5)}Ni(SCH2CH2S)](CF3CO2) (4b):
m/z 654.1080 [M-CF3CO2]+ (highest peak, Calcd. 654.1112). 31P{1H}
NMR (243 MHz, CDCl3, 85% H3PO4): δ 9.0 (br s, 7%) ppm for 2b, 6.5 (s,
56%) ppm for 4b, and 2.3 (br s, 37%) ppm for 3b.
Complexes 3c and 4c (X = NCH2CHMe2). HRESI-MS (MeOH, positive
mode) for [{(Ph2PCH2)2N(CH2CHMe2)}2Ni2(μ-SCH2CH2S)](CF3CO2)2
(3c): m/z 573.1281 [(M-2CF3CO2)/2]+ (low peak, Calcd. 573.1313) and
for [{(Ph2PCH2)2N(H)(CH2CHMe2)}Ni(SCH2CH2S)](CF3CO2) (4c): m/z
620.1226 [M-CF3CO2]+ (highest peak, Calcd. 620.1269). 31P{1H} NMR
(243 MHz, CDCl3, 85% H3PO4): δ 8.9 (s, 3%) ppm for 2c, 6.6 (s, 80%)
ppm for 4c, and 1.1 (br s, 17%) ppm for 3c.
Complexes 3d (X = CH2). HRESI-MS (MeOH, positive mode) for
[{(Ph2PCH2)2CH2}2Ni2(μ-SCH2CH2S)](CF3CO2)2 (3d): m/z 516.0709
[(M-2CF3CO2)/2]+ (highest peak, Calcd. 516.0735). 1H NMR (600 MHz,
CDCl3, TMS): δ 7.62–7.48 (m, 16H, 4 x P(C6H5-ο)2), 7.37–7.34 (m, 24H,
4 x P(C6H5-p,m)2), 3.09 (s, 4H, 2 x SCH2), 2.79 (s, 8H, 4 x PCH2), 1.98 (s,
4H, 2 x CH2CH2CH2) ppm. 31P{1H} NMR (243 MHz, CDCl3, 85%
H3PO4): δ 10.1 (br s, 15%) ppm for 2d, 5.8 (s, 85%) ppm for 3d.
Complex 2b (X = NCH2C6H5). Yield: 0.102 g (65%). Anal. Calcd. for
C
35H35NNiP2S2: C, 64.24; H, 5.39; N, 2.14%. Found: C, 64.11; H, 5.60;
N, 2.39%. 1H NMR (600 MHz, CDCl3, TMS): δ 7.63 (dd, JPH = 11.4 Hz,
JHH = 6.6 Hz, 8H, 2 × P(C6H5-ο)2), 7.37 (t, JHH = 7.2 Hz, 4H, 2 × P
(C6H5-p)2), 7.27 (t, JHH = 7.2 Hz, 8H, 2 × P(C6H5-m)2), 7.23 (t, JHH
=
2.5. X-ray crystal structure determination
7.2 Hz, 3H, NCH2(C6H5-o,p)2), 6.97 (d, JHH = 7.2 Hz, 2H, NCH2(C6H5-
m)2), 3.60 (s, 2H, NCH2C6H5), 3.30 (s, 4H, 2 × PCH2N), and 2.73 (s, 4H,
2 × SCH2) ppm. 31P{1H} NMR (243 MHz, CDCl3, 85% H3PO4): δ 9.1 (s)
ppm.
Single crystals of complexes 1b, 2a-2d and 3d suitable for X-ray
diffraction analysis were grown by slow evaporation of the CH2Cl2/n-
hexane or CH2Cl2/ petroleum ether solution at -5 ◦C or -20 ◦C. The
crystals were mounted on a Bruker-CCD diffractometer. Data were
collected at 150(2) or 293 K using a graphite monochromator with Mo
Complex 2c (X = NCH2CHMe2). Yield: 0.097 g (65%). Anal. Calcd.
for C32H37NNiP2S2⋅0.5CH2Cl2: C, 58.89; H, 5.78; N, 2.11%. Found: C,
58.66; H, 6.15; N, 2.29%. 1H NMR (600 MHz, CDCl3, TMS): δ 7.75 (dd,
JPH = 12 Hz, JHH = 7.8 Hz, 8H, 2 × P(C6H5-ο)2), 7.39 (t, JHH = 7.8 Hz,
4H, 2 × P(C6H5-p)2), 7.31 (t, JHH = 7.8 Hz, 8H, 2 × P(C6H5-m)2), 3.27 (s,
4H, 2 × PCH2N), 2.73 (s, 4H, 2 × SCH2), 2.21 (d, JHH = 7.2 Hz, 2H,
NCH2), 1.65–1.58 (m, 1H, NCH2CH), and 0.64 (d, JHH = 6.6 Hz, 6H, 2 ×
CH3) ppm. 31P{1H} NMR (243 MHz, CDCl3, 85% H3PO4): δ 8.9 (s) ppm.
Complex 2d (X = CH2). Yield: 0.063 g (47%). Anal. Calcd. for
Kɑ radiation (λ = 0.71073 Å) in the
ω-φ scanning mode. The structure
was solved by direct methods using the SHELXS-97 program and refined
by full-matrix least-squares techniques (SHELXS-97) on F2 [30].
Hydrogen atoms were located using the geometric method. Details of
crystallographic data and structure refinement for 1b, 2a-2d and 3d are
summarized in Table 1.
C
29H30NiP2S2: C, 61.83; H, 5.37%. Found: C, 61.98; H, 5.64%. 1H NMR
2.6. Electrochemical tests
(600 MHz, CDCl3, TMS): δ 7.67 (m, 8H, 2 x P(C6H5-ο)2), 7.37 (t, JHH
=
7.2 Hz, 4H, 2 x P(C6H5-p)2), 7.31 (t, JHH = 7.2 Hz, 8H, 2 x P(C6H5-m)2),
2.73 (s, 4H, 2 x SCH2), 2.33 (s, 4H, 2 x PCH2), and 2.00 (s, 2H,
CH2CH2CH2) ppm. 31P{1H} NMR (243 MHz, CDCl3, 85% H3PO4): δ
10.26 (s) ppm.
Electrochemical and electrocatalytic properties of complexes 2a-2d
and their main protonated products 3a, 4b, 4c, 3d were studied by
cyclic voltammetry (CV) in MeCN solution. As the electrolyte, n-
Bu4NPF6 was recrystallized multiple times from a CH2Cl2 solution by the
addition of hexane. All the CV measurements were recorded using a
Gamry Interface potentiostat which was connected to a glassy carbon
working electrode (3 mm diameter), a platinum wire counter electrode,
and Ag/AgCl reference electrode. CV measurements were conducted
using a three-neck electrochemical cell that was washed and dried in an
oven overnight before use. All electrochemical experiments were con-
ducted under a N2 atmosphere. The potential scale was calibrated
against the Fc/Fc+ couple and reported versus this reference system.
2.4. General procedure for protonation of complexes 2a-2d with excess
TFA to form dinuclear Ni(II)-Ni(II) dication complexes
[{(Ph2PCH2)2×}2Ni2(μ-SCH2CH2S)](CF3CO2)2 (3a-3d) and
mononuclear Ni(II) N-protonated complexes [{(Ph2PCH2)2×(H)}Ni
(SCH2CH2S)](CF3CO2) (4a-4c)
A dry CH2Cl2 (5 mL) solution of the above-obtained complexes 2a-2d
(0.1 mmol) was treated with 10 equivalents of TFA (74 μL, 1.0 mmol).
The reaction mixture was stirred at room temperature for 1 h to give a
solution colour change from orange-red to black-red. After this reaction
was completed by TLC monitor, the volume was reduced under vacuum
and n-hexane was added. Upon the slow diffusion of n-hexane into the
above-concentrated CH2Cl2 solution at -20 ◦C overnight, a black-red
solid was precipitated and dried to afford the protonated products 3a-
3d and 4a-4c accompanied by a slight amount of neutral precursors 2a-
2d.
3. Results and discussion
3.1. Synthesis and characterization of mononuclear Ni(II) dichloride
precursors [{(Ph2PCH2)2×}NiCl2] (X = CH2C5H4N-p, 1a; CH2C6H5, 1b;
CH2CHMe2, 1c; and CH2, 1d)
The diphosphine-chelate mononuclear Ni(II) precursors 1a-1d can
be readily obtained from the treatments of diphosphines (Ph2PCH2)2×
(X = CH2C5H4N-p, CH2C6H5, CH2CHMe2, and CH2) with mononickel salt
NiCl2⋅6H2O in the mixed CH2Cl2/EtOH solvent at room temperature, as
displayed in Scheme 1.
Complexes 3a and 4a (X = NCH2C5H4N-p). HRESI-MS (MeOH, pos-
itive mode) for [{(Ph2PCH2)2N(CH2C5H4N-p)}2Ni2(μ-SCH2CH2S)]
(CF3CO2)2 (3a): m/z 608.1094 [(M-2CF3CO2)/2]+ (highest peak, Calcd.
608.1109) and for [{(Ph2PCH2)2N(H)(CH2C5H4N-p)}Ni(SCH2CH2S)]
(CF3CO2) (4a): m/z 655.1038 [M-CF3CO2]+ (low peak, Calcd.
The as-prepared precursors 1a-1d are air-stable solids, in which it is
easy for 1b-1d but is difficult for 1a to be dissolved in most organic
3

X.-L. Gu et al.
Journal of Inorganic Biochemistry 219 (2021) 111449
Table 1
Details of crystallographic data and structure refinement for 1b, 2a-2d, and 3d.
Complex
1b
2a
2b
2c
2d
3d
Empirical
formula
Formula
weight
C
33H31Cl2NNiP2•CH2Cl2
C34H34N2NiP2S2•0.5CH2Cl2
C35H35NNiP2S2
C32H37NNiP2S2•CH2Cl2
C29H30NiP2S2
C56H56Ni2P4S2•2CF3CO2•2CF3CO2H
718.06
150(2)
0.71073
697.86
150(2)
0.71073
654.41
150(2)
0.71073
705.32
150(2)
0.71073
563.30
1488.52
293
Temperature
(K)
150(2)
Wavelength
(Å)
0.71073
0.71073
Crystal system
Space group
a (Å)
orthorhombic
P212121
8.0265(5)
19.9078(15)
20.9194(17)
90
monoclinic
P21
monoclinic
P21/n
monoclinic
P21/c
monoclinic
P21/n
triclinic
P-1
13.6242(12)
13.7786(12)
17.6737(16)
90
8.5386(4)
19.9641(7)
19.0182(9)
90
18.6993(16)
9.3828(7)
21.9922(17)
90
10.1801(8)
24.5226(17)
11.2237(10)
90
13.0245(7)
13.4777(7)
22.1673(11)
98.063(2)
93.394(2)
117.525(2)
3382.4(3)
2
b (Å)
c (Å)
α
(◦)
β (◦)
γ (◦)
V (Å3)
Z
90
103.897(3)
90
101.970(2)
90
121.408(6)
90
106.643(3)
90
90
3342.7(4)
4
3220.6(5)
4
3171.5(2)
4
3293.2(5)
4
2684.5(4)
4
D
calc (g cmꢀ 3
)
1.427
1.439
1.371
1.423
1.394
1.462
μ
(mmꢀ 1
)
1.021
0.943
0.870
1.000
1.014
0.798
F(000)
1480.0
1452.0
1368.0
1472.0
1176.0
1524.0
Crystal size
(mm)
0.24 × 0.18 × 0.14
0.22 × 0.16 × 0.14
0.22 × 0.18 ×
0.16
0.28 × 0.22 × 0.14
0.32 × 0.26
× 0.24
0.40 × 0.37 × 0.30
θmin, θmax (◦)
Reflections
collected/
unique
4.398, 52.796
48,734/6794
4.268, 52.89
4.63, 52.794
70,020/6499
4.67, 52.828
43,501/6743
4.494, 52.856
36,373/5487
4.596, 51.998
53,627/13217
44,241/13178
Rint
0.0748
0.0668
0.0890
0.0920
0.1154
0.0415
hkl Range
-8 ≤ h ≤ 10
ꢀ 24 ≤ k ≤ 24
ꢀ 26 ≤ l ≤ 26
1.76/0.99
ꢀ 17 ≤ h ≤ 17
ꢀ 17 ≤ k ≤ 17
ꢀ 22 ≤ l ≤ 22
1.90/0.99
ꢀ 10 ≤ h ≤ 10
ꢀ 24 ≤ k ≤ 24
ꢀ 23 ≤ l ≤ 23
0.999
ꢀ 23 ≤ h ≤ 23
ꢀ 11 ≤ k ≤ 11
ꢀ 27 ≤ l ≤ 27
0.997
ꢀ 12 ≤ h ≤ 12
ꢀ 30 ≤ k ≤ 30
ꢀ 14 ≤ l ≤ 14
0.993
ꢀ 16 ≤ h ≤ 16
ꢀ 16 ≤ k ≤ 16
ꢀ 27 ≤ l ≤ 27
0.996
Completeness
to θmax (%)
Data/
6794/0/380
13,178/7/767
6499/0/370
6743/0/372
5487/0/307
13,217/223/859
restraints/
parameters
Goodness-of-
fit (GOF) on
F2
1.032
1.012
1.055
1.197
1.030
1.125
R1/wR2[I > 2
(I)]
σ
0.0415/0.0983
0.0571/0.1066
0.45/ꢀ 0.35
0.0560/0.1421
0.0834/0.1614
0.76/ꢀ 0.83
0.0383/0.0747
0.0615/0.0830
0.40/ꢀ 0.30
0.1101/0.2705
0.1325/ 0.2814
1.79/ꢀ 1.72
0.0491/
0.1076
0.0574/0.1595
0.0768/0.1742
1.70/ꢀ 0.65
R1/wR2(all
data)
0.0909/
0.1275
Largest
1.16/ꢀ 0.71
difference
peak/ hole
(e Aꢀ 3
)
Scheme 1. Synthesis of mononuclear Ni(II) dichloride precursors 1a-1d
solvents such as CH2Cl2, EtOH, etc. Although complexes 1c and 1d are
known [31,32], all the precursors 1a-1d have been characterized by
elemental analysis, NMR (1H, 31P) spectroscopy, and by X-ray crystal-
lography for 1b. The 1H NMR spectra of 1a-1d in d6-DMSO show their
phenyl proton signals in the downfield region of δ 7.8–6.9 ppm and
their methylene proton signals in the upfield regionHof δH 4.3–3.3 ppm.
Furthermore, the 31P{1H} NMR spectra of 1a-1d in d6-DMSO display a
selected lengths and angles. It crystallizes in the orthorhombic space
group P212121 with a target molecule and a dichloromethane solvent in
the asymmetric unit (Table 1). As presented in Fig. 2, precursor 1b
features a mononuclear Ni(II) core with one chelate diphosphine and
two terminal chlorides, thus giving rise to a square-planar coordination
around Ni center. It is worth noting that a steady six-membered metal-
locycle NiPCNCP is formed between the PCNCP diphosphine and the Ni
sharp singlet at δ ca. 28 ppm for two symmetrical phosphorus atoms of
core, wherein the P-Ni-P angle (95.19 (6)o) is much lower in contrast to
P
o
–
the (Ph2PCH2)2× diphosphines chelated to central nickel atom.
Most intuitively, the molecular struture of 1b is further confirmed by
single-crystal X-ray diffraction analysis, as illustrated in Fig. 2 with
the C-N-C angle (109.1 (5) ). In addition, the Ni P lengths (ca. 2.16 Å)
–
are smaller relative to the Ni Cl lengths (ca. 2.21 Å), probably due to
the following observation that two P atoms and two Cl atoms are
4

X.-L. Gu et al.
Journal of Inorganic Biochemistry 219 (2021) 111449
the molecular structures of the previously reported mononuclear Ni(II)
dithiolate analogues are inferred only through spectroscopic studies
[27]. The single crystals of 2a-2d were grown in a mixed CH2Cl2/n-
hexane solution at low temperature for days before being subjected to
the X-ray diffraction analysis as displayed in Fig. 3. The selected lengths
and angles of 2a-2d are listed in Table 2.
All the solid-state structures of 2a-2d contain a mononuclear Ni(II)
cluster core ligated by two phosphorus atoms (i.e., P1 and P2) of the
(Ph2PCH2)2× diphosphines and two sulfur atoms (i.e., S1 and S2) of the
edt bridges (Fig. 3). The central Ni(II) cores of 2a-2d all adopt a slightly
distorted square-planar coordination geometry, which are similar to
those observed previously for some mononuclear Ni(II) analogues of the
type {(Ph2P)2NR}Ni(dithiolate) with chelating PNP ligands (labeled
PNP = (Ph2P)2NR) [33–37]. The (Ph2PCH2)2× diphosphines in 2a-2d all
adopt a chair configuration with a six-membered metallocycle of
NiP2C2N or NiP2C3, whereas their edt bridges exhibit a nearly coplanar
conformation with a five-membered metallocycle of NiS2C2. This is
possibly due to the following observation that the P(1)-Ni(1)-P(2) bite
angles (ca. 95o) of the PCNCP ligands are a little larger than the S(1)-Ni
(1)-S(2) ones (ca. 92o) of the edt bridges except for 2a. It is worth
mentioning that the dihedral angles (more than 10o) between the two
planes defined by the P-Ni-P and S-Ni-S atoms in 2a-2d with chelating
PCNCP ligands are larger than those (3o to 8o) observed in the known
mononuclear Ni(II) analogues with chelating PNP ligands [33–37]. This
finding implies that the central X groups of the (Ph2PCH2)2× diphos-
phines in 2a-2d may be closer to the Ni atom relative to those of the
(Ph2P)2NR diphosphines in the reported mononuclear Ni analogues
[33–37].
Fig. 2. Molecular structure of precursor 1b with thermal ellipsoids at 40%
probability. Hydrogen atoms and solvent molecules are omitted and phenyl
groups are represented as sticks for clarity. Selected lengths (Å) and angles
(deg): Ni(1)-P(1) 2.1648 (15), Ni(1)-P(2) 2.1607 (15), Ni(1)-Cl(1) 2.2028 (14),
Ni(1)-Cl(2) 2.2107 (14); P(1)-Ni(1)-P(2) 95.19 (6), Cl(1)-Ni(1)-Cl(2) 93.11 (5),
C(1)-N(1)-C(2) 109.1 (5).
respectively chelated and terminal to the Ni core (Fig. 2).
3.2. Synthesis and characterization of mononuclear Ni(II) dithiolate
complexes [{(Ph2PCH2)2×}]Ni(SCH2CH2S) (X = CH2C5H4N-p, 2a;
CH2C6H5, 2b; CH2CHMe2, 2c; and CH2, 2d)
3.3. Protonation studies of mononuclear Ni(II) complexes 2a-2d with
excess TFA
Although a few mononuclear Ni(II) dithiolate/diphosphine com-
plexes have been reported previously [27], protonation studies of these
complexes were not pursued and help to understand the catalytic proton
reduction steps for efficient HER by them. Thus, the protonation re-
actions of complexes 2a-2d with 10 equiv. TFA at room temperature
resulted in the unexpected formations of dinuclear Ni(II)-Ni(II) dication
As shown in Scheme 2, the room-temperature treatments of the
above-obtained precursors 1a-1d with HSCH2CH2SH in the presence of
Et3N as acid-binding agent in CH2Cl2 gave rise to a series of mono-
nuclear Ni(II) dithiolate complexes 2a-2d with chelating diphosphines
in 47–86% yields.
The new target complexes 2a-2c together with a known reference
complex 2d are air-stable solids and characterized by elemental anal-
ysis, NMR spectroscopy, as well as X-ray crystallography. The 1H NMR
spectra of 2a-2d all show a doublet of doublet at δ ca. 7.7 ppm and two
complexes [{(Ph2PCH2)2×}2Ni2(μ-SCH2CH2S)](CF3CO2)2 (3a-3d) [27]
and mononuclear Ni(II) N-protonated complexes [{(Ph2PCH2)2×(H)}Ni
(SCH2CH2S)](CF3CO2) (4a-4c) as shown in Scheme 3.
The formations of these protonated products 3a-3d and 4a-4c are
well supported by the HRESI-MS, NMR (31P, 1H) as well as FT-IR spectra,
and especially by the X-ray crystallography for 3d. First of all, the
HRESI-MS and 31P{1H} spectra for protonated products of complexes
2a-2c with PCNCP ligands display similar characteristics as follows. For
protonated product of 2a, its positive-mode HRESI-MS spectrum gives a
highest ion peak at m/z = 608.1094 and a low adjacent ion peak at m/z
= 655.1038 (Fig. S5), which are respectively ascribed to the half of
triplets at δ ca. 7.4, 7.3 ppm for the phenyl protoHns of the diphosphine
H
ligands in addition to a broad singlet at δ ca. 2.7 ppm for methylene
H
protons of the dithiolate bridges. Meanwhile, a broad singlet is observed
at δH ca.3.3 ppm for 2a-2c but at δH 2.33 ppm for 2d, being attributed to
the phosphorus-attached methylene protons of diphosphines in their 1H
NMR spectra. This finding is possibly due to the difference in the central
X moieties of the (Ph2PCH2)2× diphosphines, i.e., X = NR for 2a-2c and
X = CH2 for 2d. Additonally, the 31P{1H} NMR spectra of 2a-2c with
dication [{(Ph2PCH2)2N(CH2C5H4N-p)}2Ni2(
μ
-SCH2CH2S)]2+ (Calcd.
PCNCP ligands exhibit a sharp singlet at δ ca. 9 ppm whereas that of 2d
608.1109 [(M-2CF3CO2)/2]+) from the main dinuclear Ni(II)-Ni(II)
product 3a and the monocation [{(Ph2PCH2)2N(H)(CH2C5H4N-p)}Ni
(SCH2CH2S)]+ (Calcd. 655.1065 [M-CF3CO2]+) from a little mono-
nuclear Ni(II) N-protonated product 4a. This is well line with the cor-
responding 31P{1H} NMR spectrum (Fig. 4A) that shows one strong
phosphorus signal at δP 3.1 ppm for 3a and another a very weak P signal
P
with dppp displays a strong singlet at δP 10 ppm, being accordant with
the 31P NMR signals at δ 7–10 ppm reported previously for (diphos-
P
phine)Ni(dithiolate) [27].
Further, we managed to obtain the crystal structures of mononuclear
Ni(II) complexes 2a-2d with chelating (Ph2PCH2)2× diphosphines, since
Scheme 2. Synthesis of mononuclear Ni(II) dithiolate complexes 2a-2d
5

X.-L. Gu et al.
Journal of Inorganic Biochemistry 219 (2021) 111449
Fig. 3. Molecular structures of complexes 2a (A), 2b (B), 2c (C) and 2d (D) with thermal ellipsoids at 50% probability. Hydrogen atoms and solvent molecules are
omitted and phenyl groups are represented as sticks for clarity.
with a low adjacent ion peak at m/z = 607.1133 and 573.1281 for the
Table 2
half of dication [{(Ph2PCH2)2NR}2Ni2(
μ
-SCH2CH2S)]2+ (R = CH2C6H5
Selected lengths and angles for complexes 2a-2d.
for Calcd. 607.1157 and CH2CHMe2 for Calcd. 573.1313 [(M-2CF3CO2)/
2]+) from the minor dinuclear Ni(II)-Ni(II) products 3b and 3c. This is
well accordant with their 31P{1H} NMR spectra (Fig. 4B and C) that all
Complex
2a
2b
2c
2d
Ni(1)-P(1)
2.157 (2)
2.155 (2)
2.177 (2)
2.171 (3)
90.23 (9)
92.40 (10)
2.1484 (7)
2.1641 (7)
2.1760 (7)
2.1797 (7)
95.03 (3)
92.00 (3)
2.162 (2)
2.151 (2)
2.181 (2)
2.178 (3)
95.91 (9)
92.01 (10)
2.1787 (10)
2.1761 (10)
2.1660 (11)
2.1839 (10)
92.00 (4)
Ni(1)-P(2)
show a strong phosphorus signal at δ 6.5 ppm for 4b and 6.6 ppm for 4c
P
Ni(1)-S(1)
[38,39] in company with a weak P signal at δP 2.3 ppm for 3b and 1.1
ppm for 3c, wherein the percentage contents correspond to 56% for 4b,
80% for 4c, 37% for 3b, and 17% for 3c. However, for protonated
product of 2d with nitrogen-free diphosphine (dppp), a highest ion peak
at m/z = 516.0709, corresponding to the half of dication [{(Ph2PCH2)2N
Ni(1)-S(2)
P(1)-Ni(1)-P(2)
S(1)-Ni(1)-S(2)
91.24 (4)
-SCH2CH2S)]2+ (Calcd. 516.0735 [(M-2CF3CO2)/2]+)
at δP 8.1 ppm for 4a [38,39] with the percentage contents of 84% and
8%, respectively. In contrast, for protonated products of 2b and 2c, their
positive-mode HRESI-MS spectra (Figs. S6 and S7) present a highest ion
peak at m/z = 654.1080 and 620.1226 for monocation [{(Ph2PCH2)2N
(CH2)}2Ni2(
μ
from the sole dinuclear product 3d, is noticed in its positive-mode
HRESI-MS spectrum (Fig. S8). This is in good agreement with only a
new phosphorus signal at δ 5.8 ppm for 3d observed in its 31P{1H} NMR
P
(H)R}Ni(SCH2CH2S)]+ (R
= CH2C6H5 for Calcd. 654.1112 and
spectrum (Fig. 4D). In addition, further analysis of the FT-IR spectral
region in the region of 2700–2300 cmꢀ 1 shows no specific differences for
both neutral precursors 2a-2d and their protonated products (Fig. S9),
CH2CHMe2 for Calcd. 620.1269 [M-CF3CO2]+) from the main N-pro-
tonated mononuclear Ni(II) products 4b and 4c respectively, together
6

X.-L. Gu et al.
Journal of Inorganic Biochemistry 219 (2021) 111449
Scheme 3. Protonation reactions of 2a-2d with excess TFA (10 equivalents).
Fig. 4. Comparisons for 31P{1H} NMR spectra (A-D) of mononuclear Ni(II) complexes 2a-2d (down) versus the respective protonated products 3a-3d as well as 4a-4c
(up), respectively.
7

X.-L. Gu et al.
Journal of Inorganic Biochemistry 219 (2021) 111449
suggesting that sulfur protonation didn’t occur [40]. The high-field re-
gion 1H NMR spectra for protonated products of 2a-2d all don’t show
any proton signals (Fig. S10), indicating no formation of the Ni-
protonated species (δ - = ca. -13 ~ ꢀ 16 ppm) [27,38,39].
sꢀ 1 under N2 atmosphere by using CV technique (Fig. 6) and their
electrochemical data are given in Table 3.
As presented in Fig. 6, the cyclic voltammograms (CVs) of 2a-2d all
display a reversible reduction peak at Epc = ꢀ 2.07 (ipc/ipa = 1.0), ꢀ 2.09
(ipc/ipa = 1.0), ꢀ 2.11 (ipc/ipa = 1.0), and ꢀ 2.10 V (ipc/ipa = 1.0),
respectively, which are attributed to the Ni(II/I) redox couple [27]. In
addition, the CVs of 2a-2d collected at multiple scan rates from 0.05 to
0.3 V sꢀ 1 in 0.1 M n-Bu4NPF6/MeCN solution can be used to construct
Cottrell plots of reduction peak current (ipc) against square root of scan
More fortunately,NtihHe molecular structure of dinuclear Ni(II)-Ni(II)
complex 3d discussed above is definitely confirmed by X-ray crystal-
lography, wherein the perspective drawing of its dication
[{(Ph2PCH2)2N(CH2)}2Ni2(
μ
-SCH2CH2S)]2+ is illustrated in Fig. 5 with
selected distances and angles. This dication of 3d features a butterfly-
shaped [Ni2S2] framework with one bridging ethanedithiolate (edt)
and two chelating diphosphines (dppp), as reported for a known dinu-
rate (
ν
1/2) as shown in Fig. S11, wherein there is a linear correlationship
between ipc and ν1/2 (insert of Fig. S11). This indicates that the elec-
trochemical Ni(II/I) reduction processes in 2a-2d are diffusion limited in
MeCN solution with the similar diffusion coefficents of ~10ꢀ 5 cm2/s
[41–45], thus demonstrating they are homogeneous pre-electrocatalyst
[44,45].
clear analogue [(dppe)2Ni2(
μ
-SC3H6S)]2+ [27]. Each Ni(II) core lies in
an almost square-planar coordination geometry and its dppp ligand
displays a chair conformation of the six-membered NiP2C3 ring, which is
comparable to its mononuclear Ni(II) precursor 2d. Furthermore, the
–
–
Ni P and Ni S distances around Ni(1) are slightly shorter by 0.01 and
0.03 Å relative to those around Ni(2), respectively. This is possibly due
to the following fact that the P-Ni-P and S-Ni-S angles around Ni(1) are
Further electrocatalytic behaviors of proton reduction to H2 cata-
lyzed by complexes 2a-2d are investigated with TFA (pKMa eCN = 12.7)
[41] by using CV technique (Fig. 7) and their electrochemical data upon
10 mM TFA are listed in Table 3.
–
different from those around Ni(2). Notably, the Ni Ni distance is
2.9911(6) Å, which is significantly longer than those observed for nat-
ural [FeFe]-H2ases (2.55–2.62 Å) [8,9] but is closer to those found in
native [NiFe]-H2ases (2.5–2.9 Å) [21–23]. This finding implies that
dinuclear Ni2S2 complex 3d could be structurally considered as an active
site model of [NiFe]-H2ases.
As displayed in Fig. 7, the CVs of 2a-2d upon sequential additions of
0–10 mM TFA all show an almost identical electrocatalytic feature for
proton reduction to H2. First of all, the addition of 2 mM TFA gives rise
to a new and more positive reduction peak at Epc = ca. -1.70 V for 2a-2c
with PCNCP ligands and at Epc = ꢀ 1.65 V for 2d with dppp (Fig. 7 and
Table 3), which is accompanied by a nearly disppearence of their initial
reduction peaks at Epc = ca. -2.10 V. Furthermore, the new reduction
peak currents of 2a-2c at Epc = ca. -1.70 V and 2d at Epc = ꢀ 1.65 V
display the dramatical and linear increasement upon the consecutive
additions of acid concentration ([TFA], 2–10 mM) as seen for inserts of
Fig. 7, indicating that the typical catalytic proton reduction processes
take place at new reduction potentials of Epc = ca. -1.70 and ꢀ 1.65 V
observed above [42–51]. It is noted that the aforementioned behaviors
of 2a-2d with TFA are analogous to those observed previously for
several diphosphine-chelate dinuclear Fe2S2 complexes with strong
acids such as HOTs, HBF4, TFA [49–51]. However, in contrast to those
with excess strong acid (TFA), the CVs of 2a-2d with excess weak acid
(HOAc, 0–10 mM) show the distinct electrocatalytic feature for proton
reduction to H2 as follows. The initial reduction peak currents of 2a-2d
at1Epc = ca. -2.10 V do not almost grow but the new reduction peak
currents at Epc = ca. -2.25 V (i.e., more negative reduction potential
Therefore, these aforemetioned results have shown that protonation
of the PCNCP-chelate complexes 2a-2c produced a mixture of dinuclear
Ni(II)-Ni(II) dication complexes 3a-3c and mononuclear Ni(II) N-pro-
tonated complexes 4a-4c, whose main protonated products are assigned
to 3a, 4b, and 4c, respectively. By contrast, protonation of the dppp-
chelate complex 2d afforded a sole dinuclear Ni2S2 analogue 3d. It is
interesting to note that the resulting protonated products of 2a-2d, i.e.,
complexes 3a-3d and 4a-4c, might be involved in the following elec-
trocatalytic proton reduction by 2a-2d in the presence of TFA as a
proton source.
3.4. Electrochemical and electrocatalytic studies of mononuclear Ni(II)
complexes 2a-2d
The electrochemical properties of mononuclear Ni(II) complexes 2a-
2d are studied in 0.1 M n-Bu4NPF6/MeCN solution at a scan rate of 0.1 V
Fig. 5. Molecular structure of dication [{(Ph2PCH2)2N(CH2)}2Ni2(μ-
SCH2CH2S)]2+ from dinuclear Ni(II)-Ni(II) complex 3d with thermal ellipsoids
at 35% probability. Hydrogen atoms are omitted and phenyl groups are rep-
resented as sticks for clarity. Selected lengths (Å) and angles (deg): Ni(1)-Ni(2)
2.9911(6), Ni(1)-P(1) 2.1989(10), Ni(1)-P(2) 2.1928(11), Ni(2)-P(3) 2.1983
(10), Ni(2)-P(4) 2.1935(10), Ni(1)-S(1) 2.2047(10), Ni(1)-S(2) 2.2049(10), Ni
(2)-S(1) 2.2348(10), Ni(2)-S(2) 2.2305(10); P(1)-Ni(1)-P(2) 91.84(4), P(3)-Ni
(2)-P(4) 97.25(4), S(1)-Ni(1)-S(2) 81.10(4), S(1)-Ni(2)-S(2) 79.88(4).
Fig. 6. CVs of 1.0 mM complexes 2a (black), 2b (red), 2c (blue), and 2d (pink)
recorded in 0.1 M n-Bu4NPF6/MeCN solution at a scan rate of 0.1 V sꢀ 1. All
potentials are versus the ferrocene/ferrocenium (Fc0/+) couple. (For interpre-
tation of the references to colour in this figure legend, the reader is referred to
the web version of this article.)
8

X.-L. Gu et al.
Journal of Inorganic Biochemistry 219 (2021) 111449
ꢀ 1.65 V (dash lines of Fig. 8A, D and Table 3). In contrast, the main
mononuclear Ni(II) N-protonated products 3b and 3c show a nearly
identical initial reduction peak at Epc = ca. -1.60 V (solid lines of Fig. 8B,
C and Table 3), whose potentials are obviously more positive than those
observed for their neutral precursors 2b and 2c with 10 mM TFA at Epc
= ca. -1.70 V (dash lines of Fig. 8B, C and Table 3). It is worth
mentioning that the CVs of homologues 2a-2c all present a very similar
Table 3
Relevant electrochemical data for neutral complexes 2a-2d and their main
protonated products 3a, 4b, 4c, 3d prepared above.
Complex
aEpc (V vs. Fc+/Fc)
bEpc (V vs. Fc+/Fc)
cEpc (V vs. Fc+/Fc)
2a
2b
2c
2d
ꢀ 2.07
ꢀ 2.09
ꢀ 2.11
ꢀ 2.10
ꢀ 1.69
ꢀ 1.73
ꢀ 1.70
ꢀ 1.65
ꢀ 1.67
ꢀ 1.59
ꢀ 1.60
ꢀ 1.62
reduction potential at Epc = ca. -1.70 V to that for reference 2d (Epc
=
a
ꢀ 1.65 V) in the presence of 10 mM TFA (dash lines of Figs. 8A-8C and
Table 3). Meanwhile, the 31P{1H} spectrum of the electroactive species
in situ formed upon addition of 10 mM TFA to 2d shows only a phos-
phorus signal at δ 5.9 ppm assigned to 3d, being well consistent with
that (δP = 5.8 ppPm) observed above for the as-prepared complex 3d
(Fig. S13). Thus, these results strongly suggest the formation of the same
electroactive species 3a-3c as 3d under excess TFA and electrochemical
condition.
Epc is defined as the potential of initial reduction peak for complex 2a-2d at
0 mM TFA.
b
Epc is defined as the potential of initial reduction peak for complex 2a-2d at
10 mM TFA.
c
Epc is defined as the potential of initial reduction peak for the main pro-
tonated products 3a, 4b, 4c, and 3d obtained in the above protonation studies.
appeared upon 2 or 4 mM) sharply and linearly increase as the con-
centrations of HOAc are from 2 to 10 mM (Fig. S12). These observations
imply that the complete protonations of 2a-2d under excess TFA and
electrochemical condition gave rise to the new and similar electroactive
species 3a-3d as observed in the chemical protonation studies (Scheme
3), thus affording the new and more positive reduction peak potentials
at Epc = ca. -1.70 or ꢀ 1.65 V.
On the basis of these CV findings discussed above for 2a-2d and some
similar cases reported previously for diphosphine-chelate diiron
dithiolate complexes [49–51], the electrocatalytic processes of proton
reduction to H2 by 2a-2d are proposed at their new reduction potentials
of Epc = ca. -1.70 or ꢀ 1.65 V, wherein that of 2d as a representative of
2a-2d is presented in Scheme 4. Firstly, mononuclear neutral complex
2d (labeled [NiII]0) is completely protonated with excess TFA to form a
dinuclear Ni(II)-Ni(II) species [3d]2þ (labeled [NiII-NII]2+). Then, the
To further support the formation of the electroactive species 3a-3d
proposed during eletrocatalysis of 2a-2d with excess TFA, we tested the
CV curves of the main protonated products 3a, 4b, 4c, and 3d prepared
above, which are compared with those of 2a-2d upon 10 mM TFA as
illustrated in Fig. 8. The relevant electrochemical data of 3a, 4b, 4c, and
3d are summarized in Table 3. First of all, the main dinuclear Ni(II)-Ni
(II) dication products 3a and 3d display a similar initial reduction peak
at Epc = ꢀ 1.67 and ꢀ 1.62 V (solid lines of Fig. 8A, D and Table 3)
respectively, the potentials of which are closer to those observed for
their neutral precursors 2a and 2d upon 10 mM TFA at Epc = ꢀ 1.69 and
resultant dication [3d]2þ is reduced at ꢀ 1.65 V (but as for 2a-2c, Epc
=
ca. -1.70 V) to produce its reduced species [3d]þ (labeled [NiII-NI]+),
which is protonated by TFA to generate the hydride species [3d(
μ
μ
H)]2þ
H)]2þ
(labeled [NiIII(
μ
H)NII]2+). Afterwards, the resulting species [3d(
would be further protonated by TFA to afford the hydrogen-binding
species [3d(H2)]3þ (labeled [NiIII(H2)NII]3+), which finally accepts an
electron to form H2 and return [3d]2þ and completes a catalytic cycle.
It is therefore worth pointing out that these dinclear Ni(II)-Ni(II) species
Fig. 7. CVs of 1.0 mM 2a (A), 2b (B), 2c (C), and 2d (D) in 0.1 M n-Bu4NPF6/MeCN solution upon additions of 0–10 mM TFA at a scan rate of 0.1 V sꢀ 1. Insert: Plots
of the catalytic peak current (icat) against the added acid concentrations ([TFA]). All potentials are versus the ferrocene/ferrocenium (Fc0/+) couple.
9

X.-L. Gu et al.
Journal of Inorganic Biochemistry 219 (2021) 111449
Fig. 8. Comparisons for CVs (A-D) of complexes 2a-2d with 10 mM TFA (1.0 mM, dash line) versus the respective main protonated products 3a, 4b, 4c, and 3d
prepared above (1.0 mM, solid line).
Scheme 4. Proposed electrocatalytic processes of proton reduction to H2 by complex 2d as a representive of mononuclear Ni(II) complexes 2a-2d under excess TFA.
3a-3d, which are formed in situ from the complete protonations of 2a-2d
with excess TFA under CV condition, are all found to be electro-
catalytically active for proton reduction to H2.
Notably, the comparative studies on the chemical protonations and
electrochemical properties of 2a-2d suggest that (i) a new series of
dinuclear Ni(II)-Ni(II) dication complexes 3a-3d were unexpectedly
formed from the facile protonations of precursors 2a-2d with excees
TFA; and (ii) they as electroactive species are responsible for the cata-
lytic proton reduction to H2 under electrochemical condition and excess
strong acid (TFA). Further X-ray crystallographic analysis of 3d shows
4. Conclusions
In summary, we synthesized and characterized three new mono-
nuclear Ni(II) dithiolate complexes 2a-2c with PCNCP ligands and a
known reference analogue 2d with dppp, which are structurally related
to the Fed core of [FeFe]-H2ases and the Ni core of [NiFe]-H2ases.
–
that its Ni Ni distance is closer to those found in [NiFe]-H2ases but is
much larger than those observed in [FeFe]-H2ases. Thus, this new type
of dinuclear Ni2S2 complexes like 3a-3d could be considered as active
10

X.-L. Gu et al.
Journal of Inorganic Biochemistry 219 (2021) 111449
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Appendix A. Supplementary data
CCDC numbers 2053145 (1b), 2053146 (2a), 2053147 (2b),
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