Heteroleptic dmit nickel complexes with bis(diphenylphosphanyl)amine ligands as robust molecular electrocatalysts for hydrogen evolution

Article abstract of DOI:10.1002/aoc.6123

Four new neutral heteroleptic dmit nickel complexes bearing bis(diphenylphosphanyl)amine ligands, [RN (PPh₂)₂Ni(dmit)] (where dmit^2? = 1,3-dithiole-2-thione-4,5-dithiolate; R = (CH₂)₄CH₃ [1], (CH₂)₃OCH₃ [2], (CH₂)₂CH(CH₃)₂ [3], and CHPhCH₃ [4]), have been synthesized in moderated yields by the reactions between (n-Bu)₂Sn(dmit) and RN (PPh₂)₂NiCl₂ at room temperature. The complexes were fully characterized by elemental analysis, spectroscopy (Fourier transform infrared [FTIR], ultraviolet–visible [UV–vis], ¹H, ¹³C{¹H}, and ³¹P{¹H} nuclear magnetic resonance [NMR]), thermogravimetric analysis, and single crystal X-ray diffraction. In the crystal structures of 1–3 and 4·2CH₂Cl₂, every nickel atom adopts a slightly distorted square-planar coordination by two phosphorus atoms of the RN (PPh₂)₂ ligand and two sulfur atoms of the dmit ligand. Furthermore, the electrochemical behaviors and electrocatalytic activities of 1–4 for hydrogen evolution have also been investigated by the cyclic voltammetry using trifluoroacetic acid (TFA) as the proton source. With the addition of 120-mM trifluoroacetic acid to 0.5-mM 1–4 in MeCN, the turnover frequency values of these catalysts were estimated to be 2827–5149 s^?1, and the relevant overpotentials were 0.72–0.79 V. Density functional theory (DFT) calculations and electrochemical investigations suggest that H₂ production proceeds via a key hydride intermediate [NiH (SH)] with an adjacent protonated sulfur atom of the dmit ligand in which the chelating sulfur atoms serve as proton relays. These findings demonstrate that these heteroleptic dmit nickel complexes could serve as robust and effective molecular electrocatalysts for hydrogen evolution.

Full text of DOI:10.1002/aoc.6123

Received: 9 October 2020
DOI: 10.1002/aoc.6123
Revised: 30 November 2020
Accepted: 2 December 2020
F U L L P A P E R
Heteroleptic dmit nickel complexes with
bis(diphenylphosphanyl)amine ligands as robust molecular
electrocatalysts for hydrogen evolution
1
,2
1,3
4
2
5
Tao Li
| Bin Xie
| Jia-Xi Cao | Dong-Liang Zhang | Chuan Lai
|
2
2
4
1
Hua-Jun Fan | Bin Zhao | Wen-Yu Mou | Xiao-Xue Bai
1
School of Materials Science and
Four new neutral heteroleptic dmit nickel complexes bearing
Engineering, Sichuan University of
2−
Science and Engineering, Zigong, China
bis(diphenylphosphanyl)amine ligands, [RN (PPh ) Ni(dmit)] (where dmit = 1,-
2
2
2
School of Chemical Engineering, Sichuan
2
3
-dithiole-2-thione-4,5-dithiolate; R = (CH ) CH [1], (CH ) OCH [2], (CH ) CH
4 3 2 3 3 2 2
University of Science and Engineering,
Zigong, China
(
CH ) [3], and CHPhCH [4]), have been synthesized in moderated yields by the
3 2 3
3
reactions between (n-Bu) Sn(dmit) and RN (PPh ) NiCl at room temperature.
2 2 2 2
Sichuan Province Key Laboratory of
Comprehensive Utilization of Vanadium
and Titanium Resources, Panzhihua
University, Panzhihua, China
The complexes were fully characterized by elemental analysis, spectroscopy
1
13
1
(
Fourier transform infrared [FTIR], ultraviolet–visible [UV–vis], H, C{ H}, and
31
1
4
P{ H} nuclear magnetic resonance [NMR]), thermogravimetric analysis, and sin-
Cl , every
College of Chemistry and Environmental
Engineering, Sichuan University of
gle crystal X-ray diffraction. In the crystal structures of 1–3 and 4ꢀ2CH
nickel atom adopts a slightly distorted square-planar coordination by two phos-
phorus atoms of the RN (PPh ) ligand and two sulfur atoms of the dmit ligand.
2
2
Science and Engineering, Zigong, China
5
School of Chemistry and Chemical
2
2
Engineering, Sichuan University of Arts
and Science, Dazhou, China
Furthermore, the electrochemical behaviors and electrocatalytic activities of 1–4
for hydrogen evolution have also been investigated by the cyclic voltammetry
using trifluoroacetic acid (TFA) as the proton source. With the addition of
Correspondence
Bin Xie, School of Materials Science and
Engineering, Sichuan University of
Science and Engineering, Zigong 643000,
China.
1
20-mM trifluoroacetic acid to 0.5-mM 1–4 in MeCN, the turnover frequency
−1
values of these catalysts were estimated to be 2827–5149 s , and the relevant
overpotentials were 0.72–0.79 V. Density functional theory (DFT) calculations and
Email: xiebinqhg@sina.com.cn
electrochemical investigations suggest that H production proceeds via a key
2
Chuan Lai, School of Chemistry and
Chemical Engineering, Sichuan
hydride intermediate [NiH (SH)] with an adjacent protonated sulfur atom of the
dmit ligand in which the chelating sulfur atoms serve as proton relays. These find-
ings demonstrate that these heteroleptic dmit nickel complexes could serve as
robust and effective molecular electrocatalysts for hydrogen evolution.
University of Arts and Science, Dazhou
635000, China.
Email: 20150002@sasu.edu.cn
Funding information
K E Y W O R D S
Graduate Innovation Fund Project of
Sichuan University of Science and
Technology, Grant/Award Numbers:
y2019031, y2019023; Training Program of
Innovation and Entrepreneurship for
Undergraduates of Sichuan University of
Science and Technology, Grant/Award
Number: cx2019198; Science and
Technology Bureau of Zigong City, Grant/
Award Numbers: 2020MZGC06,
bis(diphenylphosphanyl)amine, dmit −, electrochemistry, hydrogen evolution, nickel complex
2
2019CXMZ01, 2015HX18; Open Fund
Appl Organomet Chem. 2021;35:e6123.
wileyonlinelibrary.com/journal/aoc
© 2020 John Wiley & Sons, Ltd.
1 of 17
https://doi.org/10.1002/aoc.6123

2
of 17
LI ET AL.
Project of Sichuan Province Key
Laboratory of Comprehensive Utilization
of Vanadium & Titanium Resources,
Grant/Award Number: 2018FTSZ09
1
| INTRODUCTION
dithiolene/dithiolate nickel complexes with RN (PPh2)2
ligands as hydrogen evolution catalysts has been
[
32,40]
With the consumption of fossil fuels, such as oil and
natural gas, the society is facing increasingly serious
problems about energy crisis and environmental pollu-
investigated.
2
−
1,3-Dithiole-2-thione-4,5-dithiolate (dmit ) is a con-
jugated, planar structured dithiolene ligand. This sulfur-
rich heterocyclic ligand has strong affinities binding to
metal ions and demonstrates reversible redox proper-
[1,2]
tion.
Thus, one of the most attractive strategies to
solve the current issues of environmental pollution and
energy shortage is to develop clean and sustainable
energy. In this respect, the focus has been increasingly
placed on hydrogen (H ) as an environmentally benign
energy carrier.
the traditional fossil fuels such as the highest specific
combustion enthalpy of any chemical fuel, only water as
the product, and not limited by Carnot cycle when used
in a fuel cell. It could not only alleviate the energy
crisis but also eliminate the environmental pollution
caused by the fossil fuels. The precious metal platinum
has been used as catalysts for hydrogen evolution,
which has low overpotentials for the reduction of protons
to hydrogen. However, the high cost of platinum is its
[
41]
ties.
In recent years, a considerable amount of interest
[
41–46]
has been placed on this dmit complexes
and their
chemical
[
41,47,48]
applications in organic conductors,
2
[3,4]
[49,50]
[42,43]
Hydrogen has many advantages over
sensors,
magnetic materials,
and nonlinear
As an analogous to dmit, the
[
44,49,51–53]
optical materials.
maleonitriledithiolate (mnt ) complexes also attracted
2
−
some attentions for the prominent biomimetic and redox
[5]
[54]
properties.
A
homoleptic complex (n-Bu N)
4
[Co (mnt) ] was first reported to have excellent photo-
2
and electrocatalytic activities for the reduction of protons
[
6]
[54,55]
to hydrogen.
And an electrochemical–chemical–
electrochemical–chemical mechanism (ECEC) involving
the protonation of two sulfur atoms in different mnt
ligands was proposed.
[5]
[7–9]
[56]
biggest drawback.
Research has been focused on
(n-Bu N)[Ni (mnt) ] has also
4 2
developing highly efficient and robust hydrogen evolu-
been reported to homogeneously electrocatalyze the
[6–8]
[57]
tion catalysts with earth-abundant metal
such as
copper,[20,22–24]
complexes. Among them, metal
reduction of protons to hydrogen.
Study showed that
[
10–14]
[8,15–20]
[9,21]
iron,
cobalt,
manganese,
the diprotonation occurs at two sulfur atoms from differ-
ent mnt ligands with a distorted nickel hydride interme-
diate. In addition, the homoleptic complexes of [BzPyN
(CH ) ] [Ni (mnt) ] and [4-ClBzPyN (CH ) ] [Ni (mnt) ]
[25–27]
and molybdenum
nickel stands out because (1) it resides in the same group
as Pt but on the first row of transition metals, and (2) it is
3
2 2
2
3 2
2
[
28]
an earth-abundant metal for hydrogen production.
with different nickel oxidation states can also
electrocatalyze protons reduction with the ECEC
Many heteroleptic nickel complexes with diphosphine
ligands have been extensively explored as catalysts for
[
58]
mechanism.
However, the protonation occurs at
[28–32]
hydrogen evolution.
The dithiolene ligand is usually
nickel center, which suggest that the nickel (III) complex
[58]
introduced in the Ni complex as proton relays and pro-
is the better catalyst than the nickel (II) species.
[7,17]
tonation sites.
Recently, Gan and coworkers reported
However, no nickel complexes containing dmit ligand,
either homoleptic or heteroleptic, have been currently
reported for electrocatalytic reduction of protons to
hydrogen.
that a bioinspired complex, [(dppf)Ni (bdt)], can catalyze
protons reduction to hydrogen with a turnover frequency
1
(
TOF) of 1240 s− and an overpotential requirement
[
33,34]
of 265 mV at the low HOAc concentrations.
Inspired by the effective catalytic hydrogen evolu-
tion performances of the nickel complexes containing
Similar electrocatalyst [(dppp)Ni (pdt)] demonstrated
high-efficient hydrogen evolution but was unstable
and degraded into an electrode-adsorbed film under
[
28,29,32,33]
diphosphine and dithiolate ligands,
and as a
continuation of studies on the heteroleptic nickel
[35]
[46,58,59]
the reducing and protic conditions.
Notably,
complexes,
we herein report the synthesis and
bis(diphenylphosphanyl)amine (RN (PPh ) ) revealed as
structural characterization of four novel heteroleptic
nickel complexes bearing RN (PPh2)2 and dmit
ligands, [RN (PPh ) Ni(dmit)] (R = (CH ) CH [1],
2
2
an excellent diphosphine ligand whose structure is close
to bis(diphenylphosphanyl)methane (dppm) but having
2
2
2 4
3
[8]
other advantages than dppm. The RN (PPh ) metal
(CH ) OCH3 [2], (CH ) CH(CH ) [3], and CHPhCH3
2 3 2 2 3 2
2
2
complexes are excellent catalysts for hydrogen
[4]). These complexes were characterized by elemental
analysis, Fourier transform infrared (FTIR), ultraviolet–
evolution, ethene oligomerization, and Suzuki–Miyaura
[32,36–39]
1
13
1
31
1
reaction.
However, little research on the
visible
(UV–vis),
H,
C{ H},
and
P{ H}

LI ET AL.
3 of 17
nuclear magnetic resonance (NMR) spectroscopies.
Their electrochemical properties were studied by
cyclic voltammetry (CV), and their electrocatalytic
performances for hydrogen production were investi-
gated in the presence of trifluoroacetic acid (TFA) in
MeCN. Density functional theory (DFT) computations
provide further insights into their mechanisms for
hydrogen evolution, and their stabilities for hydrogen
evolution were also evaluated in the presence
of concentrate TFA and under the electrocatalytic
conditions.
carbon electrode was polished with alumina (1 μm) on a
polishing cloth before each measurement. The solution
was purged with argon or nitrogen gas prior to measure-
ments. At the end of each experiment, ferrocene was
added as an internal standard. The potential scale was
+
calibrated against the Fc /Fc couple and reported versus
this reference system, whose potential was measured in
the cell at the end of each experiment. Catalysis studies
for hydrogen evolution were carried out by sequential
addition of TFA by syringe from solutions of the acid
in the same electrolyte. Controlled potential electrolysis
(
CPE, namely, the bulk electrolysis) experiments were
+
performed at −1.60 V versus Fc /Fc using 0.5-mM
complex 1 and 260-mM TFA in MeCN (40 mL) with
2
2
| EXPERIMENTAL
0
.1-M n-Bu NPF6 in an air-tight single compartment
4
.1 | Materials and methods
cell. The cell was sparged at least 15 min with
nitrogen atmosphere before electrolysis and then
electrolysis was allowed to run for 1.5 h. After electroly-
sis, a sample of the headspace was injected into a
GC7980 gas chromatograph equipped with a thermal
conductivity detector (TCD) to analyze the gas
composition.
All reactions and operations were carried out using stan-
dard Schlenk techniques under nitrogen atmosphere. Sol-
vents were dried and distilled by using standard
procedures. NiCl ꢀ6H O, Et N, Ph PCl, NH (CH ) OCH ,
2
2
3
2
2
2 3
3
NH (CH ) CH , NH (CH ) CH(CH ) , NH CHPhCH ,
2
2 4
3
2
2 2
3 2
2
3
CS , ZnCl , Et NBr, n-Bu SnCl , and sodium were
2
2
4
2
2
obtained commercially and used as received. Ferrocene
and n-Bu NPF were bought from commercial and puri-
2.2 | Synthesis
4
6
fied multiple times by crystallization before use, respec-
tively. RN (PPh ) , RN (PPh ) NiCl , [NEt ] [Zn (dmit) ],
2.2.1 | [CH (CH ) N(PPh ) Ni(dmit)] (1)
2
2
2 2
2
4 2
2
3
2 4
2 2
and (n-Bu) Sn(dmit) were prepared according to the
2
[36,59–63]
literature procedures.
Preparative thin layer chro-
CH (CH ) N(PPh ) NiCl (0.292 g, 0.5 mmol) in CH Cl
3 2 4 2 2 2 2 2
matography (TLC) was carried out on glass plates
(10 mL) was slowly added to a solution of (n-Bu) Sn
2
3
(26 × 20 × 0.25 cm ) coated with silica gel G. Melting
(dmit) (0.5 mmol, 0.215 g) in acetone (5 mL), and the
mixture was stirred for 2 h at room temperature and
resulted in a green solution. Solvents were removed
under vacuum, and the residue was subjected to TLC sep-
aration using CH Cl as eluent to give 1 as a green solid.
points were measured on a WRS-2C microscopic melting
point apparatus and were uncorrected. Elemental ana-
lyses were investigated on an Elementar Vario EL cube
analyzer. FTIR spectra were conducted using a NICOLET
2
2
ꢁ
6
700 infrared spectrophotometer with pressed KBr disk
Yield: 0.305 g, 86%. m.p. ( C): 240.0–240.5. Anal. Calc.
at room temperature. NMR spectra were obtained on a
(%) for C H NNiP S : C, 54.09; H, 4.40; N, 1.97; S,
32
31
2 5
Bruker AV400 NMR spectrometer with CDCl as solvent
22.56. Found (%): C, 54.34; H, 4.58; N, 1.79; S, 22.32.
3
1
13
1
−1
at room temperature. H NMR (400 MHz) and C{ H}
NMR (101 MHz) spectra were referenced to residual
FTIR (KBr disk, cm ): v
1436 (s), v
1104 (s),
C−S
C C
1
v C S 1053 (vs), v P−N−P 828 (s). H NMR (400 MHz,
CDCl , ppm): 0.59 (t, J = 7.2 Hz, 3H, CH ), 0.72–0.84,
31
1
solvent relative to tetramethyl-silane, and P{ H} NMR
162 MHz) spectra were referenced to external 85%
H PO . UV–vis absorption spectra in acetonitrile were
3
HH
3
(
0.85–0.97, and 1.03–1.11 (tm, 6H, NCH (CH ) ),
2
2 3
2.85–2.96 (m, 2H, NCH ), 7.52 (t, J
= 7.4 Hz, 8H,
3
4
2
HH
measured using a TU-1950 UV–vis spectrophotometer
with a 1-cm quartz cell at room temperature. Ther-
mogravimetric analyses were recorded on a NETZSCH
STA 409 PC/PG instrument under nitrogen atmosphere.
CV measurements were performed on a CHI660E
instrument equipped with a three-electrode cell, a 3-mm
diameter glassy carbon working electrode, a platinum
m-PhH), 7.62 (t, J
= 7.4 Hz, 4H, p-PhH), 7.83 (dd,
HH
3
13
1
JPH = 13.0 Hz, JHH = 6.8 Hz, 8H, o-PhH). C{ H} NMR
(101 MHz, CDCl , ppm): 13.84 (s, CH ), 21.99, 29.07, and
3
3
2
29.42 (3s, NCH (CH ) ), 48.86 (t, J = 4.8 Hz, NCH2),
2
2 3
PC
3
129.30 (s, p-PhC), 129.57 (t,
J
= 5.7 Hz, m-PhC),
PC
2
129.78 (s, S C CS ), 133.10 (t, J = 6.2 Hz, o-PhC),
2
2
PC
1
31
1
138.81 (t, J = 11.1 Hz, i-PhC), 221.75 (s, S C). P{ H}
PC
+
wire counter electrode, and a nonaqueous Ag/Ag
NMR (162 MHz, CDCl , 85% H PO , ppm): 58.29
3
3
4
reference electrode at room temperature. The supporting
(s, NP ). UV–vis (MeCN, λmax, nm): 226, 241, 267, 310,
2
electrolyte was 0.1-M n-Bu NPF in MeCN. The glassy
463.
4
6

4
of 17
LI ET AL.
2
.2.2 | [CH O(CH ) N(PPh ) Ni(dmit)]
2.2.4 | [CH CHPhN(PPh ) Ni(dmit)] (4)
3 2 2
3
2 3
2 2
(
2)
Complex 4 was synthesized by a similar procedure as
described for 1, but it used CH CHPhN(PPh ) NiCl
2
Complex 2 was synthesized by a similar procedure as
described for 1, but it used CH O(CH ) N(PPh ) NiCl
3
2 2
(0.309 g, 0.5 mmol) instead of CH (CH ) N(PPh ) NiCl .
3
2 3
2 2
2
3
2 4
2 2
2
(
0.5 mmol, 0.293 g) instead of CH (CH ) N(PPh ) NiCl .
Solvents were removed under vacuum, and the residue
was subjected to TLC separation using CH Cl as eluent
3
2 4
2 2
2
Solvents were removed under vacuum, and the residue
was subjected to TLC separation using CH Cl as eluent
to give 2 as a green solid. Yield: 0.299 g, 84%. m.p. ( C):
2
5
4
2
2
ꢁ
to give 4 as a green solid. Yield: 0.271 g, 73%. m.p. ( C):
241.3–242.1. Anal. Calc. (%) for C H NNiP S : C,
2
2
ꢁ
35
29
2 5
40.6–241.5. Anal. Calc. (%) for C H NNiOP S : C,
56.46; H, 3.93; N, 1.88; S, 21.53. Found (%): C, 56.44; H,
31
29
2 5
−
1
2.26; H, 4.10; N, 1.97; S, 22.50. Found (%): C, 52.51; H,
4.15; N, 1.97; S, 21.32. FTIR (KBr disk, cm ): v
1434
C C
−
1
1
.25; N, 1.76; S, 22.29. FTIR (KBr disk, cm ): v
1434
(s), v C−S 1097 (s), v C S 1053 (vs), v P−N−P 889 (s). H
NMR (400 MHz, CDCl , ppm): 1.08 (d, J = 7.1 Hz, 3H,
C C
1
(s), v C−S 1105 (vs), v C S 1051 (vs), v P−N−P 829 (s). H
3
HH
NMR (400 MHz, CDCl , ppm): 1.29–1.37 (m, 2H,
CH ), 4.52 (tq, J = 13.9 Hz, JHH = 7.0 Hz 1H, NCH),
3
3
PH
NCH CH ), 2.92 (t, J
= 5.7 Hz, 2H, NCH ), 2.98–3.12
6.62 (d, JHH = 7.3 Hz, 2H, o-CHPhH), 6.93 (t,
JHH = 7.7 Hz, 2H, m-CHPhH), 7.08 (t, JHH = 7.4 Hz, 1H,
p-CHPhH), 7.47 (dt, JPH = 15.0 Hz, JHH = 7.5 Hz, 8H,
m-PPhH), 7.52–7.64 (m, 4H, p-PPhH), 7.82 (dd,
JPH = 13.2 Hz, JHH = 6.9 Hz, 4H, o-PPhH), 7.91 (dd,
2
2
HH
2
(
m, 2H, OCH ), 3.03 (s, 3H, OCH , overlaps with the
2 3
oxomethylene group signal), 7.52 (t, JHH = 7.4 Hz, 8H,
m-PhH), 7.61 (t, JHH = 7.4 Hz, 4H, p-PhH), 7.83 (dd,
13
1
JPH = 13.0 Hz, JHH = 6.8 Hz, 8H, o-PhH). C{ H} NMR
13
1
(
101 MHz, CDCl , ppm): 29.90 (s, NCH CH ), 46.19 (t,
JPH = 13.1 Hz, JHH = 6.9 Hz, 4H, o-PPhH). C{ H} NMR
3
2
2
2
3
JPC = 4.7 Hz, NCH ), 58.70 (s, OCH ), 69.34 (s, OCH ),
(101 MHz, CDCl , ppm): 22.01 (t, J = 2.4 Hz, CH3),
2
3
2
3
PC
2
1
29.02 (s, p-PhC), 129.26 (s, S C CS ), 129.55 (t,
62.09 (t, JPC = 4.7 Hz, NCH), 128.17 (s, o-CHPhC),
128.77 (s, m-CHPhC), 128.98 (s, p-CHPhC), 129.32 (t,
JPH = 5.9 Hz, m-PPhC), 132.87, 133.00 (2s, S C CS ),
133.27 (t, J = 6.2 Hz, p-PPhC), 133.71 (t, J = 6.1 Hz,
PC PC
o-PPhC), 138.81 (t, JPC = 10.7 Hz, i-PPhC), 139.26
2
2
3
2
JPC = 5.7 Hz, m-PhC), 133.04 (t, J = 6.1 Hz, o-PhC),
1
PC
1
31
1
3
38.80 (t, J = 11.1 Hz, i-PhC), 221.50 (s, S C). P{ H}
PC
2
2
4
2
NMR (162 MHz, CDCl , 85% H PO , ppm): 58.56 (s,
3
3
4
1
NP ). UV–vis (MeCN, λmax, nm): 227, 241, 266, 309, 463.
2
3
31
1
(
t, J = 2.3 Hz, i-CHPhC), 221.42 (s, S C). P { H}
PC
NMR (162 MHz, CDCl , 85% H PO , ppm): 62.10
3
3
4
2
.2.3 | [(CH ) CH(CH ) N(PPh ) Ni
(s, NP ). UV–vis (MeCN, λmax, nm): 227, 243, 266, 311,
3
2
2 2
2 2
2
(dmit)] (3)
465.
Complex 3 was synthesized by a similar procedure as
described for 1, but it used (CH ) CH(CH ) N(PPh ) NiCl
2
2.3 | X-ray crystal structure
determinations
3
2
2 2
2 2
(
0.292 g, 0.5 mmol) instead of CH (CH ) N(PPh ) NiCl .
3
2 4
2 2
2
Solvents were removed under vacuum, and the residue
was subjected to TLC separation using CH Cl as eluent to
give 3 as a green solid. Yield: 0.287 g, 81%. m.p. ( C):
The single crystals of 1–3 and 4ꢀ2CH Cl suitable for
2
2
2
2
ꢁ
X-ray diffraction analyses were grown by slow evapora-
ꢁ
2
5
4
68.1–269.8. Anal. Calc. (%) for C H NNiP S : C,
tion of the CH Cl /hexane solutions at about −10 C. The
3
2
31
2 5
2
2
4.09; H, 4.40; N, 1.97; S, 22.56. Found (%): C, 54.47; H,
crystal data of 2 and 3 were collected on a charge-coupled
device (CCD) diffractometer by using Mo-Kα radiation
sealed X-ray tube (λ = 0.71073 Å) in the ω scanning mode
at 293(2) K, and the crystal data for 1 and 4ꢀ2CH Cl were
−1
.65; N, 1.88; S, 22.71. FTIR (KBr disk, cm ): v
1433
C C
1
(s), v C−S 1100 (s), v C S 1050 (vs), v P−N−P 851 (s). H NMR
(
400 MHz, CDCl , ppm): 0.53 (d, JHH = 6.6 Hz, 6H, 2CH3),
3
2
2
0
.88–1.01 (m, 2H, NCH CH ), 1.05–1.13 (m, 1H,
collected on a Supernova Dual with Atlas S2 diffractome-
2
2
(
8
CH ) CH), 2.83–2.99 (m, 2H, NCH ), 7.53 (t, J = 7.4 Hz,
ter by using Cu-Kα radiation sealed X-ray tube
3
2
2
HH
H, m-PhH), 7.62 (t, JHH = 7.4 Hz, 4H, p-PhH), 7.83 (dd,
(λ = 1.54184 Å) in ω scanning mode at 162.00(10) K.
13
1
[64]
JPH = 13.0 Hz, JHH = 6.8 Hz, 8H, o-PhH). C{ H} NMR
101 MHz, CDCl , ppm): 22.21 (s, CH ), 26.23 (s,
Using Olex2,
the structures were solved by direct
[65]
(
methods with the SIR2004 structure solution program
3
3
2
NCH CH ), 38.50 (s, (CH ) CH), 47.20 (t, J = 4.8 Hz,
NCH ), 129.28 (s, p-PhC), 129.58 (t, J = 5.7 Hz, m-PhC),
and refined by full-matrix least-squares minimization
2
2
3 2
PC
3
[66]
2
with the SHELXTL refinement package
on F . All
2
PC
2
129.77 (s, S C CS ), 133.10 (t, J = 6.1 Hz, o-PhC),
non-hydrogen atoms were located from the Fourier maps
and were refined anisotropically. All hydrogen atoms
were refined isotropically, with the isotropic vibration
parameters related to the non-hydrogen atom to which
2
2
PC
1
31
1
138.80 (t, J
= 11.1 Hz, i-PhC), 221.77 (s, S C). P{ H}
PC
NMR (162 MHz, CDCl , 85% H PO , ppm): 58.40 (s, NP ).
3
3
4
2
UV–vis (MeCN, λmax, nm): 226, 242, 266, 310, 463.

LI ET AL.
5 of 17
[
67]
they are bonded. Relevant crystallographic details are
summarized in Table 1.
carried out with the Gaussian 09 program.
All geome-
tries in this study were optimized using B3LYP func-
[
68,69]
tional
in combination with the 6-311G (d, p) basis
The DFT-D3 model was applied for the dispersion
[
70]
set.
2
.4 | Computational details
correction to accurately compute the London dispersion
interactions. Separate frequency calculations were carried
out to confirm the optimized geometry as local minima.
The SMD solvation model was applied to simulate the
DFT calculations were used to model the mechanisms of
the proton reduction reaction. All calculations were
TABLE 1 Crystal data and structure refinements details for 1–3 and 4ꢀ2CH
2 2
Cl
Complex
1
2
3
2 2
4ꢀ2CH Cl
Empirical formula
Formula weight
Temperature (K)
Radiation
C
32
H
31NNiP
2
S
5
C
31
H
29NNiOP
2
S
5
2(C32
H
29.5NNiP
2
S
5
)
C
35
H
29NNiP
2
S
5
2 2
ꢀ2CH Cl
710.53
712.50
293(2)
1418.03
293(2)
914.39
162.00(10)
162.00(10)
Cu-K
α
Mo-K
α
Mo-K
α
Cu-K
α
Wavelength (Å)
Crystal system
Space group
a (Å)
1.54184
0.71073
triclinic
P-1
0.71073
triclinic
P-1
1.54184
monoclinic
monoclinic
P2
1
/c
1
P2 /n
10.6451(3)
9.0121(2)
34.8130(9)
90
9.3341(5)
13.3350(7)
13.8294(7)
96.956(2)
93.676 (2)
102.477(2)
1661.07(15)
2
9.1122(5)
12.2204(6)
15.2059(8)
86.058(2)
81.427(2)
83.050(2)
1659.81(15)
1
10.4279(2)
26.2013(5)
14.7691(3)
90
b (Å)
c (Å)
ꢁ
α ( )
ꢁ
β ( )
92.792(2)
90
91.712(2)
90
ꢁ
γ ( )
3
V (Å )
3335.81(15)
4
4033.48(14)
4
Z
−
3
D
calc (gꢀcm
)
1.415
1.425
1.419
1.506
μ (mm−
1
)
4.849
1.021
1.019
6.530
F (000)
1472.0
736.0
733.0
1872.0
3
Crystal size (mm )
0.18 × 0.17 × 0.16
8.316–132.006
−12 ≤ h ≤ 12,
0.18 × 0.16 × 0.15
6.046–50.018
0.19 × 0.18 × 0.17
5.954–50.016
0.18 × 0.15 × 0.14
9.024–132.016
ꢁ
2
θ range ( )
Index ranges
−11 ≤ h ≤ 11,
−15 ≤ k ≤ 15,
−16 ≤ l ≤ 16
−10 ≤ h ≤ 10,
−14 ≤ k ≤ 14,
−18 ≤ l ≤ 18
−12 ≤ h ≤ 11,
−31 ≤ k ≤ 21,
−17 ≤ l ≤ 17
−6 ≤ k ≤ 10,
29 ≤ l ≤ 41
−
Reflections
collected/unique
9608/5797 Rint = 0.0523,
sigma = 0.0609
30,051/5842
30,083/5833
13,326/6995 Rint = 0.0468,
sigma = 0.0556
R
Rint = 0.0258,
Rint = 0.0284,
R
Rsigma = 0.0191
Rsigma = 0.0229
Data/restraints/
parameters
5797/0/371
5842/1/371
5833/48/409
6995/24/470
Completeness (%)
99.3
99.7
99.8
99.4
Goodness of fit on
F
1.184
1.085
1.035
1.037
2
Final R indices
R
1
= 0.0828,
R
1
= 0.0272,
R
1
= 0.0305,
R
1
1
= 0.0583, wR
2
2
= 0.1545
= 0.1646
(
I > 2σ[I])
wR
2
= 0.2098
wR
2
= 0.0683
wR
2
= 0.0739
R indices (all data)
R
1
= 0.0924,
R
1
= 0.0315,
R
1
= 0.0384,
R
= 0.0663, wR
wR
2
= 0.2149
wR
2
= 0.0706
2
wR = 0.0784
Largest diff peak
and hole (eꢀ Å
1.06/−0.51
0.37/−0.41
0.36/−0.32
1.30/−0.97
−3
)

6
of 17
LI ET AL.
acetonitrile solvent in the reaction. The coordinates of
optimized geometries are available in the Supporting
Information.
While the phenyl proton signals of the PPh groups for
4 exhibit one multiplet (7.52–7.64 ppm), one double trip-
let (7.47 ppm), and two double doublets (7.82 and
2
7.91 ppm). Such discrepancies are most likely due to the
presence of chiral α-carbon atom connected to nitrogen
atom of the N-1-phenylethyl bis (diphenylphosphanyl)
3
3
| RESULTS AND DISCUSSION
.1 | General features
[
40]
1
amine ligand.
The H NMR spectrum of 4 demon-
strates one doublet at 6.62 ppm and two triplets at 6.93
and 7.08 ppm for the phenyl protons of the CH CHPh
3
Previous published procedure showed the reaction
between RN (PPh ) NiCl and in situ formed Na dmit
moiety. The proton signals of the CH N fragment appear
2
one triplet at 2.92 ppm with the coupling constant of
J = 5.7 Hz for 2, one multiplet in 2.85–2.96 for 1, and one
multiplet in 2.83–2.99 ppm for 3, respectively, while the
CHN proton signal for 4 also presents one multiplet
signal in the region of 4.47–4.58 ppm.
2
2
2
2
from CH ONa and (PhCO) dmit in absolute methanol
3
2
[46]
and under nitrogen atmosphere
only produced low
yields of dmit nickel complexes. This study adopted a
[49]
modified procedure by Jeon et al. ; the complexes
13
1
[
(
RN (PPh ) Ni(dmit)] (1–4) were prepared from RN
For the C{ H} NMR spectra, the phenyl carbons of
2
2
PPh ) NiCl and (n-Bu) Sn(dmit) in CH Cl and acetone
the PPh moieties for 1–3 display one singlet around
2
2
2
2
2
2
2
solutions at room temperature with 73–86% yields, as
shown in Scheme 1. Results show that this modified syn-
thetic route for 1–4 is simple and efficient, because this
reaction route does not require anhydrous and oxygen-
free reaction conditions; 1–4 are air-stable green solids
and soluble in CH Cl , CHCl , MeCN, and DMF.
129.28 ppm and three triplets at about 129.57, 133.10,
and 138.80 ppm, respectively. The phenyl carbons of the
PPh moiety for 4 show four triplets at 129.32, 133.27,
2
133.71, and 138.81 ppm. For 1–3, one singlet in the range
of 129.26–129.78 ppm and one singlet around 221.50 ppm
are assigned to the S C CS and S C groups of the dmit
2
2
3
2
2
ligand, respectively, while the S C CS and S C carbon
2
2
signals for 4 appear two singlets (132.87 and 133.00 ppm)
and one singlet (221.42 ppm). In addition, 4 shows addi-
tional three singlets at 128.17, 128.77, and 128.98 ppm
and one triplet at 139.26 ppm, which could be assigned to
3
.2 | Spectroscopic characterizations
The structures of 1–4 have been fully characterized by
1
13
1
the elemental analysis, FTIR, UV–vis, H, C{ H}, and
the phenyl carbons in the CH CHPh unit. Moreover, the
3
31
1
P{ H} NMR spectroscopies. Their FTIR and NMR spec-
carbon signals for NCH groups of 1–3 display one triplet
2
tra are shown in Figures S1–S16.
in the range of 46.19–48.86 ppm, while the CHN group of
4 appears one triplet at 62.09 ppm.
The FTIR spectra of 1–4 all display a strong absorp-
−1
31
1
tion band in the range of 828–889 cm for the P–N–P
bending vibration in the RN (PPh ) ligands
strong absorption band at about 1051 cm for the C S
stretching vibration of the dmit ligands.
absorption bands in the range of 1097–1105 cm are
assigned to the C–S stretching vibrations of the dmit
ligands.
The H NMR spectra of 1–3 display one double dou-
blet at 7.83 ppm, two triplets at 7.52 and 7.62 ppm
assigned to the phenyl protons of the PPh2 moieties.
Correspondingly, the P{ H} NMR spectra of 1–4
demonstrate only one singlet at 59.28, 58.56, 58.40, and
62.10 ppm, respectively. These chemical shifts are consis-
tent with those of the analogous nickel complexes
(57.47–67.16 ppm) such as [PhCH N(PPh ) Ni(edt)],
[37]
and one
2
2
−
1
[71,72]
The strong
−
1
2
2 2
[CH (CH ) N(PPh ) Ni(pdt)],
and
[CH S(CH ) N
3
2 3
2 2
[40,60,61]
3
2 3
[72]
(PPh ) Ni(bdt)].
This can be assigned to the two
2
2
1
phosphorus atoms of the RN (PPh ) ligand coordinated
2
2
to nickel atom, which has been verified by X-ray crystal-
lography in the following section.
SCHEME 1 Synthetic scheme of 1–4

LI ET AL.
7 of 17
FIGURE 3 Molecular structure of 2 with displacement
FIGURE 1 Ultraviolet–visible (UV–vis) absorption spectra of
ellipsoids at the 50% probability level. Hydrogen atoms have been
omitted for clarity
1–4 in MeCN at room temperature
The UV–vis spectra of 1–4 were collected in MeCN at
isopentyl group for 3 and one CH Cl solvent molecular
2
2
room temperature and are presented in Figure 1. Their
UV–vis spectra are almost identical and show five absorp-
tion bands around 227, 241, 266, 310, and 463 nm, respec-
tively. The low intensity band at 463 nm is assigned to
the metal to ligand charge transfer (MLCT) transition
for 4ꢀ2CH Cl are in disordered states.
2
2
As shown in Figures 2–5, all complexes contain one
mononuclear nickel atom ligated by two phosphorus
atoms of RN (PPh2)2 and two sulfur atoms of dmit
ligands. Their structures are all isostructural in which
the nickel atom adopts a slightly distorted square-planar
coordination, which deviations from the mean plane
formed by P1, P2, S1, and S2 atoms. The sum of the
[73,74]
from nickel to phosphorus and nickel to sulfur.
The
rest four absorption bands may be assigned as intraligand
charge transfer (ILCT) π ! π* transitions of both R N
2
[33,59,72–74]
ꢁ
(
PPh ) and dmit ligand.
angles around the nickel atom is 359.99 (for 1), 360.06
2
2
ꢁ
ꢁ
(
for 2), 360.35 (for 3), and 360.33 (for 4ꢀ2CH Cl ),
2
2
respectively. The Ni–S and Ni–P bond lengths for 1–3
and 4ꢀ2CH Cl are comparable with those determined
3
.3 | Crystal structures
2
2
for [RN (PPh ) Ni(bdt)] (R = (CH ) OCH , (CH ) SCH ,
2
2
2 3
3
2 3
3
The molecular structures of 1–3 and 4ꢀ2CH Cl are
and CHPhCH3 [Ni–S: 2.1430–2.1567 Å; Ni–P:
[40]
2
2
shown in Figures 2–5. The selected bond lengths and
angles are listed in Table 2. The C26–C29 atoms of
2.1348–2.1701 Å]),
(dppv)Ni (dmit) (Ni–S: 2.1601
[6] and 2.1693[6] Å; Ni–P: 2.1507[6], and 2.1520
FIGURE 2 Molecular structure of 1 with displacement
ellipsoids at the 50% probability level. Hydrogen atoms have been
omitted for clarity
FIGURE 4 Molecular structure of 3 with displacement
ellipsoids at the 50% probability level. Hydrogen atoms have been
omitted for clarity

8
of 17
LI ET AL.
3.5 | Electrochemical studies
The electrochemical properties of 1–4 were measured by
CV in MeCN with 0.1 M n-Bu NPF as the supporting
4
6
electrolyte under nitrogen atmosphere. The electrochemi-
cal data are presented in Table 3. The CV for 1 displays
two sets of redox peaks within the potential windows, as
presented in Figure 7. A reversible reduction peak at
+
Epc = −1.60 V versus Fc /Fc for 1 can be ascribed to
II
I [32,35,76,77]
one-electron reduction of Ni to Ni .
The
+
irreversible oxidation peak at Epa = 0.22 V versus Fc /Fc
can be assigned to the oxidation of N-butyl
bis(diphenylphosphanyl)amine ligand.
also show one reversible reduction peak for Ni /Ni cou-
ple at Epc = −1.58, −1.58, and −1.66 V versus Fc /Fc and
[
78]
Similarly, 2–4
II I
FIGURE 5 Molecular structure of 4ꢀ2CH
displacement ellipsoids at the 50% probability level. Hydrogen
atoms and CH Cl moleculars have been omitted for clarity
2 2
Cl with
+
the irreversible oxidation event at E = 0.22–0.24 V versus
pa
2
2
+
Fc /Fc for bis(diphenyl-phosphanyl)amine ligands,
respectively (see Figures S21, S27, and S33).
Variable-scan rate studies of 1–4 demonstrate linear
relationships of the currents to the square root of the scan
rate (ν ) for the Ni /Ni events, indicating diffusion-
[
46]
[
[
(
[
7] Å),
7] Å; Ni–P: 2.1570[7] and 2.1606[9] Å),
PPh ) Ni(edt (CH ) )] (Ni–S: 2.1663[9] and 2.1681
(dppv)Ni (dmio) (Ni–S: 2.1681[7] and 2.1642
[46]
1/2
II
I
and [i-BuN
controlled processes (Figures 8, S22, S28, and
2
2
3 2
[75]
[34,40,57]
8] Å; Ni–P: 2.1506[9] and 2.1561[9] Å).
The mea-
S34),
where the electrochemical active species are
sured P1–Ni1–P2 and S1–Ni1–S2 bond angles for 1–3
and 4ꢀ2CH Cl are close to those of the analogous
freely diffusing in the solution and the calculated diffu-
−
5
sion coefficients are determined to be 1.47 × 10 ,
2
2
[32,36,40,46,59,61,75]
−5
−5
−5
2 −1
nickel complexes.
The dmit ligands for
1.47 × 10 , 1.20 × 10 , and 2.24 × 10 cm ꢀs ,
[77,79]
the complexes are all coplanar configuration duo to the
fully conjugated π bonds feature, whereas the four-
membered metallocycle Ni1P1N1P1A are almost copla-
respectively.
nar with the mean deviations of 0.046 for 1, 0.018 for 2,
3.6 | Electrocatalytic hydrogen evolution
ꢁ
0
.005 for 3, and 0.068 for 4ꢀ2CH Cl , respectively. This
2
2
suggests that the N1 atom in RN (PPh ) ligand adopts
sp
To determine the electrocatalytic activities of complexes
2
2
2
hybridization due to the influence of
1–4, the CVs of 1–4 in MeCN have been explored with
[32]
MeCN
ꢁ
= 12.7, E = 0.89 V
coordination.
the addition of TFA (pK
a
+
[77]
vs. Fc /Fc. ). As shown in Figure 9, the successive
addition of TFA to 1 in MeCN, CV curves display a signif-
3.4 | Thermogravimetric analyses
icant increased cathodic current near E = −1.60 V
versus Fc /Fc concomitant with the formation of gas
pc
+
The thermogravimetric measurements for 1–4 were
bubbles, which is confirmed to be H by gas chromatog-
2
recorded on a NETZSCH STA 409 PC/PG instrument
raphy (GC)-TCD analysis (see Figure S40). Meanwhile,
the maximum peak gradually shifts to more negative
potentials. These phenomena indicate that 1 can effec-
tively catalyze protons reduction to hydrogen in the pres-
−
1
with a flow of 100 mL min nitrogen atmosphere at a
ꢁ
−1
ꢁ
heating rate of 10.0 C min in the range of 25–700 C.
As illustrated in Figure 6, only 1 exhibits a slight weight-
ꢁ
[34,40,77]
lessness under 100 C, which may be caused by the
ence of TFA in the electrochemical condition.
evaporation of bound water; 1, 3, and 4 show two-step
A number of control experiments were also carried
out to confirm that 1 is responsible for the catalysis. For
example, the current peak of hydrogen evolution of
Epc = −2.07 V versus Fc /Fc by glassy carbon working
electrode is observed with 10-mM TFA (Figure S19).
decompositions that occur in the temperature ranges of
ꢁ
2
88.3–319.6 and 346.1–390.4 C by weight loss of 31.01
+
and 32.22 (for 1), 33.59 and 25.46 (for 3), and 17.36 and
9.00% (for 4), respectively. Whereas 2 presents only one-
3
step decomposition by weight loss of 59.79% in the range
of 314.8–410.9 C. According to the TGA analysis, the
order of thermal stability of the four complexes can be
However, the reduction peak current for 1 is observed at
Epc = −1.63 V versus Fc /Fc in the presence of 10-mM
TFA. The shift of reduction peak current of 1 suggests
ꢁ
+
summarized as follows: 4 > 3 > 2 > 1.
that the electrocatalytic activity is entirely from complex

LI ET AL.
9 of 17
ꢁ
TABLE 2 Selected bond lengths (Å) and bond angles ( ) for 1–3 and 4ꢀ2CH
2 2
Cl
1
Ni1–S1
2.152(2)
2.163(2)
1.696(5)
1.644(8)
93.93(8)
97.16(8)
97.5(3)
Ni1–P1
2.146(2)
Ni1–S2
Ni1–P2
2.130(2)
1.707(6)
73.53(7)
95.44(8)
94.1(2)
P1–N1
P2–N1
S5–C32
P1–Ni1–P2
S1–Ni1–S2
N1–P1–Ni1
N1–P2–Ni1
C30–S3–C32
S3–C32–S4
S2–C31–S4
P1–Ni1–S1
P2–Ni1–S2
P1–N1–P2
C30–S1–Ni1
C31–S2–Ni1
C31–S4–C32
S5–C32–S3
2
94.3(2)
100.3(2)
99.7(2)
97.9(3)
111.8(4)
122.3(4)
98.6(4)
123.4(4)
Ni1–S1
2.1664(5)
2.1618(5)
1.6928(15)
1.655(2)
Ni1–P1
2.1506(5)
2.1566(5)
1.6938(15)
73.682(19)
94.83(2)
Ni1–S2
Ni1–P2
P1–N1
P2–N1
S5–C31
P1–Ni1–P2
S1–Ni1–S2
N1–P1–Ni1
N1–P2–Ni1
P1–N1–P2
C29–S3–C31
S3–C31–S4
P1–Ni1–S1
P2–Ni1–S2
C30–S1–Ni1
C29–S2–Ni1
C30–S4–C31
S5–C31–S3
S3–C31–S4
3
94.94(2)
96.54(2)
93.54(5)
100.22(7)
100.32(6)
97.97(10)
123.44(13)
112.45(11)
93.30(5)
99.39(8)
97.92(9)
112.45(11)
Ni1–S1
2.1564(6)
2.1556(7)
1.6947(18)
1.654(2)
Ni1–P1
2.1479(6)
2.1493(6)
1.6942(18)
73.75(2)
Ni1–S2
Ni1–P2
P1–N1
P2–N1
S5–C32
P1–Ni1–P2
S1–Ni1–S2
N1–P2–Ni1
N1–P1–Ni1
P1–N1–P2
C30–S3–C32
S2–C31–S4
P1–Ni1–S1
P2–Ni1–S2
C30–S1–Ni1
C31–S2–Ni1
C31–S4–C32
S5–C32–S3
S3–C32–S4
96.05(2)
94.84(2)
95.71(2)
93.55(6)
100.63(8)
100.59(8)
98.14(12)
123.99(16)
112.26(14)
93.59(6)
99.10(9)
97.97(11)
122.25(14)
4
ꢀ2CH
2 2
Cl
Ni1–S1
Ni1–S2
P1–N1
2.1669(10)
2.1622(11)
1.710(3)
Ni1–P1
2.1417(10)
2.1311(10)
1.703(3)
Ni1–P2
P2–N1
S5–C35
1.652(4)
P1–Ni1–P2
S1–Ni1–S2
N1–P2–Ni1
N1–P1–Ni1
73.17(4)
P1–Ni1–S2
P2–Ni1–S1
C33–S1–Ni1
95.97(4)
95.29(4)
95.90 (4)
100.05(13)
94.83(11)
94.24(10)
(
Continues)

10 of 17
LI ET AL.
TABLE 2 (Continued)
1
C34–S2–Ni1
C33–S4–C35
S5–C35–S3
S3–C35–S4
100.25(13)
97.99(19)
123.9(3)
P1–N1–P2
C34–S3–C35
S1–C33–S4
96.51(15)
97.94(19)
122.1(2)
112.6(2)
−
3
−3
−3
3
3
.22
×
10 , 3.83
×
10 , 3.65
×
10 , and
−
3
2 −1
.63 × 10 cm ꢀs , respectively. Moreover, the standard
rinse experiments for 1–4 also indicate that the
electrocatalysis is only initiated by molecular species in
solution and not by the unknown nanoparticle materials
on the electrode surface (Figures S20, S26, S32, and
[
33,34]
S38).
These findings suggest that 1–4 all have effi-
cient reactivity as homogeneous molecular catalysts for
reduction protons, and the as-observed catalytic waves
[
34,56,57]
correspond to hydrogen evolution.
II
I
Furthermore, the i /i ratio of the Ni /Ni couple
cat pc
can be used as an indicator for the catalytic efficiency of
different catalytic complexes, where the ipc and icat are
the peak currents observed for the complex catalysts in
the absence of and in the presence of TFA. A higher
FIGURE 6 Thermogravimetric analysis curves for 1–4
[37,80]
value of icat/ipc suggests a faster catalysis reaction.
As showed in Table 3, i /i values of 1–4 with the addi-
cat pc
catalyst 1. Noticeably, the oxidation peak current is not
affected much under the acidic and electrochemical con-
ditions, as shown in Figure 9.
Similarly, 2–4 also show similar electrocatalytic
behaviors to that of 1 for the reduction of protons to
hydrogen in the presence of TFA in MeCN (Figures S23,
tion of 120-mM TFA exhibit the catalytic efficiency order
of 2 > 3 > 1 > 4. This result shows that 2 as molecular
electrocatalyst has a higher catalytic efficiency than other
complexes. Interestingly, the plots of icat/ipc for 1–4 versus
the square root of the amount of the TFA added demon-
strate linear correlations, indicating first-order depen-
dence of the catalytic rate on TFA concentration
S29, and S35). In addition, the catalytic currents (i ) for
cat
[
24,34]
1
–4 with the addition of 10-mM TFA show linear depen-
(Figures 9, S23, S29, and S35).
Also, the catalytic
1/2
dences on the square root of the scan rate (ν
)
currents reveal linear dependency on the concentrations
of 1–4 (Figures 11, S25, S31, and S37), which suggests
first-order catalytic reaction.
The TOF and the overpotential (η) of the catalytic
reaction can be used to quantitatively evaluate the
(Figures 10, S24, S30, and S36), respectively, suggesting
that their electrocatalysis is diffusion-controlled pro-
[34,40,57]
cesses.
And the corresponding diffusion coeffi-
cients for 1–4 in the presence of 10-mM TFA are
[
a]
[b]
TABLE 3 The electrochemical data
Complex
E
pc (V)
E
pa (V)
i
pc (μA)
i
cat (μA)
icat/ipc
+
of 1–4 (vs. Fc /Fc) in MeCN
1
2
3
4
−1.60
−1.58
−1.58
−1.66
+0.22
+0.24
+0.24
10.23
11.01
10.11
12.74
1545.43
1812.56
1564.34
1554.18
151.07
164.63
154.73
121.99
+0.22
a
II
I
The ipc is the cathodic peak current at Ni /Ni event observed for 0.5-mM catalyst in the absence of TFA in
+
−1
MeCN (0.1 M n-Bu
b
4
NPF
6
, glassy carbon electrode, potential vs. Fc /Fc, 100 mV.s scan rate).
The icat is the catalytic cathodic peak current for 0.5-mM catalyst observed in the present of 120-mM TFA
+
−1
4 6
in MeCN (0.1 M n-Bu NPF , glassy carbon electrode, potential vs. Fc /Fc, 100 mV.s scan rate).

LI ET AL.
11 of 17
identical conditions, the TOF values of 1–4 for hydrogen
production are 8–14 times than those of analogous nickel
complexes [RN (PPh ) Ni(mnt)], and the corresponding
2
2
overpotentials are slightly greater than those of
[
78]
[
RN (PPh ) Ni(mnt)].
The difference in catalytic
2
2
activity for hydrogen evolution is mainly caused by the
significant electronic difference between the dithiolene
ligands, namely, that dmit ligand is an electronic donor
while mnt ligand is an acceptor. The secondary reason
may come from the slightly difference for electrocatalytic
hydrogen production mechanism, which is described
below.
3.7 | Evaluation of stability for the
electrocatalytic complexes
FIGURE 7 Cyclic voltammogram of 0.5-mM 1 in MeCN (0.1 M
+
n-Bu
4
NPF
6
, glassy carbon electrode, potential vs. Fc /Fc, 100 mV.
The stability of 1–4 was evaluated and monitored with
the UV–vis absorption spectra in the presence of concen-
trate TFA in MeCN and under the electrocatalytic condi-
tions. The MeCN solutions of 1–4 are insensitive to air as
indicated by temporal evolution of their UV–vis absorp-
tion spectra (Figure S17). And as depicted in Figure S18,
the UV–vis absorption spectra of 1–4 in MeCN with
50-mM TFA were unchanged for 12 h, indicating 1–4 all
have good acid tolerance. Furthermore, the bulk electrol-
ysis experiments of 1–4 in MeCN in the presence of
s− scan rate)
1
[77,79]
catalytic activities.
The detail calculation of TOF
and the overpotential (η) are presented in the Supporting
Information, and the results are summarized in Table 4.
The TOF values of the complex-catalyzed TFA reduction
can be calculated from CVs by comparing the ratio of
i /i (see the Supporting Information), which are 4336
for 1), 5149 (for 2), 4548 (for 3), and 2827 s (for 4),
cat pc
−
1
+
(
50-mM TFA at −1.6 V versus Fc /Fc showed little change
when 120-mM TFA is added. Following the method of
of the UV–vis absorption spectra for 1.5 h, compared with
before electrolysis, and the color of the solution remained
unchanged, and no precipitation was also observed
(Figure S39). The spectroelectrochemical experiments
suggest that the catalysts for hydrogen production are
[81]
Fourmond and coworkers,
the overpotential (η)
required to attain half of the overall catalytic current
determined for 1–4 evaluate 0.79, 0.72, 0.76, and 0.78 V,
respectively (see the Supporting Information). Under the
4 6
FIGURE 8 (a) Cyclic voltammograms of 0.5-mM 1 in MeCN at different scan rate (0.1-M n-Bu NPF , glassy carbon electrode, potential
+
II
I
1/2
vs. Fc /Fc). (b) Plots of ipc and ipa for the Ni /Ni couple versus ν for 0.5-mM 1 in MeCN

12 of 17
LI ET AL.
FIGURE 9 Cyclic voltammograms of
.5-mM 1 in the presence of 0- to 120-mM TFA
0
in MeCN (0.1-M n-Bu
4
NPF
6
, glassy carbon
+
−1
electrode, potential vs. Fc /Fc, 100 mVꢀs scan
rate). Inset: Plots of the plateau current (icat/ipc
of 0.5-mM 1 against the square root of TFA
concentration
)
FIGURE 10 Cyclic voltammograms of
0
.5-mM 1 at different scan rate in the presence
of 10-mM TFA in MeCN (0.1-M n-Bu NPF
glassy carbon electrode, potential vs. Fc /Fc).
4
6
,
+
1
/2
Inset: Plots of ipc versus ν for 0.5-mM 1 in
MeCN in the presence of 10-mM TFA
likely relatively stable even under the acidic and
and robust molecular electrocatalysts for hydrogen evolu-
tion using TFA as the proton source.
[18,24,33,34]
electrochemical conditions.
electrocatalytic hydrogen evolution complex [CpCo(N N)
In contrast, the
^
+
(
MeCN)](ClO ) is unstable when using DMFH /DMF
4 2
buffer as the proton source under the electrocatalytic
3.8 | Catalytic mechanistic studies for
hydrogen evolution
[19]
conditions.
Similarly, a high-efficient hydrogen evolu-
tion electrocatalyst [(dppp)Ni (pdt)] is also unstable and
degrades to an electrode-adsorbed film under the electro-
Molecular catalysis of hydrogen evolution through the
electrochemical protons reduction can proceed through
[35]
chemical conditions.
Therefore, with the combination
[
82]
of the standard rinse experiments (Figures S20, S26, S32,
homolytic route or heterolytic route.
Based on the
and S38), all these results support that 1–4 are low-cost
abovementioned electrochemical observations and the

LI ET AL.
13 of 17
FIGURE 11 Cyclic voltammograms of
1
(0.1, 0.2, 0.3, 0.4, and 0.5 mM) in the presence
of 10-mM TFA in MeCN (0.1-M n-Bu NPF
glassy carbon electrode, potential versus Fc /Fc,
4
6
,
+
−
1
1
00 mVꢀs scan rate). Inset: Plots of icat versus
catalyst concentration (0.1, 0.2, 0.3, 0.4, and
0
.5 mM) for 1 in MeCN in the presence of
1
0-mM TFA
TABLE 4 The values of the
turnover frequency (TOF),
TOF (s−1
2
−1 [b]
(cm ꢀs−1) b
3.22 × 10
3.83 × 10−3
3.65 × 10−3
3.63 × 10−3
2
Complex
)
η (V)
0.79
0.72
0.76
0.78
D
0
(cm ꢀs
)
D
1
1.47 × 10−
5
−3
1
2
3
4
4336
5149
4548
2827
overpotential (η), and diffusion
[a]
1.47 × 10−5
1.20 × 10−
2.24 × 10−
0 1
coefficient (D and D ) for 1–4
5
5
a
The data of the turnover frequency (TOF) and overpotential (η) were calculated by the methods of
Supporting Information.
b
0 1
The diffusion coefficients of D (without TFA) and D (with 10-mM TFA) were determinated using the
Randles–Sevcik equation (see Supporting Information).
SCHEME 2 The possible catalytic
mechanism for protons reduction by 2

14 of 17
LI ET AL.
−
−
FIGURE 12 Single electron occupied highest occupied molecular orbital (HOMO) of [2] (left) and HOMO of [2(SH)] (right) for
catalyst 2 by using B3LYP/6-311G(d, p)
[
34,56]
previously studies,
a heterolytic ECEC mechanism
second electron reduction of [2(SH)] affords monoanionic
−
is proposed for hydrogen production of 1–4 in the pres-
ence of TFA. The following experimental results support
the ECEC mechanism: (a) The catalytic peak current is
proportion to the square root of the TFA concentration;
species [2(SH)] . However, the subsequent second pro-
−
tonation of [2(SH)] leads only to protonate on the nickel
ion, namely, [2(NiH)(SH)]. The HOMO orbital analysis
−
of [2(SH)] (Figure 12) reveals only 7% contribution of S1
(
b) the UV–vis absorption spectra for 1–4 are not affected
and S2 atoms, 27% contribution from P1 and P2 atoms,
and 52% contribution from Ni atom. This is different
from [n-Bu N][Ni (mnt) ], where the diprotonations
by the presence of TFA; (c) the catalytic peak is observed
II
I
at a potential close to the Ni /Ni reduction potential.
DFT calculations are also employed to investigate the
possible reaction pathways for hydrogen evolution by cat-
alysts. Scheme 2 illustrates the reaction pathway of the
proposed ECEC mechanism with catalyst 2 as a model.
Firstly, one-electron reduction of neutral complex 2 forms
a monoanionic species [2] at −1.63 V versus Fc /Fc.
Secondly, the protonation of [2] at sulfur or nickel
atoms by TFA could yield either [2(NiH)] or [2(SH)]
4
2
occur on two sulfur atoms from two different mnt ligands
−
and yields a [Ni (mntH)(mntH)] species. Interestingly,
−
this [Ni (mntH)(mntH)] species is more stable than the
[(NiH)(mntH)(mnt)]− species.
[57]
Close examining the
second protonation species [2(NiH)(SH)] shows that the
−
+
II
S–H bond is syn to the Ni –H bond, which is
−
similar to the key intermediates [(CoH)(SH)] of the
2
−
2−
2−
dithiolene cobalt complexes with qpdt , bdt , or tdt
ligands.
[
17,55]
(where S represent the dmit ligand), respectively. The
Lastly, [2(NiH)(SH)] produces hydrogen
calculation results indicate that the most favorable
protonation site of [2] is on one of the chelating sulfur
from the hydride on the nickel ion and the proton on the
sulfur atom and regenerates catalyst 2.
−
atoms of the dmit ligand. The nickel-protonated species
−
1
[
2(NiH)] is less stable by 36 kJꢀmol
than sulfur-
protonated species [2(SH)]. In this single electron
4 | CONCLUSION
occupied highest occupied molecular orbital (HOMO) of
−
[
2] , indeed, S1 and S2 atoms contribute 37%, Ni atom
In summary, four new heteroleptic dmit nickel com-
plexes with bis(diphenylphosphanyl)amine ligands,
[RN (PPh ) Ni(dmit)] (1–4), were synthesized with satis-
contributes 23%, and P1 and P2 atoms contribute 19%,
respectively (Figure 12). This result suggests that the
chelating sulfur atoms serve as the proton relays. The sul-
fur protonation of 2 is similar to the homoleptic complex
2
2
factory yields by the reactions of (n-Bu) Sn(dmit) and RN
2
(PPh ) NiCl at room temperature. These four complexes
2
2
2
[
n-Bu N][Ni (mnt) ] but in contrast to the heteroleptic
were characterized by elemental analysis, spectroscopy
4
2
1
13
1
31
1
complexes [RN (PPh ) Ni(mnt)]. For example, DFT study
of [n-Bu N][Ni (mnt) ] by Zarkadoulas and coworker
(FTIR, UV–vis, H,
C{ H}, and
P{ H} NMR),
2
2
thermogravimetric analysis, and single crystal X-ray
4
2
showed that the protonation occurs on the sulfur atom of
diffraction. The crystal structures for 1–3 and 4ꢀ2CH Cl
2
2
2
−[57]
the dianion [Ni (mnt)2]
; while the study on
reveal that the nickel atom lies at the center but a slightly
distorted square-planar geometry. In addition, the elec-
trochemical properties of 1–4 have been measured by
means of CV in MeCN. The investigation results suggest
[
RN (PPh ) Ni(mnt)] by Mou et al. revealed that the neu-
2
2
tral nickel species is reduced to the monoanion, followed
[78]
by the protonation on the nickel atom.
Thirdly, a

LI ET AL.
15 of 17
that all complexes are robust molecular catalysts and can
efficiently electrocatalyze protons reduction to hydrogen
with TFA as the proton source. A possible ECEC mecha-
nism for electrocatalytic hydrogen evolution was also
examined. The DFT calculations show that the chelating
sulfur atoms of the dmit ligand are preferred protonation
sites than the nickel center; thus, the chelating sulfur
atoms may serve as proton relays. And only the second
protonation forms a [NiH (SH)] hydride intermediate.
This study demonstrates that the dmit nickel complexes
represent a new group of active catalysts for the reduc-
tion of protons to hydrogen.
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[
ACKNOWLEDGEMENTS
16304.
This work is supported by grants from the Open
Fund Project of Sichuan Province Key Laboratory of
Comprehensive Utilization of Vanadium & Titanium
Resources (2018FTSZ09), the Science and Technology
Bureau of Zigong City (2015HX18, 2019CXMZ01, and
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Dalton Trans. 2016, 45, 14754.
2
020MZGC06), the Training Program of Innovation and
Entrepreneurship for Undergraduates of Sichuan Univer-
sity of Science and Technology (cx2019198), and the
Graduate Innovation Fund Project of Sichuan University
of Science and Technology (y2019023 and y2019031).
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18] H. L. Sun, Y. Z. Han, H. T. Lei, M. X. Chen, R. Cao, Chem.
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[
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T. L. Groy, M.-H. Baik, A. K. Jones, R. J. Trovitch, Inorg.
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[27] H. I. Karunadasa, C. J. Chang, J. R. Long, Nature 2010, 464,
Tao Li: Conceptualization; investigation; visualization.
Bin Xie: Conceptualization; funding acquisition; meth-
odology; project administration; resources; supervision.
Jia-Xi Cao: Investigation; methodology; visualization.
Dong-Liang Zhang: Data curation; investigation.
Chuan Lai: Resources; supervision. Hua-Jun Fan:
Resources. Bin Zhao: Formal analysis; supervision.
Wen-Yu Mou: Validation. Xiao-Xue Bai: Investigation.
[
[
[
[
CONFLICT OF INTEREST
The authors declare that they have no known competing
financial interests or personal relationships that could have
appeared to influence the work reported in this paper.
1329.
[
[
[
28] J. W. Wang, W. J. Liu, D. C. Zhong, T. B. Lu, Coord. Chem.
Rev. 2019, 378, 237.
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ORCID
Bin Xie https://orcid.org/0000-0002-9345-5084
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How to cite this article: Li T, Xie B, Cao J-X,
et al. Heteroleptic dmit nickel complexes with bis
diphenylphosphanyl)amine ligands as robust
molecular electrocatalysts for hydrogen evolution.
Appl Organomet Chem. 2021;35:e6123. https://doi.
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