Function of 7,7,8,8-tetracyanoquinodimethane (TCNQ) on electrocatalytic hydrogen generation catalyzed by N,N′-benzene bis(salicylideniminato)nickel(II)

Article abstract of DOI:10.1016/j.poly.2016.05.064

In the presence of trimethylamine, the reaction of N,N′-benzene bis(salicylidenimine) (H₂L) with NiCl₂·6H₂O affords a nickel(II) complex, [NiL] (1), and the reaction of H₂L with NiCl₂·6H₂O

Full text of DOI:10.1016/j.poly.2016.05.064

Polyhedron 117 (2016) 300–308
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Polyhedron
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Function of 7,7,8,8-tetracyanoquinodimethane (TCNQ) on
electrocatalytic hydrogen generation catalyzed by N,N0-benzene bis
(salicylideniminato)nickel(II)
Dan Xue a, Qi-Ying Lv b, Chen-Neng Lin a, Shu-Zhong Zhan a,
⇑
a College of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou 510640, China
b School of Chemistry & Chemical Engineering, Huazhong University of Science and Technology, Wuhan 430074, China
a r t i c l e i n f o
a b s t r a c t
In the presence of trimethylamine, the reaction of N,N0-benzene bis(salicylidenimine) (H2L) with
NiCl2ꢀ6H2O affords a nickel(II) complex, [NiL] (1), and the reaction of H2L with NiCl2ꢀ6H2O and TCNQ pro-
vides a TCNQ compound, [LNi(TCNQ)] (2). Electrochemical studies show that 1 electrocatalyzes hydrogen
evolution, both from acetic acid with a turnover frequency (TOF) of 53.25 moles of hydrogen per mole of
catalyst per hour at an overpotential (OP) of 941.6 mV (in DMF), and from an aqueous buffer solution (pH
7.0) with a TOF of 462.33 moles of hydrogen per mole of catalyst per hour at an OP of 836.7 mV. 2 can
electrocatalyze hydrogen evolution, both from acetic acid with a TOF of 62.56 moles of hydrogen per
mole of catalyst per hour at an OP of 941.6 mV (in DMF), and from an aqueous buffer solution (pH 7.0)
with a TOF of 909.58 moles of hydrogen per mole of catalyst per hour at an OP of 836.7 mV. To testify
that TCNQ plays a vital role in determining the catalytic activities of the nickel complex, we have system-
atically studied the electrocatalytic activities of 1 and 2, to provide a possible catalytic mechanism for
hydrogen generation by the nickel complexes. This observation suggests that the presence of TCNQ is
a key structural feature for eliciting proton or water reduction catalysis. This can be attributed to nega-
tively charged TCNQ, formed from 2, that can stabilize the low oxidation state of the nickel ion.
Ó 2016 Elsevier Ltd. All rights reserved.
Article history:
Received 9 March 2016
Accepted 25 May 2016
Available online 5 June 2016
Keywords:
Nickel(II) complex
TCNQ
Molecular electrocatalyst
Water reduction
Hydrogen evolution
1. Introduction
been developed as electrocatalysts for the reduction of protons or
water to form H2. Despite much progress in water reduction catal-
Hydrogen is one of the most ideal sources for energy in the
future, because of its numerous advantages such as recyclability
and pollution-free use [1–3]. Water is the only waste-free elec-
tron-source substrate that could sustain the scale of the process
required to supply our energy demands. Thus water splitting is
an important and simple method for hydrogen production with
high purity and in large quantities [4]. The generation of hydrogen
by electrochemical reduction of a proton source, such as water,
represents an attractive approach for storing the electrical energy
transiently produced by renewable energy sources [5,6]. To
increase the reaction rate, it is necessary to use an efficient hydro-
gen evolution reaction electrocatalyst. Therefore, many research
groups, including ours, have designed molecular catalysts by
employing abundant metals, and several complexes of nickel
[7,8], cobalt [9–13], copper [14–17] and molybdenum [18,19] have
ysis, major improvements in several areas, including lowering
overpotentials, increasing catalyst durability and using earth-
abundant elements, are needed before efficient electrocatalytic
water reduction can be realized. Especially, structural complexity,
insolubility in aqueous media and low pH conditions severely limit
their practical utility [20].
Our interest focuses on the design of molecular electrocatalysts
that are water soluble and can efficiently catalyze water reduction.
To the best of our knowledge, there is as yet no report on electro-
chemical water reduction by TCNQ compounds. As we know, TCNQ
is electron poor and forms numerous donor–acceptor compounds
[21], where radical anions are stabilized by charge and spin delo-
calization, forming numerous stable electron transfer salts. Gener-
ally, compounds containing TCNQ exhibit metal-like electrical
conductivity [22] and ferromagnetic ordering [23]. Reported here
is an electrocatalyst based on one nickel-TCNQ complex, [LNi
(TCNQ)] (2), for hydrogen evolution from both acetic acid and
aqueous buffer solution, as well as the effect of TCNQ on the
catalytic properties of the nickel(II) complex. We hope this can
⇑
Corresponding author.
E-mail address: shzhzhan@scut.edu.cn (S.-Z. Zhan).
http://dx.doi.org/10.1016/j.poly.2016.05.064
0277-5387/Ó 2016 Elsevier Ltd. All rights reserved.

D. Xue et al. / Polyhedron 117 (2016) 300–308
301
establish a new method for the improvement in electrocatalytic
hydrogen production by structural modification of the catalyst.
10.0 mmol), NiCl2ꢀ6H2O (1.17 g, 5.0 mmol) and TCNQ (1.02 g,
5.0 mmol) in 50 ml CH2Cl2 with stirring. The color of the solution
changed from yellow to deep-green, and it was allowed to react
for 3 h at room temperature, and then it was filtered. On slow
evaporation at room temperature for several days, deep green crys-
tals appeared. The crystals were collected and dried in vacuo
(1.26 g, 43.8% based on TCNQ). Anal. Calc. for C32H18N6NiO2: C,
66.82; H, 2.80; N, 14.61. Found: 66.86; H, 2.79; N, 14.64%. IR band
2. Experimental section
2.1. Materials and physical measurements
The nickel complex [LNi] (1) was prepared by using the litera-
ture procedure [24]. Elemental analyses for C, H and N were
obtained on a Perkin-Elmer analyzer model 240. An ESI-MS exper-
iment was performed on a Bruker Daltonics Esquire 3000 spec-
trometer by introducing samples directly into the ESI source
using a syringe pump. IR spectra were obtained as KBr pellets on
a Bruker 1600 FT-IR spectrometer from 4000 to 400 cmꢁ1. Elec-
tronic spectra were recorded on a Lambda-900 spectrophotometer.
Cyclic voltammograms were obtained on a CHI-660E electrochem-
ical analyzer under N2 using a three-electrode cell in which a
glassy carbon electrode was the working electrode, an Ag/AgNO3
or saturated Ag/AgCl electrode was the reference electrode and a
platinum wire was the auxiliary electrode. In organic media, the
ferrocene/ferrocenium (1+) couple was used as an internal stan-
dard and a solution of 0.10 M [(n-Bu)4N]ClO4 was used as the sup-
porting electrolyte. Controlled-potential electrolysis (CPE) in DMF
was conducted using an air-tight glass double compartment cell
separated by a glass frit. The working compartment was fitted with
a glassy carbon plate and an Ag/AgNO3 reference electrode. The
auxiliary compartment was fitted with a Pt gauze electrode.
The working compartment was filled with 50 mL of 0.10 M
[(n-Bu4N)]ClO4 DMF solution with acetic acid, while the auxiliary
compartment was filled with 35 mL of 0.10 M [(n-Bu4N)]ClO4
DMF solution, resulting in equal solution levels in both compart-
ments. Both compartments were purged for 1 h with N2 and cyclic
voltammograms (CVs) were recorded as controls. Controlled-
potential electrolysis (CPE) in aqueous media was conducted using
an air-tight glass double compartment cell separated by a glass frit.
The working compartment was fitted with a glassy carbon plate
and an Ag/AgCl reference electrode. The auxiliary compartment
was fitted with a Pt gauze electrode. The working compartment
was filled with 50 mL of 0.25 M buffer, while the auxiliary com-
partment was filled with 35 mL phosphate buffer solution. Adding
nickel complexes, both compartments were purged for 1.5 h with
nitrogen and CVs were recorded as controls. After electrolysis, a
0.5 mL aliquot of the headspace was removed and replaced with
0.5 mL of CH4. The headspace sample was injected into the gas
chromatograph (GC). GC experiments were carried out with an
Agilent Technologies 7890A gas chromatography instrument.
(KBr pellets, cmꢁ1
) m: 2217.
2.4. Crystal structure determination
X-ray analysis of the complex [LNi(TCNQ)] (2) was carried out
with a Bruker Smart Apex II DUO area detector using graphite
monochromated Mo Ka radiation (k = 0.71073 Å) at room temper-
ature. All empirical absorption corrections were applied using the
SADABS program [25]. The structure was solved using direct meth-
ods and the corresponding non-hydrogen atoms were refined
anisotropically. All the hydrogen atoms of the ligands were placed
in calculated positions with fixed isotropic thermal parameters and
included in the structure factor calculations in the final stage of the
full-matrix least-squares refinement. All calculations were per-
formed using the SHELXTL computer program [26]. Crystallo-
graphic data for complex 2 are given in Table 1 and selected
bond lengths and angles are listed in Table 2.
3. Results and discussion
3.1. Synthesis and characterization of the nickel complexes [LNi] (1)
and [LNi(TCNQ)]ꢀMeOH (2ꢀMeOH)
Typically, the reaction of the TCNQ molecule with transition
metal centers leads to di-, tri- or tetranuclear nitrile-bonded
r
compounds, to compounds with side-on coordination, or to the
p
corresponding ion pair compounds without direct coordination
[27–31]. However, when the NiII complex NiL was used as a metal
source to react with TCNQ, an unprecedented TCNQ-compound 2
(Scheme 1) resulted, which is soluble in both aqueous media
(1.5 ꢂ 10ꢁ3 mol Lꢁ1) and general organic solvents, such as CH3OH
and CH3CN, etc. The infrared spectrum of 2 shows one peak at
2217 cmꢁ1, which is assigned to the m(CN) band of the TCNQ mole-
cule of 2 (Fig. S1), indicating that the four cyano groups are in the
same state.
Table 1
Crystallographic data for [LNi(TCNQ)]ꢀMeOH (2ꢀMeOH).
2.2. Synthesis of [LNi] (1)
Empirical formula
Formula weight
k (Å)
Crystal system
Space group
Volume (Å3)
a (Å)
C33H22N6O3Ni
609.27
0.71073
To a solution of o-phenylenediamine (1.08 g, 10 mmol) in
methanol (20 ml), salicylaldehyde (2.45 g, 20 mmol) was added,
and the mixture was refluxed for 8 h. To the above yellow mixture
was added triethylamine (2.00 g, 20 mmol) and NiCl2ꢀ6H2O (2.34 g,
10 mmol) with stirring for 20 min, the color of the solution changed
from yellow to brown. On slow evaporation at room temperature
for several days, brown crystals appeared. The crystals were col-
lected and dried in vacuo (3.15 g, 85%). Anal. Calc. for C20H14N2NiO2:
C, 64.75; H, 3.26; N, 7.55;. Found: 64.86; H, 3.76; N, 7.54%.
triclinic
ꢀ
P1
1382.61(13)
11.2467(7)
11.6531(5)
11.7725(6)
73.1900(10)
69.628(2)
b (Å)
c (Å)
a
(°)
b (°)
c
(°)
81.457(2)
Z
2
Dc(Mg mꢁ3
F(000)
)
1.464
628
2.3. Synthesis of [LNi(TCNQ)] (2)
h range for data collection
Data/restraints/parameters
Goodness-of-fit (GOF) on F2
6.252–50.052
4873/0/390
1.091
R1 = 0.0342, wR2 = 0.0718
R1 = 0.0728, wR2 = 0.0875
A solution of o-phenylenediamine (0.54 g, 5.0 mmol) and salicy-
laldehyde (1.23 g, 10.0 mmol) in 50 ml methanol was refluxed with
stirring for 8 h, whereupon a yellow powder was obtained. To the
above yellow mixture was added triethylamine (1.00 g,
Final R indices [I > 2
r
(I)]
R indices (all data)

302
D. Xue et al. / Polyhedron 117 (2016) 300–308
Table 2
3.2. Electrochemical studies
The selected bond lengths (Å) and angles (°) for [LNi(TCNQ)]ꢀMeOH (2ꢀMeOH).
To study the electrochemical properties of TCNQ, 1 and 2, cyclic
voltammograms (CVs) were conducted in DMF solution with
0.10 M [(n-Bu)4N]ClO4 as the supporting electrolyte. Fig. 2a repre-
sents the CV of TCNQ in DMF. The first reduction potential of TCNQ
is presumably at 0.278 V, and two typical reversible redox peaks
for TCNQ are found at the potentials ꢁ0.315 and 0.278 V versus
Ag/AgNO3, respectively. Complex 1 shows a quasi-reversible redox
couple at ꢁ1.41 V versus Ag/AgNO3, which can be assigned to that
of NiII/NiI (Fig. 2b). Compared to TCNQ, the TCNQ-nickel complex 2
displays two reversible redox couples at ꢁ0.30 and 0.275 V versus
Ag/AgNO3, which are assigned to those of TCNQ, and a quasi-rever-
sible NiII/I redox couple at ꢁ1.39 V versus Ag/AgNO3 (Fig. 2c). Note,
the potential of NiII/I is shifted by 0.02 V as a result of the introduc-
tion of TCNQ. From Fig. S5, all the current responses of the redox
events at ꢁ0.37 V for TCNQ, ꢁ1.46 V for 1 and ꢁ1.41 V for 2 show
a linear dependence on the square root of the scan rate, indicative
of a diffusion-controlled process, with the electrochemically active
species freely diffusing in solution.
To investigate the effect of the temperature of the media on the
electrochemical behaviors of TCNQ and the nickel complexes, CVs
were measured in the range 298–328 K. For 1, the current strength
significantly increases near ꢁ1.46 V on increasing the temperature
from 298 to 328 K (Fig. S6a). Note, both current strengths increase
near ꢁ1.4 and ꢁ0.37 V with increasing temperature from 298 to
328 K for 2 (Fig. S6b). Similar to 2, the current strength for TCNQ
also increases at ꢁ0.37 V with increasing temperature from 298
to 328 K, and the potential of TCNQ/TCNQꢁ moves negative by
about 40 mV from ꢁ0.37 to ꢁ0.41 V (Fig. S6c).
Ni1–O1
Ni1–N5
C24–C25
C24–C28
N1–C29
N3–C31
1.8444(16)
1.851(2)
1.440(4)
1.369(4)
1.346(4)
1.139(3)
Ni1–O2
Ni1–N6
C25–C26
C28–C31
N2–C30
N4–C32
1.8391(19)
1.8526(19)
1.338(4)
1.431(4)
1.140(3)
1.128(3)
O2–Ni1–N6
O2–Ni1–N5
O1–Ni1–N5
95.45(9)
178.59(9)
95.14(8)
O2–Ni1–O1
O1–Ni1–N6
N5–Ni1–N6
83.58(8)
178.18(9)
85.85(9)
Complex 2ꢀMeOH crystallizes in the triclinic system, space
ꢀ
group P1 with two formula units present per unit cell. As shown
in Fig. 1 (top), the X-ray analysis reveals that the crystal structure
of 2ꢀMeOH consists of one Ni(II) complex [NiL], one TCNQ molecule
and one methanol molecule. In the [NiL] unit, the central nickel
atom is coordinated by two oxygen atoms and two nitrogen atoms
of the ligand (L2ꢁ) in a distorted square geometry. The Ni–O bond
lengths fall in the range 1.8391(19)–1.8444(16) Å, and the O–Ni–O
angle is 83.58(8)°. The Ni–N bond lengths are 1.851(2) and 1.8526
(19) Å, and the N–Ni–N angle is 85.85(9)°. In the three-dimensional
packing arrangement, the [NiL] unit and TCNQ molecule lie in the
same plane (Fig. 1 (bottom)).
The CBN bond lengths of TCNQ in 2ꢀMeOH range from 1.128(3)
to 1.14(3) Å, and the average length (1.136 Å) is similar to free
TCNQ (1.140 Å) [32]. To test the degree of charge transfer between
the TCNQ molecule and [LNi], Kistenmacher’s equation was used:
the relationship
determined from neutral TCNQ (
(TCNQ)2
= ꢁ1.0005) [34], where c, b and d are the TCNQ bond
lengths. The charge of the TCNQ group in complex 2 was estimated
from Kistenmacher relationship as
q
= A[c/(b + d)] + B (A = ꢁ41.667, B = 19.833) was
q
= 0) [33] and [Ni([14]aneN4)]
(
q
3.3. Catalytic hydrogen evolution from acetic acid in DMF
q
= ꢁ0.061 (Scheme 2 and
Table 3), suggesting that TCNQ is in the neutral state.
As shown in Fig. S4a, the absorption spectrum of compound 2
exhibits two peaks at 420 (the
TCNQ) and 740 nm (a charge-transfer involving radical anions
(TCNQ/TCNQꢁ), which were found in the free TCNQ [35,36]
(Fig. S4b).
From Fig. 3a, it can be seen that the catalytic currents near
ꢁ1.48 V increase with increasing proton concentration (the acetic
acid concentration was increased from 0.0 to 22.80 mM). This indi-
cates that hydrogen evolution electrocatalyzed by 1 requires the
reduction of Ni(II) to Ni(I) and protonation. Interestingly, when
the acetic acid concentration increases from 0.0 to 22.80 mM
p ?
p⁄ transition of the ring in
OH
HO
NH2
2
OH
+
H2L
N
N
C
NH2
O
H
Et3N NiCl2 6H2O.
NC
NC
CN
CN
TCNQ
O
O
O
O
[NiL] 1
Ni
Ni
N
N
N
N
[NiL(TCNQ)] 2
Scheme 1. Synthesis of the nickel complexes [LNi] 1 and [LNi(TCNQ)] 2.

D. Xue et al. / Polyhedron 117 (2016) 300–308
303
Fig. 1. ORTEP drawing of complex 2 with thermal ellipsoids at the 50% probability level (hydrogen atoms are not shown) (top); Molecular packing arrangement of 2 (bottom).
(Fig. 3a), the potential of NiII/NiI becomes more positive by about
50 mV, from ꢁ1.48 to ꢁ1.43 V, and the onset of the catalytic wave
also becomes more positive by about 430 mV compared to that in
the absence of 1. For 2, the catalytic current near ꢁ1.45 V also
increases with an increasing proton concentration (the acetic acid
concentration was increased from 0.0 to 22.80 mM) (Fig. 3b), and
the potential of NiII/NiI becomes slightly more negative by about
9 mV, from ꢁ1.456 to ꢁ1.465 V. Interestingly, when the acetic acid

304
D. Xue et al. / Polyhedron 117 (2016) 300–308
N
N
N
(a)
10
C
C
C
d
*
b
C
c
C
5
C
N
0
-5
Scheme 2. Schematic representation of 7,7,8,8-tetracyanoquinodimethane.
Table 3
Important structural features in several TCNQ complexes.
-10
-15
Compounds
Bond distances (Å)
References
b
c
d
TCNQ
1.448(4)
1.374(3)
1.441(4)
[30]
-1.0
-0.5
0.0
0.5
1.0
[Ni(N4cisdiene)](TCNQ)2
TCNQ
[LNi(TCNQ)] 2
TCNQ
Potential(VνsAg/AgNO)
1.410
1.410
1.400
[31]
3
1.433(5)
1.365(5)
1.431(5)
this work
*
6
3
(b)
concentration increases from 0.0 to 22.80 mM (Fig. 2b), the onset
of the catalytic wave moves to a higher potential from ꢁ1.28 to
ꢁ1.24 V versus Ag/AgNO3. For TCNQ, the catalytic current near
ꢁ0.45 V remains almost constant with increasing proton concen-
tration (the acetic acid concentration was increased from 0.0 to
22.80 mM) (Fig. 3c), indicating that TCNQ does not have activity
for proton reduction. Thus, the introduction of 1 to TCNQ is essen-
tial for catalytic activity.
0
-3
-6
-9
To testify that TCNQ plays a vital role in determining the cat-
alytic activity of the nickel complex, bulk electrolysis of 1 and 2
were conducted in DMF solutions with acetic acid at variable
applied potentials using a glassy carbon plate electrode in a dou-
ble-compartment cell. Fig. 4 shows the total charges of bulk elec-
trolysis of 1 and 2 in the presence of acid, and the charge
significantly increased when the applied potential was more nega-
tive. When the applied potential was ꢁ1.45 V versus Ag/AgNO3, the
maximum charge reached 98 and 115 mC during 2 min of electrol-
ysis, respectively (Fig. 4a and b). A CPE experiment under the same
potential without 1 or 2 gave a charge of only 6 mC (Fig. 4c), show-
ing that the nickel complexes do indeed serve as effective hydro-
gen producers under such conditions. Assuming every catalyst
molecule was distributed only on the electrode surface and every
electron was used for the reduction of protons, according to Eqs.
(1) [13] and (2) [37], we calculated the TOF for the catalysts as
reaching a maximum of 53.25 for 1 and 62.56 for 2 moles of hydro-
gen per mole of catalyst per hour at an overpotential of 941.6 mV,
respectively (Eqs. (S1) and (S2)). Remarkably, 2 exhibits more
activity than 1, indicating that the introduction of TCNQ into 1 is
essential for more efficient activity. These results also suggest that
the nickel complexes with more positive NiII/I redox potentials
show a higher activity for hydrogen generation, which is different
from the reported results [38].
-2.0
-1.5
-1.0
-0.5
0.0
0.5
1.0
Potential(Vνs Ag/AgNO3)
6
3
(c)
*
0
-3
-6
-2.0 -1.5 -1.0 -0.5
0.0
0.5
1.0
TOF ¼
D
C=ðFꢃnꢃ1nꢃ2tÞ
ð1Þ
Potential (Vνs Ag/AgNO3)
Overpotential ¼ Applied potential ꢁ EHꢄA
Fig. 2. Cyclic voltammograms of 2.33 mM TCNQ (a), 2.33 mM [LNi] (1) (b) and
1.86 mM [LNi(TCNQ)] (2) (c). Conditions: room temperature, 0.10 M [n-Bu4N]ClO4
as the supporting electrolyte, glassy carbon working electrode (1 mm diameter), Pt
counter electrode, Ag/AgNO3 reference electrode, scan rate 0.10 V/s, ferrocene
internal standard (⁄).
¼ Applied potential ꢁ ðEꢄHþ ꢁ ð2:303RT=FÞpKaHA
Þ
ð2Þ
where
DC is the charge from the catalyst solution during CPE minus
the charge from solution without catalyst during CPE; F is Faraday’s
constant, n1 is the number of moles of electrons required to gener-
ate one mole of H2, n2 is the number of moles of catalyst in solutio,
and t is the duration of the electrolysis.
Generally, hydrogen evolution reactions occur via metal-
hydride species, which have been demonstrated in several catalytic
systems, such as diiron mimics of iron-only hydrogenases

D. Xue et al. / Polyhedron 117 (2016) 300–308
305
0.10
8
4
(a)
*
Applied potential Overpotential
(a)
341.6 mV
391.6 mV
491.6 mV
641.6 mV
791.6 mV
941.6 mV
-0.85 V
-0.90 V
-1.00 V
-1.15 V
-1.30 V
-1.45 V
0.08
0.06
0.04
0.02
0.00
0
cHOAc
0.000 mM
0.999 mM
1.996 mM
3.984 mM
9.901 mM
22.80 mM
-4
-8
-12
0
0
0
20
40
60
80
80
80
100
100
100
120
-2.0
-1.5
-1.0
-0.5
s Ag/AgNO3)
0.0
0.5
1.0
Time (s)
Potential (V
ν
0.12
0.10
0.08
0.06
0.04
0.02
0.00
Applied potential Overpotential
(b)
241.6 mV
391.6 mV
541.6 mV
741.6 mV
941.6 mV
-0.75 V
-0.90 V
-1.05 V
-1.25 V
-1.45 V
(b)
*
cHOAc
20
10
0
0.000 mM
0.999 mM
1.996 mM
3.984 mM
9.901 mM
22.80 mM
-10
-20
20
40
60
120
Time (s)
-2.0
-1.5
-1.0
-0.5
0.0
0.5
1.0
Potential (V vs Ag/AgNO3)
(c)
0.006
0.005
0.004
0.003
0.002
0.001
0.000
(c)
4
2
*
0
cHOAc
0.000 mM
0.999 mM
1.996 mM
3.984 mM
9.901 mM
22.80 mM
-2
-4
-6
20
40
60
120
Time (s)
Fig. 4. Charge build-up versus time from electrolysis of 0.10 M [n-Bu4N]ClO4 with
5.36 M complex 1 (a), 5.36 M complex 2 (b) and 2.0 mM acetic acid in DMF under
various applied potentials. All data have been deducted blank. (c) Charge build-up
versus time from electrolysis of 0.10 M [n-Bu4N]ClO4 in DMF at ꢁ1.45 V versus Ag/
AgNO3.
l
l
-1.0
-0.5
0.0
0.5
1.0
Potential (V vs Ag/AgNO3)
Fig. 3. Cyclic voltammograms of a 2.23 mM solution of [NiL] (1) (a), a 2.23 mM
solution of [LNi(TCNQ)] (2) (b) and a 2.23 mM solution of TCNQ (c), with varying
concentrations of acetic acid in DMF. Conditions: 0.10 M [n-Bu4N]ClO4 as the
supporting electrolyte, scan rate: 0.10 V/s, glassy carbon working electrode (1 mm
diameter), Pt counter electrode, Ag/AgNO3 reference electrode, ferrocene internal
standard (⁄).
clooctane; X = OMe, Me, CH2P(O)(OEt)2, Br, CF3) [41], cobalt macro-
cyclic complexes [3] and [Co(dmgBF2)2L] (dmg2ꢁ = the dimethyl-
glyoximato dianion; L = CH3CN or N,N0-dimethylformamide) [42].
Based on the above analyses, we propose a presumed mechanism
depicted in Scheme 3 for the generation of hydrogen from acetic acid
mediated by 1. One-electron reduction of [LNiII] gives a putative
[LNiI]ꢁ species. Addition of a hydrogen proton yields the [LNiIII-H]
[3,39,40], molecular nickel catalysts [Ni(Pp2hN2C6H4X)2](BF4)2
(Pp2hNC26H4X = 1,5-di(para-X-phenyl)-3,7-diphenyl-1,5-diphosphacy-

306
D. Xue et al. / Polyhedron 117 (2016) 300–308
species, a highly reactive intermediate. Further one-electron reduc-
tion of the [LNiII-H] species affords H2 and regenerates the starting
[LNiII]. In order to confirm these results, we also tried to apply H2L
only as a catalyst for hydrogen generation (Fig. S7). Obviously, there
was no catalyst effect for hydrogen generation when compared to
[LNi] (1). Although the relative contributions are indistinguishable
in this analysis, we suspect that these processes are complementary
H2 evolution pathways.
10
0
(a)
c[LNi]
-10
-20
-30
0.000
μ
μ
μ
μ
μ
M
0.580
1.594
1.739
2.319
M
M
M
M
3.4. Catalytic hydrogen evolution in aqueous media
We further explored the electrochemical behaviors of [LNi] (1)
and [LNi(TCNQ)] (2) in buffered aqueous solutions at varying
pHs, where pH = 4.5–7.0, which is the range associated with cat-
alytic water reduction, a much more attractive medium for the sus-
tainable generation of hydrogen. As shown in Fig. 5a, in the
absence of 1, a catalytic current was not apparent until a potential
of ꢁ1.48 V versus Ag/AgCl was attained. Upon addition of complex
1, the onset of the catalytic current was observed at about ꢁ1.25 V
versus Ag/AgCl, showing that addition of complex 1 can reduce the
potential, and the current strength increased with increasing con-
-2.0
-1.5
-1.0
-0.5
0.0
0.5
Potential (V vs Ag/AgCl)
(b)
0
-100
-200
-300
-400
-500
centrations of 1 from 0.580 to 2.319 lM (Fig. 5a). From Fig. 5b, the
pH
onset of this catalytic current is clearly influenced by the solution
pH, the applied potential declined with increasing pH, evidencing
the involvement of a proton in the initial stage of the electrochem-
ical catalysis. From Fig. 6a, with addition of 2, the onset of the cat-
alytic current was observed at about ꢁ1.05 V versus Ag/AgCl, and
the current strength increased significantly with increasing con-
7.0
6.5
6.0
5.5
5.1
4.5
centration of 2 from 0.580 to 2.319 lM (Fig. 6a). From Fig. 6b, 2
shows a pH-dependent peak, which is responsible for the catalytic
water reduction, and the applied potential declined with increas-
ing pH.
To further confirm that TCNQ plays a vital role in hydrogen pro-
duction catalyzed by the nickel complex in aqueous media, bulk
electrolysis was measured in a 0.25 M aqueous buffer solution
(pH 7.0) containing 1 or 2. When the applied potential was
ꢁ1.45 V versus Ag/AgCl, the maximum charge was only 23 mC
for 2 min of electrolysis in the absence of complex 1 (Fig. 7a).
Under the same conditions, the charge reached 822 mC during
2 min of electrolysis in the presence of 1 (Fig. 7b), accompanied
by evolution of a gas, which was confirmed as H2 by gas
-2.0
-1.5
-1.0
-0.5
0.0
0.5
Potential (V vs Ag/AgCl)
Fig. 5. (a) Cyclic voltammograms of complex
(b) Cyclic voltammograms of complex (0.232
Conditions: 0.25 M phosphate buffered solution (pH 7.0), glassy carbon working
electrode (1 mm diameter), Pt wire counter electrode, Ag/AgCl reference electrode.
1
at different concentrations.
lM) at different pH values.
1
chromatography. According to Fig. S8a, ꢅ7.25 mL of H2 was pro-
duced over an electrolysis period of 1 h with a Faradaic efficiency
of 96.53% for H2 (Fig. S8-b). According to Eqs. (1) and (3) [9,43],
we calculated the TOF for the catalyst as reaching a maximum of
462.33 moles of hydrogen per mole of catalyst per hour at an over-
potential of 837.6 mV (Eq. (S3)), where
O
O
e-
H2
NiII
N
N
H+
Overpotential ¼ Applied potential ꢁ EðpHÞ
-
-
H
¼ Applied potential ꢁ ðꢁ0:059pHÞ
ð3Þ
O
O
O
O
NiII
NiI
Fig. 7c shows the total charge of the bulk electrolysis of 2 in a
0.25 M buffer solution (pH 7.0), and the charge significantly
increased when the applied potential was set to be more negative.
When the applied potential was ꢁ1.45 V versus Ag/AgCl, the max-
imum charge reached 1599 mC during 2 min of electrolysis in the
presence of 2 (Fig. 7c), accompanied by a large amount of gas bub-
bles, which was confirmed as H2 by gas chromatography. Accord-
ing to Fig. S9a, ꢅ9.918 mL of H2 was produced over an
electrolysis period of 1 h with a Faradaic efficiency of 98.68% for
H2 (Fig. S9-b). According to Eqs. (1) and (3), we also calculated
the TOF for the catalyst as reaching a maximum of 909.58 moles
of hydrogen per mole of catalyst per hour at an overpotential of
836.7 mV (Eq. (S4)). Remarkably, complex 2 is almost two times
N
N
N
N
H
H+
O
O
e-
NiIII
N
N
Scheme 3. Proposed mechanism for proton reduction catalyzed by [LNi].

D. Xue et al. / Polyhedron 117 (2016) 300–308
307
(a)
(a)
0.025
0
-100
-200
-300
-400
-500
c[LNi(TCNQ)]
0.020
0.000
0.580
1.169
1.594
1.739
2.319
μ
μ
μ
μ
μ
μ
M
M
M
M
M
M
0.015
0.010
0.005
0.000
0
20
40
60
Time (s)
80
100
120
-2.0
-1.5
-1.0
-0.5
0.0
0.5
Potential (V vs Ag/AgCl)
Applied potential Overpotential
(b)
537.6 mV
587.6 mV
637.6 mV
687.6 mV
737.6 mV
787.6 mV
837.6 mV
-1.15 V
-1.20 V
-1.25 V
-1.30 V
-1.35 V
-1.40 V
-1.45 V
0.8
0.6
0.4
0.2
0.0
(b)
0
-150
-300
-450
-600
-750
pH
7.0
6.5
6.0
5.5
4.5
0
20
40
60
Time (s)
80
100
120
-2.0
-1.5
-1.0
-0.5
0.0
0.5
Potential (V vs Ag/AgCl)
Applied potential Overpotential
(c)
1.5
1.2
0.9
0.6
0.3
0.0
-0.85 V
-1.00 V
-1.15 V
-1.30 V
-1.35 V
-1.40 V
-1.45 V
237.6 mV
387.6 mV
537.6 mV
687.6 mV
737.6 mV
787.6 mV
837.6 mV
Fig. 6. (a) Cyclic voltammogram (CV) of complex 2 at different concentrations. (b)
CVs of complex 2 (0.289 M) at different pH values. Conditions: 0.25 M phosphate
buffered solution (pH 7.0), glassy carbon working electrode (1 mm diameter), Pt
wire counter electrode, Ag/AgCl reference electrode.
l
more effective than complex 1. To the best of our knowledge, this
value is significantly higher than some reported molecular cata-
lysts based on nickel complexes for electrochemical hydrogen pro-
duction from neutral water. For example, a similar nickel complex
supported by 2,3-bis(2-hydroxybenzylideneimino)-2,3-butene-
dinitrile shows a TOF of 206 moles of H2 per mole of catalyst per
hour at an overpotential of 836.6 mV [8] and a dinickel complex
exhibits a turnover number of 100 mol of H2 per mole of catalyst
at an overpotential of 820 mV [44]. This indicates that complex 2
is a highly active catalyst, probably because of its unique structure
and properties: (1) [LNi(TCNQ)] (2) is soluble in water. (2) TCNQꢁ,
formed from 2 (Scheme 4) can stabilize a low oxidation state of
nickel. This observation suggests that the presence of TCNQ is
the key structural feature for eliciting proton and water reduction
catalysis.
0
20
40
60
Time (s)
80
100
120
Fig. 7. (a) Charge build-up versus time from electrolysis of a 0.25 M buffer solution
at ꢁ1.45 V versus Ag/AgCl. Charge build-up versus time from electrolysis of a
0.25 M buffer solution (pH 7.0) with 5.36 lM [LNi] (1) (b), 5.36 lM [LNi(TCNQ)] (2)
(c) under various applied potentials. All data have been deducted blank.
To prove that both complexes 1 and 2 act as homogeneous elec-
trocatalysts, we measured the dependence of the catalytic current
on the concentration of 1 or 2. From Figs. 5a and 6a, the catalytic
currents were indeed dependent on the concentrations of com-
plexes 1 and 2. Additionally, several other pieces of evidence sug-
gest that these nickel complexes are homogeneous catalysts, as
follows. (1) There is no evidence for a heterogeneous electrocat-
alytic deposit. For example, the electrode was rinsed with water
and electrolysis at ꢁ1.45 versus Ag/AgCl was run for an additional
2 min in 0.25 M phosphate buffer at pH 7.0 with no nickel complex
present in solution. During this period, ca. 23 mC of charge was
passed, a similar magnitude as observed for electrolyses conducted
with freshly polished electrodes. (2) No discoloration of the
electrodes was observed during cyclic voltammetry or bulk
electrolysis.

308
D. Xue et al. / Polyhedron 117 (2016) 300–308
NC
NC
CN
CN
NC
CN
2e-
O
O
O
O
NiI
NiII
N
N
N
N
NC
CN
Scheme 4. The formation of [LNiIꢀTCNQꢁ].
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In this paper, we have described two nickel(II) complexes, that
are easily obtained from the reactions of a simple nickel salt with a
Schiff base ligand and which have been characterized by physico-
chemical and spectroscopic methods. Both complexes can electro-
catalyze hydrogen generation from acetic acid and aqueous buffer
solution. Electrochemical studies show that the introduction of
TCNQ into the nickel complex [LNi] can improve the catalytic effi-
ciency for hydrogen production, and this nickel complex, with a
more positive NiII/I redox potential, exhibits higher activity for
hydrogen generation. Our ongoing efforts are focused on modifying
the Schiff base ligand to give related water-soluble complexes with
higher activity for further functional studies, with an emphasis on
chemistry relevant to sustainable energy cycles.
Acknowledgements
This work was supported by the National Science Foundation of
China (No. 20971045 and 21271073).
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Appendix A. Supplementary data
CCDC 772098 contains the supplementary crystallographic data
for this paper. These data can be obtained free of charge via http://
www.ccdc.cam.ac.uk/conts/retrieving.html, or from the Cambridge
Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ,
UK; fax: (+44) 1223-336-033; or e-mail: deposit@ccdc.cam.ac.uk.
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.poly.2016.05.064.
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