Neutral heteroleptic nickel complexes incorporating maleonitriledithiolate and bis(diphenylphosphanyl)amine as robust molecular electrocatalysts for hydrogen evolution

Article abstract of DOI:10.1016/j.ica.2020.119587

Five neutral heteroleptic maleonitriledithiolate nickel complexes [RN(PPh₂)₂Ni(mnt)] bearing bis(diphenylphosphanyl)amine ligands (where mnt^2? = maleonitriledithiolate; R = CH₃O(CH₂)₃ (1), CH₃S(CH₂)₃ (2), (S)-CH₃CHPh (3), (CH₃)₂CH(CH₂)₂ (4), and p-CH₃C₆H₄ (5)) have been synthesized and fully characterized by elemental analysis, FTIR, NMR (¹H, ¹³C, and ³¹P) spectroscopy, UV–Vis spectrum, single crystal X-ray diffraction, thermogravimetric analysis, and density functional theory (DFT) calculation. The nickel atom in 1, 2, 4, and 5?1.5CH₂Cl₂ adopts a slightly distorted square-planar coordination environment with two phosphorus atoms of RN(PPh₂)₂ and two sulfur atoms of mnt^2?, respectively, while the nickel atom in 3 exhibits a pseudo square pyramid with HP₂S₂ chromophore. Furthermore, the electrochemical properties for 1–5 were also investigated by cyclic voltammetry. With the addition of 120 mM trifluoroacetic acid (TFA) to 1–5 in MeCN, the turnover frequency (TOF) values are estimated to be 368.59–585.17 s^?1 with the corresponding overpotential (η) values of 0.59–0.73 V. The electrochemical studies indicate that all complexes can be used as low-cost robust molecular eletrocatalysts for the reduction protons to hydrogen in the presence of TFA and a possible ECEC mechanism for electrocatalytic hydrogen evolution has been also proposed.

Full text of DOI:10.1016/j.ica.2020.119587

Journal Pre-proofs
Research paper
Neutral heteroleptic nickel complexes incorporating maleonitriledithiolate and
bis(diphenylphosphanyl)amine as robust molecular electrocatalysts for hydro‐
gen evolution
Wen-Yu Mou, Tao Li, Bin Xie, Dong-Liang Zhang, Chuan Lai, Cheng-Long
Deng, Jia-Xi Cao, Xiao-Xue Bai, Xiao-Qiang Liu
PII:
DOI:
Reference:
S0020-1693(20)30152-3
https://doi.org/10.1016/j.ica.2020.119587
ICA 119587
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Inorganica Chimica Acta
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Revised Date:
Accepted Date:
26 January 2020
8 March 2020
8 March 2020
Please cite this article as: W-Y. Mou, T. Li, B. Xie, D-L. Zhang, C. Lai, C-L. Deng, J-X. Cao, X-X. Bai, X-Q.
Liu, Neutral heteroleptic nickel complexes incorporating maleonitriledithiolate and
bis(diphenylphosphanyl)amine as robust molecular electrocatalysts for hydrogen evolution, Inorganica Chimica
Acta (2020), doi: https://doi.org/10.1016/j.ica.2020.119587
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Neutral heteroleptic nickel complexes incorporating
maleonitriledithiolate and
bis(diphenylphosphanyl)amine as robust molecular
electrocatalysts for hydrogen evolution
Wen-Yu Mou a, b, Tao Li , Bin Xie
c
a, b, d, *
, Dong-Liang Zhang , Chuan Lai e, *,
c
c
b
a
b
Cheng-Long Deng , Jia-Xi Cao , Xiao-Xue Bai , Xiao-Qiang Liu
a
Scholl of Materials Science & Engineering, Sichuan University of Science & Engineering, Zigong
43000, China
6
b
College of Chemistry & Environmental Engineering, Sichuan University of Science & Engineering,
Zigong 643000, China
c
School of Chemical Engineering, Sichuan University of Science & Engineering, Zigong 643000,
China
d
Sichuan Province Key Laboratory of Comprehensive Utilization of Vanadium & Titanium Resources.
Panzhihua University, Panzhihua 617000, China
e
School of Chemistry & Chemical Engineering, Sichuan University of Arts & Science, Dazhou
6
35000, China
ABSTRACT
Five neutral heteroleptic maleonitriledithiolate nickel complexes [RN(PPh ) Ni(mnt)]
2
2
2
-
bearing bis(diphenylphosphanyl)amine ligands (where mnt = maleonitriledithiolate;
R = CH O(CH ) (1), CH S(CH ) (2), (S)-CH CHPh (3), (CH ) CH(CH ) (4), and
3
2 3
3
2 3
3
3 2
2 2
p-CH C H (5)) have been synthesized and fully characterized by elemental analysis,
3
6
4
1
13
31
FTIR, NMR ( H, C, and P) spectroscopy, UV-Vis spectrum, single crystal X-ray
diffraction, thermogravimetric analysis, and density functional theory (DFT)
calculation. The nickel atom in 1, 2, 4, and 51.5CH Cl adopts a slightly distorted
2
2
square-planar coordination environment with two phosphorus atoms of RN(PPh2)2
2
-
and two sulfur atoms of mnt , respectively, while the nickel atom in 3 exhibits a
pseudo square pyramid with HP S chromophore. Furthermore, the electrochemical
2
2
*
Corresponding authors at: Scholl of Materials Science & Engineering, Sichuan University of
Science & Engineering, Zigong 643000, China; School of Chemistry & Chemical Engineering,
Sichuan University of Arts & Science, Dazhou 635000, China.
E-mail address: xiebindhg@sina.com.cn (Bin Xie); lc860801@163.com (Chuan Lai)

properties for 1 - 5 were also investigated by cyclic voltammetry. With the addition of
20 mM trifluoroacetic acid (TFA) to 1 - 5 in MeCN, the turnover frequency (TOF)
1
-1
values are estimated to be 368.59-585.17 s with the corresponding overpotential ()
values of 0.59-0.73 V. The electrochemical studies indicate that all complexes can be
used as low-cost robust molecular eletrocatalysts for the reduction protons to
hydrogen in the presence of TFA and a possible ECEC mechanism for electrocatalytic
hydrogen evolution has been also proposed.
Keywords: nickel complex; maleonitriledithiolate; bis(diphenylphosphanyl)amine;
electro- chemistry; hydrogen evolution
*
Corresponding authors at: Scholl of Materials Science & Engineering, Sichuan University of
Science & Engineering, Zigong 643000, China; School of Chemistry & Chemical Engineering,
Sichuan University of Arts & Science, Dazhou 635000, China.
E-mail address: xiebindhg@sina.com.cn (Bin Xie); lc860801@163.com (Chuan Lai)

1
. Introduction
Global dependence on fossil fuels for electricity production, transportation, and
as reagents for the chemical industry is increasing today. However, the inefficient use
of vast fossil fuels has caused serious and dangerous climate alteration, such as
greenhouse gas emissions, sulfurous fogs, and acid rains [1]. Systematical and
continued research and development of clean renewable energy is crucial to
effectively reduce or even eliminate greenhouse gas emissions and environmental
pollution issues [2-9]. Hydrogen is widely preferred as having potential to provide a
clean, renewable, and sustainable energy in the near future due to its energy security,
reliability, low operating cost, constant power production, clean emissions, and high
electrical efficiency [10]. On our planet, hydrogen is not available in free form, but
mainly exists in compounds such as hydrocarbons and water. At present, hydrogen is
predominantly produced by non-electrochemical steam reforming of hydrocarbons -
an energy intensive and greenhouse gas emission intensive process [11]. Hence,
electrocatalytic hydrogen production from water or acid medium has been widely
considered to be an alternative route without CO emissions, which will play a vital
2
role in future energy storage and energy delivery systems [4, 8, 12, 13]. To improve
the efficiency for hydrogen production, one of the challenging works for overcoming
this issue would be to develop alternative electrocatalysts which are inexpensive,
highly active, abundant, environmentally friendly, and importantly stable enough for
long term operations in commercial electrolyzer composed of earth-abundant
materials [5, 8, 14]. On this basis, a large number of late-transition metal complexes,
such as nickel [14-18], cobalt [5, 19-22], copper [19, 23-25], and zinc [19, 26], have
been developed as molecular catalysts for hydrogen evolution.
Remarkably, dithiolate complexes have been widely harnessed as molecular
catalysts for hydrogen evolution [27-29], which often contain another chelating
*
Corresponding authors at: Scholl of Materials Science & Engineering, Sichuan University of
Science & Engineering, Zigong 643000, China; School of Chemistry & Chemical Engineering,
Sichuan University of Arts & Science, Dazhou 635000, China.
E-mail address: xiebindhg@sina.com.cn (Bin Xie); lc860801@163.com (Chuan Lai)

bidentate ligand, such as diphosphine [27, 28], diimine [30-33], and aminophosphine
34]. Bis(diphenyl- phosphanyl)amines (RN(PPh ) ), as excellent aminodiphosphine
[
2
2
ligands, have also been widely used in the design and fabrication of molecular
catalysts [35-40], especially high-performance catalysts for hydrogen production
2
-
[
41-43]. More specifically, the metal complexes of maleonitriledithiolate (mnt )
ligand, as one of the most important metal-dithiolene complexes, have attracted
considerable interests from scientists over the past five decades due to the novel
properties and applications in the areas of organic electric conductors [44], organic
magnets [45-47], non-linear optics [48], dyes [49, 50], and catalysis [51, 52]. These
unique properties arise principally from the special planar molecular geometries and
n-
delocalizing electron structures of [M(mnt) ] (n = 1 or 2) and their reversible redox
2
behaviors. More recently, researchers have found that the mnt complexes can also be
used as photo- or electrocatalysts for hydrogen production [51-54]. These complexes,
in general, are ion-pair complexes Q [M(mnt) ] [53] and the catalytic activities are
n
2 m
related to the oxidation states of [M(mnt)2]n- anions [52, 55], for example, the
electrocatalyst [4-ClBzPyNMe ][Ni(mnt) ] (4-ClBzPyNMe2
-chlorobenzyl-4-dimethylaminopyridinium) for hydrogen production is more active
than [BzPyNMe ] [Ni(mnt) ] (BzPyNMe = 1-benzyl-4-(dimethylamino)pyridinium)
=
2
2
4
2
2
2
2
[
52]. Thus, much work has gone into developing the synthesis and properties of the
neutral heteroleptic mnt complexes [LM(mnt)] (L = diphosphine [56, 57], diimine
50, 58], dithione [59]). To date, however, there is no report on the neutral heteroleptic
[
mnt complexes [LM(mnt)] that can catalyze proton reduction to hydrogen. The
intriguing properties of the mnt complexes strongly encourage us to investigate the
electrocatalytic activities for hydrogen evolution of the neutral heteroleptic complexes
[
RN(PPh ) Ni(mnt)].
2 2
As part of our researches for constructing efficient nickel-based complex
catalysts with diphosphine ligands for hydrogen evolution [43, 60, 61], herein, we
*
Corresponding authors at: Scholl of Materials Science & Engineering, Sichuan University of
Science & Engineering, Zigong 643000, China; School of Chemistry & Chemical Engineering,
Sichuan University of Arts & Science, Dazhou 635000, China.
E-mail address: xiebindhg@sina.com.cn (Bin Xie); lc860801@163.com (Chuan Lai)

further report our continual effort to the synthesis and structural characterization of
five new neutral heteroleptic nickel complexes [RN(PPh ) Ni(mnt)] (R =
2
2
CH O(CH ) (1), CH S(CH ) (2), (S)-CH CHPh (3), (CH ) CH(CH ) (4), and
3
2 3
3
2 3
3
3 2
2 2
p-CH C H (5)). Furthermore the electrochemical properties for 1 - 5 were studied by
,
3
6
4
means of cyclic voltammetry, and their electrocatalytic performances for hydrogen
evolution were investigated with trifluoroacetic acid (TFA) as the proton source.
2
. Experimental
2
.1. General materials and methods
Solvents were distilled from appropriate drying agents. Na mnt, Ph PCl,
2
2
CH O(CH ) NH , CH S(CH ) NH , (S)-CH CHPhNH , (CH ) CH(CH ) NH , and
3
2 3
2
3
2 3
2
3
2
3 2
2 2
2
p-CH C H NH were obtained commercially and used as received. n-Bu NPF was
3
6
4
2
4
6
recrystallized multiple times from a CH Cl solution by the addition of hexane.
2
2
RN(PPh ) and RN(PPh ) NiCl (R = CH O(CH ) , CH S(CH ) , (S)-CH CHPh,
2
2
2 2
2
3
2 3
3
2 3
3
(
CH ) CH(CH ) , p-CH C H ) were prepared according to literature procedures [41,
3 2 2 2 3 6 4
6
2]. Preparative TLC was carried out on glass plates (26  20  0.25 cm) coated with
silica gel H (10-40 µm). Elemental analyses were performed on an Elementar Vario
EL analyzer. FTIR spectra were recorded with a Thermo Nicolet 6700 FTIR
spectrophotometer in KBr disk at room temperature. NMR spectra were obtained on a
1
13
Brucker AV400 NMR spectrometer. H NMR (400 MHz) and C NMR (101 MHz)
3
1
spectra were referenced to proteo solvent impurities, and P NMR (162 MHz) spectra
were referenced to 85 % phosphoric acid. UV-Vis absorption spectra were measured
using a TU-1950 UV-Visible spectrophotometer with a 1 cm quartz cell at room
temperature. Cyclic voltammogram measurements were performed under nitrogen
atmosphere at room temperature on a CHI660E electrochemical work station
*
Corresponding authors at: Scholl of Materials Science & Engineering, Sichuan University of
Science & Engineering, Zigong 643000, China; School of Chemistry & Chemical Engineering,
Sichuan University of Arts & Science, Dazhou 635000, China.
E-mail address: xiebindhg@sina.com.cn (Bin Xie); lc860801@163.com (Chuan Lai)

equipped with a standard three-electrode single cell, in which 3 mm diameter glassy
carbon was the working electrode, a platinum wire was the auxiliary electrode, a
+
non-aqueous Ag / Ag was the reference electrode, with 0.1 M n-Bu NPF in MeCN
4
6
as the supporting electrolyte. Glassy carbon electrodes were polished before
experiments with 0.3 m alumina slurry interspersed by washings with purified water,
and they were then rinsed and sonicated to remove residual alumina. A freshly
polished working electrode was used for every voltammogram recorded. 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) experiments were performed at -1.54 V vs Fc / Fc using
1
.0 mM complex 1 and 30 mM TFA in MeCN (40 mL) with 0.1 M n-Bu NPF in an
4 6
air-tight single compartment cell. The cell was sparged at least 15 min with nitrogen
atmosphere before electrolysis, and then electrolysis was allowed to run for 3 h. After
electrolysis, a sample of the headspace was injected into a GC7980 gas
chromatograph equipped with a thermal conductivity detector to analyze the gas
composition.
2
2
.2. Synthesis
.2.1. Synthesis of [CH O(CH ) N(PPh ) Ni(mnt)] (1)
3
2 3
2 2
Na mnt (0.1860 g, 1 mmol) was added to a CH Cl solution (20 mL) of
2
2
2
CH O(CH ) N(PPh ) NiCl (0.5712 g, 1 mmol). The mixture was stirred at room
3
2 3
2 2
2
temperature for 2 h, and the mixture was then washed with water to remove NaCl.
The organic solution was dried using CaCl and the solvent was removed under
2
reduced pressure. The residue was subjected to preparative TLC separation using
*
Corresponding authors at: Scholl of Materials Science & Engineering, Sichuan University of
Science & Engineering, Zigong 643000, China; School of Chemistry & Chemical Engineering,
Sichuan University of Arts & Science, Dazhou 635000, China.
E-mail address: xiebindhg@sina.com.cn (Bin Xie); lc860801@163.com (Chuan Lai)

CH OH: CH Cl = 10:1 as eluent and complex 1 (0.5736 g, 87.6%) was obtained as
3
2
2
an orange solid from main orange band. Elemental Anal. Calcd. for C H N NiOP S
2 2
3
2
29
3
(FM 656.35): C, 58.55; H, 4.45; N, 6.40. Found: C, 58.21; H, 4.62; N, 6.29 %. FTIR

1
(
KBr disk, cm ): 
3055 (w),  C-H 2929 (w), 2869 (w),  CN 2200 (s),  C=C 1486
=
C-H
1
(
s), 1436 (s),  C-CN 1150 (vs),  C-S 1106 (vs),  P-N-P 837 (s),  C-P 748 (s). H NMR
(
400 MHz, CDCl ppm): 1.32-1.39 (m, 2H, CH ), 2.94 (t, J = 5.4 Hz, 2H, NCH ),
3
,
2
2
3
.04 (s, 3H, OCH , overlaps with OCH group signal), 2.99-3.17 (m, 2H, OCH ), 7.54
3 2 2
(t, J = 7.3 Hz, 8H, m-PPhH), 7.64 (t, J = 7.2 Hz, 4H, p-PPhH), 7.80 (dd, J = 13.0, 6.6
1
3
1
Hz, 8H, o-PPhH). C { H} NMR (101 MHz, CDCl , ppm): 29.82 (s, CH ) , 46.15 (t,
3
2
2J PC = 4.7 Hz, NCH ), 58.62 (s, OCH ), 69.20 (s, OCH ), 116.47 (s, CN), 126.62 (t,
2
3
2
3J PC = 11.5 Hz, m-PPhC), 128.81 (t, 2J PC = 24.7 Hz, p-PPhC), 129.59 (t, 2J PC = 5.8
Hz, o-PPhC), 132.94 (t, 1J PC = 6.2 Hz, i-PPhC), 133.23 (s, C=C of mnt ). P { H}
2-
31
1
NMR (162 MHz, CDCl , 85 % H PO , ppm): 58.32 (s, NP ). UV-Vis (CH Cl , max,
3
3
4
2
2
2
nm): 236, 276, 290, 328, 367.
2
.2.2. Synthesis of [CH S(CH ) N(PPh ) Ni(mnt)] (2)
3 2 3 2 2
Complex 2 was synthesized in a fashion similar to that of 1 using
CH S(CH ) N(PPh ) NiCl instead of CH O(CH ) N(PPh ) NiCl . The residue was
3
2 3
2 2
2
3
2 3
2 2
2
subjected to preparative TLC using CH OH:CH Cl = 20:1 as eluent. From the main
3
2
2
band, complex 2 (0.6365 g, 95 %) was obtained as an orange solid. Elemental Anal.
Calcd. for C H N NiP S (FM 672.41): C, 57.16; H, 4.35. N, 6.25. Found: C, 57.03;
3
2
29
3
2 3

1
H, 4.54; N, 6.24 %. FTIR (KBr disk, cm ): 
3052 (w),  C-H 2919 (w), 2846 (w),
=
C-H
 CN 2202 (s),  C=C 1489 (s), 1437 (s),  C-CN 1145 (vs),  C-S 1104 (vs),  P-N-P 864
1
(
s),  C-P 747 (s). H NMR (400 MHz, CDCl , ppm): 1.26-1.36 (m, 2H, CH ), 1.74 (s,
3
2
3
H, SCH ), 2.01 (t, J = 6.5 Hz, 2H, SCH ), 3.05-3.16 (m, 2H, NCH ), 7.53 (t, J = 7.5
3 2 2
Hz, 8H, m-PPhH), 7.63 (t, J = 7.4 Hz, 4H, p-PPhH), 7.79 (dd, J = 13.1, 6.8 Hz, 8H,
1
3
1
o-PPhH). C { H} NMR (101 MHz, CDCl , ppm): 15.54 (s, CH ), 28.49 (s, SCH ),
3
2
3
2
3
3
1.28 (s, SCH ), 47.51 (t, J = 4.7 Hz, NCH ), 116.55 (s, CN), 126.73 (t, J
=
PC
2
PC
2
*
Corresponding authors at: Scholl of Materials Science & Engineering, Sichuan University of
Science & Engineering, Zigong 643000, China; School of Chemistry & Chemical Engineering,
Sichuan University of Arts & Science, Dazhou 635000, China.
E-mail address: xiebindhg@sina.com.cn (Bin Xie); lc860801@163.com (Chuan Lai)

1
1.7 Hz, m-PPhC), 128.89 (t, 4J PC = 24.7 Hz, p-PPhC), 129.76 (t, 2J PC = 5.8 Hz,
o-PPhC), 133.04 (t, 1J PC = 6.2 Hz, i-PPhC), 133.41 (s, C=C of mnt ). P { H} NMR
2-
31
1
(
162 MHz, CDCl , 85 % H PO , ppm): 58.77 (s, NP ). UV-Vis (CH Cl , max, nm):
3
3
4
2
2
2
2
32, 275, 290, 326, 367.
2
.2.3. Synthesis of [(S)-CH CHPhN(PPh ) Ni(mnt)] (3)
3 2 2
The same procedure as that for 1 was followed, but (S)-CH CHPhN(PPh ) NiCl
2
3
2 2
was used in place of CH O(CH ) N(PPh ) NiCl . The residue was subjected to
3
3 2
2 2
2
preparative TLC using CH OH:CH Cl = 20:1 as eluent, from the main band, complex
3
2
2
3
(0.5237 g, 76 %) was obtained as a pale orange solid. Elemental Anal. Calcd. for
C H N NiP S (FM 688.39): C, 62.81; H, 4.25; N, 6.11. Found: C, 62.63; H, 4.34;
3
6
29
3
2 2

1
N, 6.21 %. FTIR (KBr disk, cm ): 
3060 (w),  C-H 2989 (w), 2929 (w), 2852
=
C-H
(w),  CN 2198 (s),  C=C. 1476 (s), 1437 (s),  C-CN 1152 (m),  C-S 1092 (vs),  P-N-P
1
8
86 (s),  C-P 743 (s). H NMR (400 MHz, CDCl , ppm): 1.08 (d, J = 7.0 Hz, 3H,
3
CH ), 4.49-4.60 (m, 1H, NCH), 6.62 (d, J = 7.4 Hz, 2H, o-PhH), 6.92 (t, J = 7.7 Hz,
3
2
H, m-PhH ), 7.07 (t, J = 7.4 Hz, 1H, p-PhH), 7.47 (dd, J = 18.9, 7.6 Hz, 8H,
m-PPhH), 7.52-7.64 (m, 4H, p-PPhH), 7.76 (dd, J = 13.4, 6.9 Hz, 4H, o-PPhH), 7.84
1
3
1
(
dd, J = 13.3, 6.9 Hz, 4H, o-PPhH). C { H} NMR (101 MHz, CDCl , ppm): 22.01
3
2
3
3
(
t, J = 2.6 Hz, CH ), 62.14 (t, J = 2.6 Hz, NCH), 116.70 (s, CN), 126.52 (t, J
PC
3
PC
=
11.2 Hz, m-PPhC), 128.15 (s, o-PhC), 128.86 (s, p-PhC), 128.97 (s, m-PhC),
PC
2
-
4
1
29.21-129.41 (m, i-PhC), 133.07 (s, C=C of mnt ), 133.19 (t, J
= 6.2 Hz,
PC
p-PPhC), 133.59 (t, 2J PC =6.3 Hz, o-PPhC), 139.01 (t, 2J PC = 2.5 Hz, i-PPhC). 31P
NMR (162 MHz, CDCl , 85 % H PO , ppm): 61.69 (s, NP ). UV-Vis (CH Cl , max,
3
3
4
2
2
2
nm): 239, 280, 292, 326, 367.
2
.2.4. Synthesis of [(CH ) CH(CH ) N(PPh ) Ni(mnt)] (4
3 2 2 2 2 2
Complex 4 was synthesized in a fashion similar to that of 1 using
CH ) CH(CH ) N(PPh ) NiCl instead of CH O(CH ) N(PPh ) NiCl . The residue
(
3
2
2 2
2 2
2
3
2 3
2 2
2
was subjected to preparative TLC using CH OH:CH Cl = 20:1 as eluent. From the
3
2
2
*
Corresponding authors at: Scholl of Materials Science & Engineering, Sichuan University of
Science & Engineering, Zigong 643000, China; School of Chemistry & Chemical Engineering,
Sichuan University of Arts & Science, Dazhou 635000, China.
E-mail address: xiebindhg@sina.com.cn (Bin Xie); lc860801@163.com (Chuan Lai)

main band, complex 4 (0.419 g, 65 %) was obtained as an orange solid. Elemental
Anal. Calcd. for C H N NiP S (FM 654.38): C, 60.56; H, 4.78; N, 6.42. Found: C,
3
3
31
3
2 2

1
6
2
0.83; H, 4.84; N, 6.31%. FTIR (KBr disk, cm ): 
3059 (w),  C-H 2977 (m),
=
C-H
928 (w), 2871 (w),  CN 2201 (s),  C=C 1484 (s), 1436 (s),  C-CN 1138 (s),  C-S 1105
1
(
vs),  P-N-P 855 (m),  C-P 747 (s). H NMR (400 MHz, CDCl , ppm): 0.53 (d, J = 6.6
3
Hz, 6H, CH ), 0.89-0.98 (m, 2H, CH ), 1.05-1.15 (m, 1H, CH), 2.90-3.02 (m, 2H,
3
2
NCH ), 7.53 (t, J = 7.5 Hz, 8H, m-PPhH), 7.63 (t, J = 7.5 Hz, 4H, p-PPhH), 7.78 (dd,
2
1
3
1
J = 13.0, 6.9 Hz, 8H, o-PPhH). C { H} NMR (101 MHz, CDCl , ppm): 22.15 (s,
3
2
CH ), 26.15 (s, CH), 38.42 (s, CH ), 47.22 (t, J = 4.7 Hz, NCH ), 116.58 (s, CN),
3
2
PC
2
1
26.73 (t, 3J PC = 11.6 Hz, m-PPhC), 129.04 (t, 4J PC = 24.7 Hz, p-PPhC), 129.68 (t, 2J
5.8 Hz, o-PPhC), 133.00 (t, 1J PC = 6.1 Hz, i-PPhC), 133.33 (s, C=C of mnt ), P
2-
31
=
PC
{1
H} NMR (162 MHz, CDCl , 85 % H PO , ppm): 57.98 (s, NP ). UV-Vis (CH Cl ,
3
3
4
2
2
2
max, nm): 233, 276, 290, 326, 367.
2
.2.5. Synthesis of [p-CH C H N(PPh ) Ni(mnt)] (5)
3 6 4 2 2
Complex 5 was synthesized in a fashion similar to that of 1 using
p-CH C H N(PPh ) NiCl instead of CH O(CH ) N(PPh ) NiCl . The residue was
3
6
4
2 2
2
3
2 3
2 2
2
subjected to preparative TLC using CH OH:CH Cl = 10:1 as eluent. From the main
3
2
2
band, complex 5 (0.6523 g, 97 %) was obtained as an orange solid. Elemental Anal.
Calcd. for C H N NiP S (FM 674.38): C, 62.33; H, 4.04; N, 6.23. Found: C, 62.01;
3
5
27
3
2 2

1
H, 4.14; N, 6.15 %. FTIR (KBr disk, cm ): 
3052 (w),  C-H 2919 (w), 2846 (w),
=
C-H
 CN 2202 (s),  C=C 1508 (s), 1489 (s), 1437 (s),  C-CN 1149 (vs),  C-S 1101 (vs), 

1 1
9
03 (vs),  C-P 746 (s) cm . H NMR (400 MHz, CDCl , ppm): 2.17 (s, 3H,
P-N-P
3
CH ), 6.35 (d, J = 8.4 Hz, 2H, o-C H ), 6.82 (d, J = 8.3 Hz, 2H, m-C H ), 7.49 (t, J =
3
6
4
6
4
7
8
.0 Hz, 8H, m-PPhH), 7.61 (t, J = 7.0 Hz, 4H, p-PPhH), 7.78 (dd, J = 13.1, 6.9 Hz,
H, o-PPhH). 13C { H} NMR (101 MHz, CDCl , ppm): 21.10 (s, CH ), 116.52 (s,
1
3
3
CN), 125.70 (t, 3J PC = 3.0 Hz, o-NC H ), 126.90 (t, J
3
= 11.4 Hz, m-PPhC),
PC
6
4
1
29.09 (t, 4J PC = 24.2 Hz, p-PPhC), 129.57 (t, 2J PC = 5.8 Hz, o-PPhC), 130.37 (s,
*
Corresponding authors at: Scholl of Materials Science & Engineering, Sichuan University of
Science & Engineering, Zigong 643000, China; School of Chemistry & Chemical Engineering,
Sichuan University of Arts & Science, Dazhou 635000, China.
E-mail address: xiebindhg@sina.com.cn (Bin Xie); lc860801@163.com (Chuan Lai)

i-C H CH ), 133.36 (s, C=C of mnt ), 133.53 (t, J = 6.3 Hz, i-PPhC), 137.05 (t, 4J
2
-
1
6
4
3
PC
3
1
1
=
6.2 Hz, m-NC H ), 137.67 (s, i-NC H ), P { H} NMR (162 MHz, CDCl , 85 %
6 4 6 4 3
PC
H PO , ppm): 62.23 (s, NP ). UV-Vis (CH Cl , max, nm): 231, 276, 291, 326, 367.
3
4
2
2
2
2
.3. X-ray crystal structure determinations
The single crystals of 1 - 5 suitable for X-ray diffraction analysis were grown by
slow evaporation of CH Cl / n-hexane solutions at about -10 C. The single crystal
2
2
X-ray diffraction data collection were carried out on a Bruker APEX-II CCD
diffractometer that was equipped with graphite monochromated Mo- (λ = 0.71073

Å, for 1) or Cu- radiation (λ = 1.54184 Å, for 2 - 4 and 51.5CH Cl ) using φ-ω

2
2
scanning mode. A multi-scan absorption correction was applied with the use of
SADABS. The structures were solved by direct method using SHELXS-97 [63] and
2
refined by full-matrix least-squares procedure based on F using SHELXL-2013 [64].
All of the hydrogen atoms were generated geometrically and refined isotropically
using the riding model. All non-hydrogen atoms were refined with anisotropic
displacement parameters. Details of crystal data, data collections, and structure
refinements are summarized in Table 1. Selected bond lengths and bond angles are
listed in Table 2. The CH Cl solvent molecules of 51.5CH Cl are in disorder.
2
2
2
2
2
.4. Computational details
All quantum-chemical calculations were carried out with the Gaussian 09
program [65]. ChemCraft and GaussView were used for the graphical presentation.
Geometry optimization and vibrational frequencies were calculated using B3LYP
functional and the 6-311g (d, p) basis set, under tight convergence criteria, with
symmetry imposed. Electron density profiles of the frontier molecular orbitals (FMO)
*
Corresponding authors at: Scholl of Materials Science & Engineering, Sichuan University of
Science & Engineering, Zigong 643000, China; School of Chemistry & Chemical Engineering,
Sichuan University of Arts & Science, Dazhou 635000, China.
E-mail address: xiebindhg@sina.com.cn (Bin Xie); lc860801@163.com (Chuan Lai)

include HOMO and LUMO.
3
. Results and discussion
3
.1. Synthesis and spectroscopic characterization
As shown in Scheme 1, the reactions of Na mnt with RN(PPh ) NiCl in CH Cl
2
2
2 2
2
2
at room temperature result in the formation of complexes 1 - 5 in satisfactory yields.
1
- 5 are all orange and air-stable solids. They have been fully characterized by
elemental analysis, FTIR, NMR ( H, 13C, and 31P), and UV-Vis absorption spectra
1
(see Fig. 1 and Figs. S1-S20).
Scheme 1.
In the FTIR spectra of 1 - 5, the weak bands in the ranges of 3052-3060 and
-1
2
846-2989 cm are due to the stretching vibration frequencies of C-H in the phenyl
and alkyl groups for RN(PPh ) ligand, respectively. The strong band at about 2200
2
2
-1
2-
cm is attributed to the CN stretching band from mnt ligand [66-68]. The bands of
-1
1
476-1508 cm are attributable to the C=C modes for the PPh moieties, while the
2
2
-
-1
C=C stretching band of mnt ligand lies at about 1437 cm [68-70]. The C-CN and
2
-
C-S stretching bands for mnt ligand appear in the regions of 1138-1152 and
-1
1
092-1106 cm , respectively [67]. And the P-N-P and C-P stretching bands for
-1
RN(PPh ) ligand are found in the ranges of 837-903 [40] and 743-748 cm [66, 70,
2
2
7
1], respectively.
1
The H NMR spectra of 1, 2, 4, and 5 show one double doublet in the region of
7
.78-7.80 ppm and two triplets in the regions of 7.49-7.54 and 7.61-7.64 ppm which
assigned to the phenyl protons of the PPh moieties, whereas the phenyl proton
2
signals of the PPh groups for 3 exhibit three double doublets (7.47, 7.76, and 7.84
2
ppm) and one multiplet (7.52-7.64 ppm), which is most likely due to the presence of
*
Corresponding authors at: Scholl of Materials Science & Engineering, Sichuan University of
Science & Engineering, Zigong 643000, China; School of Chemistry & Chemical Engineering,
Sichuan University of Arts & Science, Dazhou 635000, China.
E-mail address: xiebindhg@sina.com.cn (Bin Xie); lc860801@163.com (Chuan Lai)

the
chiral
-carbon
center
connected
to
nitrogen
atom
of
N-1-phenylethylbis(diphenylphosphanyl)amine ligand. The phenyl proton signals of
the (S)-Ph(CH )CHN unit for 3 demonstrate one doublet at 6.62 ppm and two triplets
3
at 6.92 and 7.07 ppm, respectively, while the aromatic proton signals in the
p-CH C H N moiety for 5 display two doublets at 6.35 and 6.82 ppm. The proton
3
6
4
signals of CH N fragments appear a triplet at 2.94 ppm (for 1), a multiplet in the
2
regions of 3.05-3.16 ppm (for 2), and 2.90-3.02 ppm (for 4), while the CHN proton
signal for 3 presents a multiplet in the region of 4.49-4.60 ppm. For 1, the singlet
signal for CH O at 3.04 ppm apparently overlaps with the multiple signals for CH O
3
2
in the region of 2.99-3.17 ppm. The methyl and methylene protons in the CH SCH
3
2
fragment for 2 appear a singlet at 1.74 ppm and a triplet at 2.01 ppm, respectively.
1
3
Correspondingly, the C NMR spectra of 1 - 5 show two characteristic singlets
at ca. 116.55 and 133.33 ppm assigned to the CN and C=C fragments of mnt2-
ligand, respectively, similar to those appearing in the nickel complexes of mnt or
i-mnt ligands [57, 72]. All of the phenyl carbons of the PPh units for 1 - 5 display
2
four triplets in the region of 126.52-139.01 ppm. In contrast to those of 1, 2, and 4, the
13C NMR spectrum of 3 shows four additional signals of one multiplet (129.21-129.41
ppm) and three singlets (128.15, 128.86, and 128.97 ppm) which assigned to the
phenyl group in the (S)-Ph(CH )CHN unit, and 5 also presents four additional signals
3
of two singlets (130.37 and 137.67 ppm) and two triplets (125.70 and 137.05 ppm)
which attributed to the aromatic group in the p-CH C H N unit. Moreover, the carbon
3
6
4
signals for NCH groups of 1, 2, and 4 display a triplet at about 47.22 ppm with the
2
2
coupling constant of J 4.7 Hz, while 3 shows one triplet at the lower field of 62.14
PC
ppm with 2JPC 2.6 Hz for the NCH group.
3
1
In addition, the P NMR spectra of 1 - 5 display one set of resonance signals.
The resonance signals observed at 58.32, 58.77, 61.69, 57.98, and 62.23 ppm for 1 - 5
can be assigned to bis(diphenylphosphanyl)amine, respectively. This indicates that the
*
Corresponding authors at: Scholl of Materials Science & Engineering, Sichuan University of
Science & Engineering, Zigong 643000, China; School of Chemistry & Chemical Engineering,
Sichuan University of Arts & Science, Dazhou 635000, China.
E-mail address: xiebindhg@sina.com.cn (Bin Xie); lc860801@163.com (Chuan Lai)

two phosphorus atoms of bis(diphenylphosphanyl)amine ligands are coordinated to
nickel atom, which has been verified by X-ray crystallography.
As shown in Fig. 1, the UV-Vis absorption spectra of 1 - 5 in MeCN are quite
similar to each other and show the common interesting spectroscopic features. Three
broad and intense absorption bands appear at ca. 234, 276, and 290 nm with two
shoulders on the low-energy sides of 326 and 367 nm. The three intense bands in the
region of 231-292 nm can be attributed to the  → * transitions of
2
-
bis(diphenylphosphanyl)amine and mnt ligands, and the two shoulder bonds can be
ascribed to the LMCT transitions [47, 52, 68, 73].
Fig. 1.
3
.2. Description of crystal structures
Table 1.
Fig. 2.
Fig. 3.
As shown in Figs. 2-3 and Figs. S21-24, all complexes contain one nickel atom
ligated by two phosphorus atoms P1 and P2 of bis(diphenylphosphanyl)amine ligands
2
-
and two sulfur atoms S1 and S2 of mnt ligands, respectively. More interestingly, all
of the nickel atom in 1, 2, 4, and 51.5CH Cl adopts a slightly distorted square-planar
2
2
coordination geometry with the maximum deviations of 0.060 (for 1), 0.048 (for 2),
.050 (for 4), and 0.028 Å (for 51.5CH Cl ) from the least squares planes defined by
0
2
2
P1, P2, S1, and S2 atoms and 359.98 (for 1), 360.06 (for 2), 360.00 (for 4), and
60.06  (for 51.5CH Cl ) of the sum of the bond angles around the nickel atom,
3
2
2
respectively. However, the configuration of the NiS P core for 3 exhibits a greatly
2
2
distorted from the square-planar geometry, which is most likely due to the presence of
the chiral -carbon atom of N-1-phenylethyl bis(diphenylphosphanyl)amine ligand
*
Corresponding authors at: Scholl of Materials Science & Engineering, Sichuan University of
Science & Engineering, Zigong 643000, China; School of Chemistry & Chemical Engineering,
Sichuan University of Arts & Science, Dazhou 635000, China.
E-mail address: xiebindhg@sina.com.cn (Bin Xie); lc860801@163.com (Chuan Lai)

and the intramolecular anagostic interaction between the ortho-hydrogen of one
phenyl group of the PPh unit and the nickel atom (see below). The Ni(mnt) unit for 2
2
-
4 and 51.5CH Cl demonstrates distinctly a planar configuration with the maximum
2
2
deviations of 0.029 (for 2), 0.045 (for 3), 0.024 (for 4), and 0.018 Å (for 51.5CH Cl )
2
2
2
-
from the corresponding mean plane, and the planes for mnt ligand are tilted slightly
at an angle of 0.00, 3.06, and 2.11  with respect to the corresponding NiP S square
2
2
plane for 2, 4, and 51.5CH Cl , respectively, which similar to those of the ion-pair
2
2
homogeneous complexes Q [Ni(mnt) ] (m = 1 or 2; n = 1 or 2; Q =
m
2 n
2
+
organoammonium, [M(phen) ] , or organophosphonium) [47, 70, 73], the ion-pair
3
heteroleptic
complex
[4-ClBz-1-APy][Ni(mnt)(dmit)]
dmit2-
(4-ClBz-1-APy
=
=
p-chlorobenzyliden-1-aminopyridinium;
2
-thioxo-1,3-dithiole-4,5-dithiolate) [74], and the neutral heteroleptic complexes
LNi(mnt)] (L = dppe or 2PPh ) [57]. However, for 1, the maximum deviation up to
[
3
2
-
0
.153 Å is observed from the mean plane of mnt , resulting in bigger angle 8.51 
2
-
between the NiP S square plane and the mnt mean plane. Moreover, it is worth
2
2
noting that the four-membered metallocycles of NiP N are planar configurations with
2
maximum deviations of 0.044, 0.045, 0.074, and 0.043 Å for N1 atom from the least
square planes of 1, 2, 4, and 51.5CH Cl , respectively, but the N1 atom locates at
2
2
0
.374 Å from the plane defined by P1, P2, N1, and Ni1 for 3. For 1 - 4 and
1.5CH Cl , as showed in Table 2, the Ni1-S bond lengths of 2.1462(7)-2.1564(6) Å
5
2
2
and S1-Ni1-S2 bond angles of 93.95(4)-94.65(7)  are comparable to those of the
heteroleptic complex [(dppe)Ni(mnt)] [57], and the Ni1-P bond lengths of
2
.1369(17)-2.1623(7) Å and the P1-Ni1-P2 bond angles of 72.54(6)-73.77(4)  are
2
-
very close to those of [RN(PPh ) Ni(S R’)] (The R’S ligand is a type of dithiolate
2
2
2
2
ligand) [35-37].
Table 2.
Fig. 4.
*
Corresponding authors at: Scholl of Materials Science & Engineering, Sichuan University of
Science & Engineering, Zigong 643000, China; School of Chemistry & Chemical Engineering,
Sichuan University of Arts & Science, Dazhou 635000, China.
E-mail address: xiebindhg@sina.com.cn (Bin Xie); lc860801@163.com (Chuan Lai)

Notably, as depicted in Fig. 4, 3 displays an intramolecular anagostic interaction
with H2···Ni1 distance of 2.810 Å and C2-H2···Ni1 angle of 118.39 , which is very
similar to those of the dithiolate nickel complexes with diphosphine ligand [61, 75].
Considering the ortho-hydrogen atom of the phenyl ring on the PPh moiety occupies
2
a vacant axial position of the nickel centre, the coordination geometry about the nickel
centre may be assumed to be a pseudo square pyramid with the HP S chromophore.
2
2
3
.3. Thermal analysis
The thermal stabilities for 1 - 5 were evaluated by thermogravimetric analysis
under nitrogen atmosphere in the range of 30-850 C. As depicted in Fig. 5, 1 and 2
undergo three-step of sequential decompositions with the weight losses of 63.44, 6.47
and 4.01 % for 1 in the ranges of 287-449, 449-616 and 629-840 C, and 53.12, 9.89
and 7.25 % for 2 in the ranges of 315- 445, 445-650 and 665-840 C, respectively.
The first two stage of weight loss process for 1 is attributed to the decomposition of
CH O(CH ) N(PPh ) ligand (Calcd. 69.91 %), and the total weight loss of 70.26 %
3
2 3
2 2
for 2 from 315 to 840 C corresponds to the removal of the CH S(CH ) N(PPh )
3
2 3
2 2
ligand (Calcd. 70.38 %). 4 displays a two-step decomposition by weight losses of
7.98 and 4.62 % in the ranges of 265-532 and 532-850 C, respectively, in which the
weight loss between 265 and 532 C corresponds to the loss of
CH ) CH(CH ) N(PPh ) (Calcd. 69.51 %). A five-step of sequential decomposition
6
(
3
2
2 2
2 2
by total weight loss of 59.50 % for 3 is observed in the region of 154-850 C, whereas
undergoes a four-step of continuous decomposition by total weight loss of 59.63 %
5
from 312 to 816 C. It is worthy to mention that only the first step decomposition
process for 1, 2, 4, and 5 shows a significant rapid weight loss. Therefore, the order of
thermal stability for 1 - 5 is as follows: 2 ~ 5 > 1 > 4 > 3.
Fig. 5.
*
Corresponding authors at: Scholl of Materials Science & Engineering, Sichuan University of
Science & Engineering, Zigong 643000, China; School of Chemistry & Chemical Engineering,
Sichuan University of Arts & Science, Dazhou 635000, China.
E-mail address: xiebindhg@sina.com.cn (Bin Xie); lc860801@163.com (Chuan Lai)

3
.4. Electrochemistry
The redox behaviors of 1 - 5 were studied in MeCN with 0.1 M n-Bu NPF as
4
6
the supporting electrolyte by cyclic voltammetry (CV) under nitrogen atmosphere at
room temperature. All complexes reveal an apparently similar electrochemical
behavior, and the electrochemical data for 1 - 5 are listed in Table 3. The cyclic
voltammogram of 1, as displayed in Fig. 6, demonstrates one reversible reduction
+
peak at E -1.54 V vs Fc / Fc, which can be ascribed to the one-electron reduction of
pc
II
I
Ni to Ni [16, 43, 52]. And 2 - 5 also show a similar reduction peak at E -1.53,
pc
-1.60, -1.55, and -1.50 V, respectively (see Figs. S26-S29).
Fig. 6.
Table 3.
+
Meanwhile, 1 - 5 present an irreversible oxidation peak at ca. E +0.60 V vs Fc
pa
/
Fc, which is positively shifted by ca. 20 mV relative to the corresponding free
bis(diphenylphosphanyl)amine ligands (Fig. S24), resulting from the lesser electron
donation of bis(diphenylphosphanyl)amine ligands in these complexes. Therefore,
these anodic oxidation peaks can be assigned to the corresponding
bis(diphenylphosphanyl)amine ligands. Remarkably, the substituent effect of
bis(diphenylphosphanyl)amine on reduction potentials is observed, comparing the
reduction potentials of 1 - 5 in MeCN obtained under the same conditions. The ease
order of reduction, based on the substituent, is as follows: 5 > 2 ~ 1 ~ 4 > 3. The
reduction potential of 3 is the most negative in comparison to other complexes, which
can be explained by the fact that the electron density at the nickel core is slightly
increased due to the electron-donating properties of the methyl and phenyl groups and
renders 3 more difficult to reduce. The reduction potential of 5, on the contrary, is
least negative, resulting from the p- conjugative electron withdrawing effect of the
*
Corresponding authors at: Scholl of Materials Science & Engineering, Sichuan University of
Science & Engineering, Zigong 643000, China; School of Chemistry & Chemical Engineering,
Sichuan University of Arts & Science, Dazhou 635000, China.
E-mail address: xiebindhg@sina.com.cn (Bin Xie); lc860801@163.com (Chuan Lai)

p-methylphenyl group. However, the reduction potentials for 1, 2, and 4 have roughly
the same value, suggesting that the nature of the corresponding
bis(diphenylphosphanyl)amine ligand has little electronic impact upon the nickel
centre.
1
/2
The peak currents for 1 - 5 against the square root of the scan rate ( ) display
linear relationships, as shown in Fig. 7 and Figs. S30-S33, revealing that the redox
processes are diffusion-controlled ones, with the electrochemical active species freely
-8
-8
diffusing in the solution and the diffusion coefficients of 3.80  10 , 1.81  10 , 2.20
-8
-8
-8
2 -1

10 , 1.50  10 , and 1.87  10 cm s , respectively [16, 61]. Such distinctive
potential prompt possible usage of these complexes as electrocatalysts for hydrogen
evolution [76].
Fig. 7.
3
.5. Density functional theory (DFT) calculation
The ground-state structures of 1 - 5 were optimized, and the obtained structural
parameters agree well with those of the X-ray crystal structures (1 as an example, see
Table S1). As depicted in Fig. 8 and Fig. S65, the calculated frontier molecular
orbitals (FMO) for all complexes are similar, in which the major components of the
HOMO orbitals for 1 - 5 are nickel and mnt bonding orbital characters, while the
LUMO orbitals are predominantly N(PPh ) moiety * character in nature but also
2
2
has some nickel-sulfur anti-bonding component. Interestingly, as shown in Table S2,
the LUMO energy of 5 is slightly lower than those of 1 - 4. Since the LUMO energies
are directly correlated to the reduction potentials, this suggests that 5 will be more
easily reduced than other complexes, which is consistent with the electrochemical
experiments.
Fig. 8.
3
.6. Electrocatalytic proton reduction
*
Corresponding authors at: Scholl of Materials Science & Engineering, Sichuan University of
Science & Engineering, Zigong 643000, China; School of Chemistry & Chemical Engineering,
Sichuan University of Arts & Science, Dazhou 635000, China.
E-mail address: xiebindhg@sina.com.cn (Bin Xie); lc860801@163.com (Chuan Lai)

TFA (pKaMeCN = 12.7 [77]) was selected as the proton source to test the
behaviors of elctrocatalytic reduction protons to hydrogen by 1 - 5, respectively. As
+
shown in Fig. 9, the cathodic currents around E -1.54 V vs Fc / Fc for 1 grow
pc
remarkably with sequential addition of TFA, along with a slight negative shift, which
is characteristic of electrocatalytic reduction protons to hydrogen [43, 52, 61, 78].
Controlled experiment using TFA without 1 is conducted under similar experimental
conditions; the current peaks of hydrogen evolution by glassy carbon working
+
electrode are observed at E -1.88 and -2.48 V vs Fc / Fc (Fig. S34). However, the
pc
+
reduction current is observed at ca. E -1.54 V vs Fc / Fc in the presence of 1 and
pc
TFA, indicating that the catalysis results entirely from 1. Moreover, the negative
standard rinse experiments indicate that the electrocatalysis is initiated by molecular
species in solution and not by the formation of ill-defined nanoparticulate materials at
the electrode surface (Fig. S39) [79, 80, 81, 82]. We further confirmed the
electrocatalytic hydrogen evolution by controlled potential electrolysis of 1 (1.0 mM)
+
at -1.54 V vs Fc / Fc in the presence of 30 mM TFA in MeCN for 3 h with
incorporating of gas chromatographic analysis (Fig. S64). Therefore, 1 as a molecular
catalyst can electrocatalyze protons reduction to hydrogen. Similarly, 2 - 5 also
perform the quite similar electrocatalytic behaviors to that of 1 for reduction protons
to hydrogen in the presence of TFA, as depicted in Figs. S35-S38.
The kinetics of the catalysis by 1 - 5 is probed by cyclic voltammetry. As shown
in Figs. S48-S52, the cathodic currents for 1 - 5 in the presence of 10 mM TFA
increase with sequential addition of catalyst and display linear relationships,
respectively, demonstrating that the reaction is first order dependence on the catalyst
concentration at fixed TFA concentration (10 mM) [32]. And the plots of the ratio of
the catalytic current in the presence of TFA to the reductive peak current of the
relevant complex in the absence of TFA (i / i ) for 1 - 5 versus the square root of
cat
pc
*
Corresponding authors at: Scholl of Materials Science & Engineering, Sichuan University of
Science & Engineering, Zigong 643000, China; School of Chemistry & Chemical Engineering,
Sichuan University of Arts & Science, Dazhou 635000, China.
E-mail address: xiebindhg@sina.com.cn (Bin Xie); lc860801@163.com (Chuan Lai)

TFA concentration provide linear correlations, suggesting that the catalytic reaction
for hydrogen evolution is first order response in TFA concentration, as depicted in the
insets of Fig. 9 and Figs. S35-S38 [32, 79], and the corresponding bimolecular
3
3
3
3
3
catalytic rate constants of 5.4010 , 5.2710 , 6.10 10 , 5.4010 , and 5.3710
-1
-1
-1
M .s can be estimated from the slope of the linear plots at a scan rate of 100 mV.s ,
respectively.
Fig. 9.
Fig. 10.
Furthermore, as shown in Fig. 10 and Figs. S44-S47, the plots of catalytic
currents for 1 - 5 with the addition of 10 mM TFA against the square root of the scan
rate display linear relationships, respectively, revealing that the catalytic currents are
determined by the diffusion speed of TFA to the electrode surface. And the diffusion
-7
-7
coefficients of 1 - 5 in the presence of 10 mM TFA are 6.19  10 , 3.67  10 , 7.42 
-7
-8
-8
2 -1
1
0 , 4.52  10 , and 3.15  10 cm s , respectively (Table 3).
In order to evaluate the electrocatalytic abilities for hydrogen production by 1 - 5,
the ratio of the catalytic current (i ) in the presence of TFA to the reduction current
cat
(ipc) of the relevant complex in the absence of TFA can be used as a marker to
compare the catalytic efficiency of different complexes, in which higher values of icat /
ipc indicate fast catalysis [43]. As showed in Table 3, icat / i values of catalytic
pc
reduction perks for 1 - 5 with the addition of 120 mM TFA exhibit the order of 4 > 3
>
5 > 2 > 1. This result shows that 4 as a molecular catalyst has a higher
electrocatalytic efficiency to compare other complex catalysts. The turnover
frequency (TOF) and overpotential () of the catalytic hydrogen evolution reaction
are also used to quantitatively evaluate the catalytic activities of the electrocatalysts
[79, 80, 83]. The TOF of the complex-catalyzed TFA reduction can be calculated
from CVs by comparing the ratio of icat / i (see the Supplementary Information) [15,
pc
7
9]. When 120 mM TFA is added, as depicted in Table 3, the TOF values for 1 - 5 are
*
Corresponding authors at: Scholl of Materials Science & Engineering, Sichuan University of
Science & Engineering, Zigong 643000, China; School of Chemistry & Chemical Engineering,
Sichuan University of Arts & Science, Dazhou 635000, China.
E-mail address: xiebindhg@sina.com.cn (Bin Xie); lc860801@163.com (Chuan Lai)

-1
estimated to be 368.59-585.17 s . Following the method of Fourmond and co-workers
[84], the overpotential required to attain half of the overall catalytic current was
determined under all experimental conditions evaluated (see the Supplementary
Information). The  values of 0.59, 0.63, 0.73, 0.69, and 0.69 V for 1 - 5 are
estimated with the addition of 120 mM TFA, respectively. As a result, 1 - 5 should be
regarded as molecular electrocatalysts for hydrogen production in the presence of
TFA.
Based on the above-mentioned electrochemical data for 1 - 5, a heterolytic
reaction pathway might be the dominant pathway, because there are two observations
which meet diagnostic criteria for a heterolytic reaction pathway [79]: the
proportionalities of the icat / i to the square root of TFA concentration (see the insets
pc
of Fig. 8 and Figs. S35-S38) and the plots of the half-wave potentials of the catalytic
peaks for 1 - 5 at positive values relative to the standard potentials of the
corresponding catalyst giving constants (Fig. S58) [85]. In addition, the UV-Vis
absorption spectra of 1 - 5 are not affected by the presence of TFA (Figs. S59-S63)
and initial protonation mechanism can be ruled out as a major pathway before 1 - 5
II
I
are reduced. On the basis of the observations catalytic wave around Ni / Ni reduction
potential and the previously reported cases [52, 79, 80, 83], a possible ECEC (where
E and C correspond to one-electron reductions and protonation, respectively) catalytic
mechanism can be proposed for the electrocatalytic hydrogen production by 1 - 5 in
the presence of TFA, although more evidence is still needed for an accurate
mechanism. The proposed ECEC catalytic mechanism of 1 as a representative
complex is depicted in Scheme 2. One-electron reduction of neutral complex 1
II
0
(
[(PNP)Ni (mnt)] , where the PNP ligand is (CH O(CH ) N(PPh ) ) forms a putative
3 2 3 2 2
-
I
-
monoanion species 1 ([(PNP)Ni (mnt)] ), and then protonation of the nickel atom of
1-
by TFA gives a nickel hydride species 1H ([(PNP)Ni H(mnt)] ). Another
II
0
one-electron attachment to 1H generates a reduced monoanion species 1H-
*
Corresponding authors at: Scholl of Materials Science & Engineering, Sichuan University of
Science & Engineering, Zigong 643000, China; School of Chemistry & Chemical Engineering,
Sichuan University of Arts & Science, Dazhou 635000, China.
E-mail address: xiebindhg@sina.com.cn (Bin Xie); lc860801@163.com (Chuan Lai)

I
-
-
(
[(PNP)Ni H(mnt)] ). Finally, 1H further receives another proton to produce
molecular H and regenerate the starting 1 to complete a cycle mechanism.
2
Scheme 2.
3
.7. Electrochemical stability of complexes
A variety of experimental methods were utilized to investigate the stability of
complexes under electrocatalytic. As shown in Figs. S53-S57, the color of the
solutions of 1 - 5 remains unchanged and precipitates are not observed under
continuous potential cycling after 20 minutes, respectively, indicating that 1 - 5 are
stable under electrochemical condition. Next, 1 - 5 in MeCN with excess TFA can be
stable for 12 h in air environment, as monitored by UV-Vis absorption spectra as a
function of time (Figs. S59-S63), indicating that 1 - 5 display high degree of both air
and TFA tolerance. And more importantly, electrolysis of 1 - 5 in MeCN in the
+
presence of 30 mM TFA for a 3 h period at a potential of -1.54 V vs Fc / Fc,
compared with before electrolysis, no change of the MeCN solutions and the
corresponding UV-Vis absorption spectra at all are observed throughout the CPE
experiments, respectively (Figs. S59- S63). Therefore, with the combination of the
standard rinse experiments (see Figs. S39-S43), these results indicate that 1 - 5 are
low-cost robust molecular electrocatalysts for hydrogen evolution using TFA as the
proton source.
Conclusions
In summary, the synthesis of five new neutral heteroleptic mnt complexes
[
RN(PPh ) Ni(mnt)] with bis(diphenylphosphanyl)amine ligands are synthesized and
2 2
*
Corresponding authors at: Scholl of Materials Science & Engineering, Sichuan University of
Science & Engineering, Zigong 643000, China; School of Chemistry & Chemical Engineering,
Sichuan University of Arts & Science, Dazhou 635000, China.
E-mail address: xiebindhg@sina.com.cn (Bin Xie); lc860801@163.com (Chuan Lai)

1
13
1
31
1
fully characterized by elemental analysis, FTIR, UV-Vis, H, C { H} and P { H}
NMR spectra, and thermogravimetric analysis. The nickel atom in 1, 2, 4, and
5
1.5CH Cl lies at the centre of a slightly distorted square-planar environment in
2
2
their crystals, respectively. However, the coordination geometry of the nickel centre
for 3 may be considered a pseudo square pyramid of HP S chromophore resulting
2
2
from the intramolecular anagostic interaction. Furthermore, the electrochemical
investigations for 1 - 5 in MeCN with TFA as a proton source show that the TOF
values of the electrocatalytic hydrogen evolution are 368.59-585.17 s-1 and the
corresponding overpotential values fall within the range of 0.59-0.73 V. A possible
ECEC mechanism for electrocatalytic hydrogen evolution has been also proposed.
Most importantly, the electrochemical investigations indicate that all complexes can
be used as low-cost robust molecular electrocatalysts for the reduction protons to
hydrogen in the presence of TFA under electrochemical condition, respectively.
CRediT authorship contribution statement
Wen-Yu Mou: Writing - original draft, Conceptualization, Validation. Tao Li:
Investigation, Conceptualization, Visualization. Bin Xie: Writing - review & editing,
Supervision, Funding acquisition. Chuan Lai: Supervision, Funding acquisition.
Dong-Liang Zhang: Investigation, Formal analysis, Conceptualization. Cheng-Long
Deng: Methodology, Investigation, Writing - original draft. Jia-Xi Cao: Investigation.
Xiao-Xue Bai: Investigation. Xiao-Qiang Liu: Supervision.
Declaration of Competing Interest
*
Corresponding authors at: Scholl of Materials Science & Engineering, Sichuan University of
Science & Engineering, Zigong 643000, China; School of Chemistry & Chemical Engineering,
Sichuan University of Arts & Science, Dazhou 635000, China.
E-mail address: xiebindhg@sina.com.cn (Bin Xie); lc860801@163.com (Chuan Lai)

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.
Acknowledgements
Financial support for this work was provided by the Department of Science and
Technology of Sichuan Province (No. 2016JY0048 and 2018103), the Science and
Technology Bureau of Zigong City (No. 2015HX18 and 2019CXMZ01), the Training
Program of Innovation and Entrepreneurship for Undergraduates of Sichuan
University of Science and Technology (No. cx2019198), the Graduate Innovation
Fund Project of Sichuan University of Science and Technology (No. y2019023 and
y2019031), and the Open Fund Project of Sichuan Province Key Laboratory of
Comprehensive Utilization of Vanadium & Titanium Resources (No. 2018FTSZ09).
Appendix A. Supplementary data
CCDC 1859833 (for 1), 1857669 (for 2), 1857670 (for 3), 1857671 (for 4), and
1
857672 (for 51.5CH Cl ) contain the supplementary crystallographic data for this
2
2
paper.
These
data
can
be
obtained
free
from
Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: (+44)
of
charge
via
http://www.ccdc.cam.ac.uk/conts/retrieving.html,
or
the Cambridge
1
223-336-033; or e-mail: deposit@ccdc.cam.ac.uk.
Supplementary data associated with this article can be found, in the online
version, at http://dx.doi.org/10.1016/j.ica.xxxx.xx.xxx.
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Corresponding authors at: Scholl of Materials Science & Engineering, Sichuan University of
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Sichuan University of Arts & Science, Dazhou 635000, China.
E-mail address: xiebindhg@sina.com.cn (Bin Xie); lc860801@163.com (Chuan Lai)

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Corresponding authors at: Scholl of Materials Science & Engineering, Sichuan University of
Science & Engineering, Zigong 643000, China; School of Chemistry & Chemical Engineering,
Sichuan University of Arts & Science, Dazhou 635000, China.
E-mail address: xiebindhg@sina.com.cn (Bin Xie); lc860801@163.com (Chuan Lai)

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Corresponding authors at: Scholl of Materials Science & Engineering, Sichuan University of
Science & Engineering, Zigong 643000, China; School of Chemistry & Chemical Engineering,
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Corresponding authors at: Scholl of Materials Science & Engineering, Sichuan University of
Science & Engineering, Zigong 643000, China; School of Chemistry & Chemical Engineering,
Sichuan University of Arts & Science, Dazhou 635000, China.
E-mail address: xiebindhg@sina.com.cn (Bin Xie); lc860801@163.com (Chuan Lai)

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Corresponding authors at: Scholl of Materials Science & Engineering, Sichuan University of
Science & Engineering, Zigong 643000, China; School of Chemistry & Chemical Engineering,
Sichuan University of Arts & Science, Dazhou 635000, China.
E-mail address: xiebindhg@sina.com.cn (Bin Xie); lc860801@163.com (Chuan Lai)

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*
Corresponding authors at: Scholl of Materials Science & Engineering, Sichuan University of
Science & Engineering, Zigong 643000, China; School of Chemistry & Chemical Engineering,
Sichuan University of Arts & Science, Dazhou 635000, China.
E-mail address: xiebindhg@sina.com.cn (Bin Xie); lc860801@163.com (Chuan Lai)