Electro- and photocatalytic hydrogen evolution by a cobalt complex based on a tripodal iminopyridine ligand

Article abstract of DOI:10.1016/j.poly.2014.07.031

A cobalt complex based on the tripodal iminopyridine type ligand (2E)-2-N-(pyridin-2-ylmethylidene)-1-N,1-N-bis({2-[(E)-(pyridin-2-ylmethylidene) amino]phenyl})benzene-1,2-diamine (L1), namely, [CoL1](BF₄) ₂ (1a), was synthesized and characterized by FT-IR, UV-Vis spectroscopy, magnetic moment and elemental analysis. X-ray structural study of 1a shows that the complex is coordinatively saturated and contains the uncoordinated bridgehead nitrogen atom. Electrochemical studies reveal that the complex 1a has an electrocatalytic proton reduction activity in acetonitrile, when using acetic acid as a proton source with i_cat/i_p ～ 3.3. Therefore we estimate that the complex 1a could give the highest TOF of 2.3 s^-1 for the reduction of proton to hydrogen. Furthermore, under visible-light irradiation, an optimized H₂ evolution system containing 1a (0.1 mM), [Ir(ppy)₂(bpy)](PF₆) (0.1 mM), and 10 vol% TEA in CH₃CN-H₂O (1/1, v/v) mixed solvents at pH 10 gives a highest turnover number of 34. For comparison, an electron-withdrawing ester group (CO₂CH₂CH₃) is introduced into the pyridine moiety of the ligand as in L2, however, which leads to its corresponding cobalt complex 2a less active than the complex 1a for electro- and photocatalytic H₂ evolution. Additionally, the control experiment establishes that the isostructural zinc complex [ZnL1](BF ₄)₂ (1b) is ineffective for H₂ generation.

Full text of DOI:10.1016/j.poly.2014.07.031

Polyhedron 81 (2014) 639–645
Contents lists available at ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
Electro- and photocatalytic hydrogen evolution by a cobalt complex
based on a tripodal iminopyridine ligand
⇑
⇑
Xiaowei Song , Huimin Wen, Chengbing Ma, Changneng Chen
State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
A cobalt complex based on the tripodal iminopyridine type ligand (2E)-2-N-(pyridin-2-ylmethylidene)-1-
Received 14 June 2014
Accepted 16 July 2014
Available online 24 July 2014
N,1-N-bis({2-[(E)-(pyridin-2-ylmethylidene)amino]phenyl})benzene-1,2-diamine (L1), namely, [CoL1](BF
4 2
)
(1a), was synthesized and characterized by FT-IR, UV–Vis spectroscopy, magnetic moment and elemental
analysis. X-ray structural study of 1a shows that the complex is coordinatively saturated and contains the
uncoordinated bridgehead nitrogen atom. Electrochemical studies reveal that the complex 1a has an elec-
Keywords:
Cobalt
Tripodal-iminopyridine
Hydrogen evolution
Electrocatalysis
Photocatalysis
trocatalytic proton reduction activity in acetonitrile, when using acetic acid as a proton source with icat
/
ꢁ1
i
p
ꢀ 3.3. Therefore we estimate that the complex 1a could give the highest TOF of 2.3 s for the reduction
of proton to hydrogen. Furthermore, under visible-light irradiation, an optimized H
containing 1a (0.1 mM), [Ir(ppy) (bpy)](PF ) (0.1 mM), and 10 vol% TEA in CH CN–H
solvents at pH 10 gives a highest turnover number of 34. For comparison, an electron-withdrawing ester
group (CO CH CH ) is introduced into the pyridine moiety of the ligand as in L2, however, which leads to
its corresponding cobalt complex 2a less active than the complex 1a for electro- and photocatalytic H
evolution. Additionally, the control experiment establishes that the isostructural zinc complex
ZnL1](BF (1b) is ineffective for H generation.
2
evolution system
2
6
3
2
O (1/1, v/v) mixed
2
2
3
2
[
4
)
2
2
Ó 2014 Elsevier Ltd. All rights reserved.
1
. Introduction
Hydrogen production through the reduction of water represents
transition metal complexes of tripodal iminopyridine type ligands
tend to be coordinatively saturated [14,15]. Striking precedents
using hexacoordinated cobalt complexes as electrocatalysts for
hydrogen generation have been reported previously, such as hexa-
mine cobalt complexes and boron-capped tris(glyoximato) cobalt
complexes [16,17]. Additionally, as suggested by proposed struc-
tures of the [FeFe]-hydrogenase active site and DuBois-type nickel
catalyst, the introduction of pendant bases within the multidentate
ligand could also influence the catalytic process and proton trans-
fer reaction [18].
a fascinating approach to providing clean energy needed for sus-
tainable development globally [1,2]. In recent years, the explora-
tion of efficient, inexpensive molecular catalysts for electro- and
photocatalytic H
2
evolution has been a subject of intense work
[
2,3]. Among various catalysts, the Co-based complexes of different
types of ligand scaffold are rightfully attracting increased attention
owing to their rich redox chemistry and catalytic power for
H
2
evolution [4]. In particular, an increasing number of imino
Directly inspired by cobalt bis(iminopyridine) complex for elec-
trocatalytic proton reduction [6], we have investigated synthesis,
characterizations, crystal structures, electro- and photocatalytic
group-containing ligands (Fig. 1) such as NNN pincer ligand [5],
bis(iminopyridine) [6], bisglyoxime [7–10], and diimine-dioxime
[
11] have been utilized to couple with the metal center for catalytic
H
2
evolution of two coordinatively saturated cobalt complexes
proton reduction [12].
[CoL1](BF (1a) and [CoL2](BF (2a) based on tripodal imino-
4
)
2
4 2
)
We are particularly interested in investigating the tripodal imi-
nopyridine ligands because of the ability of this ligand type to sup-
port metal complexes with novel electronic properties, which are
also easily tailored by the possibility of the introduction of various
substituents on both the amine and aldehyde [13]. The first row
pyridine ligands (2E)-2-N-(pyridin-2-yl-methylidene)-1-N,1-N-
bis({2-[(E)-(pyridin-2-ylmethylidene)amino]phenyl})benzene-1,2-
diamine (L1) and ethyl-6-[(E)-N-{2-[bis({2-[(E)-{[5-(ethoxycar
bonyl)pyridine-2-yl]methylidene}amino]phenyl})amino]phenyl}-
carboximidoyl]pyridine-3-carboxylate (L2). As a comparison with
the cobalt complexes, the isostructural zinc complex [ZnL1](BF
1b) has also been studied. Drawings of the complexes are
presented in Fig. 1.
4 2
)
(
⇑
Corresponding authors.
E-mail addresses: songxiaowei@fjirsm.ac.cn (X. Song), ccn@fjirsm.ac.cn
(
C. Chen).
http://dx.doi.org/10.1016/j.poly.2014.07.031
277-5387/Ó 2014 Elsevier Ltd. All rights reserved.
0

6
40
X. Song et al. / Polyhedron 81 (2014) 639–645
Fig. 1. Imino group-containing ligands reported in the literature and synthetic procedure for the complexes in the present work.
H), 3.31 (bs, 6 H). 13C NMR (101 MHz, CDCl
25.65, 125.56, 118.84, 116.46. Anal. Calc. for C18
H, 6.25; N, 19.30. Found: C, 74.20; H, 6.44; N, 19.57%.
) d 141.27, 132.41,
: C, 74.46;
2
. Experimental
3
1
18 4
H N
2.1. Materials and methods
All reactions were carried out under a dry nitrogen atmosphere
4 2
2.3. Synthesis of [CoL1](BF ) (1a)
by using Schlenk techniques and vacuum-line systems unless
otherwise specified. The solvents were dried, distilled, and
degassed prior to use except that those for spectroscopic measure-
A solution of 0.145 g (0.5 mmol) of tris(2-aminophenyl)amine
in 5 mL acetonitrile was treated with 2-pyridinecarboxaldehyde
0.160 g, 1.5 mmol). After stirring for 15 min at room temperature,
a solution of [Co(H O) ](BF (0.170 g, 0.5 mmol) in acetonitrile
10 mL) was added. The resulting red solution was heated at
ments were of spectroscopic grade. [Co(bpy)
rochelated tris(glyoximato)cobalt [Co(dmg(BF)2/3
Ir(ppy) (bpy)](PF ) [20], Tris(2-nitrophenyl)amine [21] and 5-eth-
]Cl
3 2
[19], BF-clath-
(
)
3
](BF [17],
4
)
2
6
4 2
)
[
2
6
(
oxycarbonyl-2-pyridinecarboxaldehyde [22,23] were synthesized
by the synthetic procedures described in the literature. Other
chemicals in this work were purchased from commercial sources
and used without further purification. Elemental analyses were
5
0 °C. After 10 h, an orange powder was precipitated by addition
of diethyl ether (20 mL) to the reaction solution. The precipitate
was collected by filtration, washed with diethyl ether (3 ꢃ 5 mL),
and recrystallized by diethyl ether diffusion to yield 0.350 g of
1
carried out on a Vario MICRO Elemental Analyser. H NMR and
ꢁ1
red crystals (81%). Selected IR (KBr, cm ): 1637 (
m
C
@
N
). Absorption
1
3
C
NMR spectra were performed on
a
Bruker Avance III
ꢁ1
ꢁ1
spectrum in acetonitrile: k, nm; (
15(ꢀ52000). eff (300 K): 4.60
for CoC36 O: C, 51.22; H, 3.94; N, 11.61. Found: C,
ꢂ3H
0.91; H, 4.13; N, 11.76%.
e
, M cm ): 194(ꢀ200000),
(
400 MHz) spectrometer. The variable temperature magnetic
3
l
l
B
.
l
eff (2 K): 4.11 . Anal. Calc.
l
B
susceptibility measurements of 1a and 2a were performed in the
temperature range of 2–300 K. Infrared spectra were recorded on
a Perkin–Elmer Spectrum One FT-IR spectrometer with KBr pellets.
UV–Vis absorption spectra were measured on a Perkin-Elmer
Lambda 35 UV–Vis spectrophotometer.
H
27
2
B F
8
N
7
2
5
4 2
2.4. Synthesis of [ZnL1](BF ) (1b)
1
b was prepared in an analogous manner to 1a using Zn(BF
4 2
)
(
0.120 g, 0.5 mmol) to give 0.390 g (88%) of product as yellow crys-
2
.2. Synthesis of tris(2-aminophenyl)amine
ꢁ1
tals. Selected IR (KBr, cm ): 1633 (
m
@
C N
). Absorption spectrum in
, M cm ): 192(170000), 276(41000),
34(29000). H NMR (400 MHz, CD CN) d 8.52 (s, 3H), 8.26–8.19
m, 3H), 7.91 (d, J = 7.7 Hz, 3H), 7.71–7.63 (m, 6H), 7.39 (td,
J = 7.7, 1.4 Hz, 3H), 7.28–7.17 (m, 6H), 7.10 (dd, J = 7.9, 1.3 Hz,
ꢁ1
ꢁ1
acetonitrile: k, nm; (
e
The preparation of tris(2-aminophenyl)amine was analogous to
1
3
(
3
that reported in the literature with a slight modification of
reducing agent used [24]. To suspension of tris(2-nitro-
phenyl)amine(7.6 g, 20 mmol) and catalyst (1.0 g, 10% Pd/C) in eth-
anol(250 mL) was added NH NH O(12.0 g, 240 mmol). The
a
3
4
H). Anal. Calc. for ZnC36
.21; N, 12.32. Found: C, 50.33; H, 4.25; N, 12.22%.
H
27
B
2
F
8
N
7
ꢂ4H
2
OꢂCH
3
CN: C, 50.17; H,
2
ꢂH
2 2
mixture was refluxed for 4 h under nitrogen atmosphere, cooled
down to room temperature and filtered through a pad of Celite.
The solvent was evaporated to give an off-white solid. The result-
ing solid was washed with ethanol and diethyl ether for 2 times
and dried overnight under vacuum to yield 4.6 g (80%) of tris(2-
4 2
2.5. Synthesis of [CoL2](BF ) (2a)
2a was prepared in an analogous manner to 1a using
5-ethoxycarbonyl-2-pyridinecarboxaldehyde (0.270 g, 1.5 mmol)
to give 0.390 g (88%) of product as red crystals. Selected IR (KBr,
aminophenyl)amine as a white solid. 1H NMR (400 MHz, CDCl
) d
.05–6.94 (m, 3 H), 6.90 (dd, J = 7.8, 1.4 Hz, 3 H), 6.80–6.63 (m, 6
3
7

X. Song et al. / Polyhedron 81 (2014) 639–645
641
cmꢁ1): 1730 (
trile: k, nm; (
eff (300 K): 4.46
CoC45
ꢂ4H
C, 50.16; H, 4.52; N, 9.34%.
m
e
@
) 1637 (
m
@
). Absorption spectrum in acetoni-
, M cm ): 194(190000), 244(49000), 327(43000).
eff (2 K): 3.69 Anal. Calc. for
O: C, 50.12; H, 4.39; N, 9.09. Found:
C
O
C
N
2
N as carrying gas. The amount of hydrogen generated was deter-
ꢁ
1
ꢁ1
mined by the external standard method. Hydrogen dissolved in the
solution was not measured and the slight effect of the hydrogen
generated on the pressure of the Schlenk bottle was neglected
for calculation of the volume of hydrogen gas.
l
l
B
.
l
B
l .
H
39
2
B F
8
N
7
O
6
2
2.6. Crystal structure determination
3
. Results and discussions
Single crystals of 1a, 1b, and 2a, suitable for X-ray diffraction
3.1. Synthesis and spectroscopic characterizations
analysis, were grown by slow diffusion of diethyl ether into the
acetonitrile solution of the complexes. The X-ray single crystal data
were collected on the Agilent Supernova Dual (Cu at zero) Atlas dif-
fractometer. Crystal data collection, refinement and reduction
were accomplished with the CrysAlisPro, Agilent Technologies,
and Version 1.171.36.28. The crystal structures were solved by
direct methods with SHELXS-97 and refined by using the SHELXL-97
crystallographic software package. All non-hydrogen atoms were
refined anisotropically. The hydrogen atoms were added in a riding
model. Details of crystal data are summarized in Table 1.
The tris(2-aminophenyl)amine can be prepared in high yield by
Pd-catalyzed reduction of tris(2-nitrophenyl) amine in the pres-
ence of hydrogen [24]. However, the reduction reaction proceeded
under the high pressure condition. In the present study, we used
hydrazine hydrate as a reducing agent to produce tris(2-aminophe-
nyl)amine without pressure vessel at ambient conditions. The
ligands L1 and L2 were made via the Schiff base condensation of
1
equiv of tris(2-aminophenyl)amine with 3 equiv of either 2-pyr-
idinecarboxaldehyde or 5-ethoxycarbonyl-2-pyridinecarboxalde-
hyde and used immediately without isolation. Complex
formation was straightforward in acetonitrile using metal salts of
2.7. Electrochemistry
2
+
2+
Co and Zn . The complexes formed quickly as evidenced by
immediate changes in the colors of the reaction mixtures and could
be isolated by concentrating the reaction mixture, followed by pre-
cipitation with diethyl ether. The obtained powders were recrystal-
lized by slow diffusion of diethyl ether and yielded the crystalline
material of red 1a, yellow 1b and red 2a, respectively. As shown in
Electrochemical measurements were made using a CH instru-
ment Model 630A electrochemical workstation. The cyclic voltam-
metry experiments were conducted on in a three electrode cell
including a glassy carbon working electrode, a Pt wire auxiliary
electrode, and a Ag/AgCl reference electrode under nitrogen atmo-
sphere. The potential was reported relative to the internal refer-
Fig. 2a, the FT-IR spectra of complexes 1a and 1b exhibit a band at
+
ence of Fc /Fc = 0.00 V. The supporting electrolyte was 0.1 M
ꢁ1
1
637 and 1633 cm , respectively, which are assigned to the
4 6
Bu NPF in acetonitrile.
stretching vibration of azomethine group (CH@N) [25]. As
expected, the FT-IR spectrum of complex 2a is very similar to that
ꢁ
1
2
.8. Photocatalysis
of 1a, except for the appearance of an intense band at 1730 cm
assigned to the stretching vibration of the C@O group. The UV–
Vis absorption spectra for the complexes shows the existence of
2 6
In a typical procedure, [Ir(ppy) (bpy)]PF (0.4 mg, 0.5 lmol), 1a
0.4 mg, 0.5 lmol) and 5 mL acetonitrile aqueous solution (1/1, v/
v) containing 10 vol% TEA were added to a Schlenck bottle. The
mixture was magnetically stirred under nitrogen atmosphere for
5 min. The system was freeze–pump-thaw degassed for three
times and then warmed to room temperature prior to irradiation.
The reaction solution was irradiated at 25 °C using a Xe lamp
300 W) with a cutoff filter (k > 400 nm). The gas phase of the reac-
(
intense absorption bands in the ultraviolet region, which may be
⁄
due to the ligand-centered
p
?
p
transitions (Fig. 2b) [15]. The
room temperature effective magnetic moments for 1a and 2a indi-
1
cate that the two complexes are of the high-spin type with three
1
0
unpaired electrons (Fig. 2c). Because of the d configuration for
Zn(II), a solid state diamagnetic moment was observed for complex
1
(
1
b, which is also confirmed by the H NMR spectrum of Zn com-
tion system was analyzed on a GC 7900 instrument with a 5 Å
molecular sieve column, a thermal conductivity detector, and using
plex 1b [13]. The data of elemental analyses agree well with the
proposed composition of the three complexes.
3.2. Description of the crystal structures
Table 1
Crystal data and structure refinement details for complexes 1a, 2a and 1b.
The crystal structures of complexes 1a, 2a and 1b are given in
Fig. 3 with selected bond lengths and angles listed in Table S1,
Complex
1aꢂ2CH
3
CN
2aꢂ2CH
3
CN
1bꢂ2CH
3
CN
which are established by means of single crystal X-ray diffraction.
Molecule formula
Formula weight
T (K)
Crystal system
Space group
a (Å)
C
40
H
33
B
2
CoF
N
8 9
C
49
H
45
B
2
CoF
8 9
N O
6
40 33 2 8 9
C H B F N Zn
878.74
100.00(10)
triclinic
ꢁ
For all three complexes, each of them consists of two BF
4
anions
872.30
99.99(14)
triclinic
P1ꢀ
12.9725(5)
13.1330(6)
13.8450(5)
79.598(3)
73.448(3)
61.917(4)
1991.79(14)
2
890
1.454
0.0737/0.1943
0.0863/0.2048
1.052
1088.49
100.01(11)
monoclinic
and a dicationic metal center that is six-coordinated by the imino-
pyridine nitrogen atoms. For complex 1a, the transoid N–Co–N
angels are in the range of 170.51–173.75° and are smaller than
the ideal 180°. The cisoid N–Co–N angels ranging from 76.34° to
P2
1
/c
P1ꢀ
15.8616(5)
13.4399(3)
23.7714(7)
90
103.168(3)
90
4934.3(2)
4
2236
12.2851(6)
13.7665(6)
13.8646(6)
83.662(4)
69.677(4)
64.512(5)
1982.74(15)
2
896
1.472
0.0480/0.1003
0.06670.1150
1.030
b (Å)
c (Å)
1
00.45° also deviate from the ideal 90° causing a slightly distorted
a
(°)
b (°)
(°)
octahedral geometry around the Co center, which is closer in
geometry to the tren-based tripodal iminopyridine cobalt complex
c
[
13,15]. An equatorial plane is defined by N(2), N(3), N(5) and N(6)
3
V (Å )
atoms, while N(4) and N(7) occupy the axial positions. The Co atom
is well located at the equatorial sphere. The Co–N bond distances
along the axis are 2.145(3) and 2.156(3) Å, which are slightly
longer than those in the mean plane (maximal value of
Z
F(000)
calc (g cm 3
ꢁ
)
r
1.465
D
R
R
1
/wR
/wR
2
(I > 2
(all data)
(I))
0.0593/0.1555
0.0697/0.1638
1.023
1
2
2
.129(3) Å). The average CoꢁNimine bond distance of 2.129(6) Å is
almost equal to the average CoꢁNpyridine bond distance of
.136(1) Å. Notably, the bridgehead nitrogen atom remains
Goodness-of-fit
GOF)
(
2

6
42
X. Song et al. / Polyhedron 81 (2014) 639–645
Fig. 3. Crystal structures of 1a, 2a and 1b (the hydrogen atoms, solvent molecules
and BF-
anions are omitted for clarity).
4
Fig. 2. (a) FT-IR spectra of prepared 1a, 2a and 1b; (b) UV–Vis absorption spectra of
ꢁ
5
ꢁ5
ꢁ5
1
a (1 ꢃ 10 M), 2a (1 ꢃ 10 M) and 1b (1 ꢃ 10 M) in acetonitrile; (c) variable
temperature effective magnetic moment measurements of 1a and 2a.
3.3. Electrochemistry properties
uncoordinated with the Co(1)ꢂ ꢂ ꢂN(1) distance of 3.009(9) Å. No sig-
Cyclic voltammetry studies were performed in acetonitrile with
Bu NPF as supporting electrolyte under nitrogen atmosphere. The
4 6
nificant hydrogen bond or
p–p
stacking was observed in the crystal
structure. The single-crystal X-ray diffraction study performed on
the Zn complex 1b reveals that the six-coordinated center is iso-
structural to 1a with an octahedral geometry. In contrast, the
recently reported tren-based tripodal iminopyridine zinc complex
tends toward the trigonal antiprismatic geometry [13].
electrochemical redox processes of complexes 1a, 2a and 1b are
+
shown in Fig. 4a. The reduction potentials are versus Fc /Fc and
are summarized in Table 2. Each of the complexes exhibits multi-
ple redox processes, and therefore this is consistent with the rich
electrochemical behavior that is observed in other divalent first
row transition metal complexes based on iminopyridine ligands
[13,15,26]. The complex 1a displays three reversible reduction
events at ꢁ1074, ꢁ1711 and ꢁ1898 mV. For the redox-inactive
zinc complex 1b, three reductions events are also observed at
ꢁ1310, ꢁ1606, and ꢁ1954 mV, although the last reduction peak
is not obvious. As noted, however, no significant difference in
reduction events was seen between the different metal complexes.
Therefore, we could assign all reductions for 1a and 1b as ligand-
based events like the discussion in the literature [13]. According
to the redox properties of the cobalt bis(iminopyridine) complex,
The geometry around the Co center in the complex 2a agrees
closely with that in complex 1a. However, owing to the introduc-
2 2 3
tion of electron-withdrawing ester groups (–CO CH CH ) into the
pyridine moiety, the CoꢁNpyridine bonds are longer by about
0
.06 Å compared to CoꢁNimine bonds. The carbonyl oxygen atoms
O(5) and O(3) in the pendent ester groups point away from the
metal center, but the atom O(1) adopts the opposite direction.
Moreover, the ester groups also render the distance (2.795(1) Å)
between the bridgehead nitrogen atom and the Co center in com-
plex 2a shorter than that in complex 1a (3.009(9) Å).

X. Song et al. / Polyhedron 81 (2014) 639–645
643
= 100 mV s 1); electrochemical response of
ꢁ
Fig. 4. (a) Cyclic voltammograms of 0.5 mM solutions of 1a, 2a and 1b (top to bottom) measured in acetonitrile (0.1 M NBu
4
PF
6
,
m
1
ꢁ
0
4 6
.5 mM 1a (b), 2a (c) and 1b (d) to addition of acetic acid (0, 2, 4, and 6 equiv) in acetonitrile (0.1 M NBu PF , m = 100 mV s ).
Å– Å +
Table 2
bridgehead nitrogen atom to give the [CoL1 H ] species, followed
Å
Redox potentials observed in CH
3
CN (0.1 M NBu
4
PF
6
) for complexes 1a, 2a and 1b.
by elimination of a hydrogen atom (H ) [30]. Finally, two hydrogen
E1/2 (red1)
E1/2 (red2)
E1/2 (red3)
atoms recombine and generate H
while the cobalt radical species
Complex
2
Åꢁ +
[
CoL1 ] is released. Fig. 4c shows a slight current enhancement
1
2
a
a
ꢁ1.074
ꢁ0.878
ꢁ1.310
ꢁ1.711
ꢁ1.314
ꢁ1.898
ꢁ1.538
observed near the ligand-centered reduction (ꢁ1538 mV) with
increasing acid concentration from 2 to 6 mM. The reason for this
phenomenon may be attributed to the reduced protonation of
a
a
1
b
ꢁ1.606
ꢁ1.954
a
Irreversible reduction wave.
2
(Åꢁ) 0
[
CoL2
] , which is extremely stabilized by the electron-with-
drawing ester groups.
the electrochemical reductions of 1a possibly yield ligand-centered
radical species [6]. As the introduction of the electron-withdrawing
ester groups in ligand results in a less electron-rich cobalt complex
2
þ
½
½
½
CoL1ꢄ þ eꢁ ! ½CoL1ꢂꢁꢄþ
ð1Þ
ð2Þ
2
a, the three reduction peaks display significantly anodic shifts rel-
0
CoL1ꢂꢁꢄþ þ eꢁ ! ½CoL12ðꢂꢁÞ
ꢄ
ative to 1a. Surprisingly, the potentials of the last two reductions
are shifted more positively (397 and 360 mV, respectively) than
the first reduction (196 mV).
To evaluate possible electrochemical activities for the reduction
of protons, cyclic voltammograms of compounds 1a, 2a, and 1b
were recorded in the presence of various amounts of acetic acid.
Among these three complexes, addition of acetic acid triggers a rel-
atively significant current enhancement only for 1a near
0
2
ðꢂꢁÞ
ꢂꢁ
þ
ꢂꢁ
ꢂ
þ
CoL1
ꢄ þ H ! ½CoL1 H ꢄ
ð3Þ
ð4Þ
½CoL1 H ꢄ ! 2½CoL1ꢂꢁꢄþ þ H
ꢂ
þ
2
2
The redox-inactive Zn(II) complex 1b does not show any current
ꢁ
1711 mV, which match with the second ligand-centered reduc-
response to acetic acid (Fig. 4d). These results indicate that the cat-
alytic activity for 1a not only arises from the ligand-based redox
activity but also has a close relation with the metal center that
has an obvious effect on the stability of reduced species. The cobal-
t(II) center with the unfilled shell of d-electrons may delocalize the
tion. The height of the reduction peak shows further increase with
the sequential increments of acid concentration (Fig. 4b), which is
a clear indication of electrocatalytic behavior of proton reduction
2 p
to H [27]. As shown in the Fig. 4b, the icat/i shows a strong depen-
dence on acid concentration, but this ratio becomes independent of
the acid concentration above approximately 4 mM. In view of the
2
(Åꢁ) 0
negative charge of the diradical species [CoL1
] to stabilize the
diradical species, which is a crucial intermediate for protonation in
catalytic H2 production process. Unlike the cobalt(II) center, the
zinc(II) center has a full shell of d-electrons and leads to the dirad-
highest value of icat/i
p
(ꢀ3.3), we estimate that the complex 1a gave
ꢁ1
the highest TOF of 2.3 s with overpotential of 250 mV using ace-
tic acid [28,29]. A plausible mechanism of the electrocatalytic pro-
ton reduction for complex 1a is proposed in eqs. (1–4). The
2
(Åꢁ) 0
ical species ([ZnL1
] ) of complex 1b unstable, which is also
suggested by the second irreversible reduction process for the
complex 1b (Fig. 4a). Moreover, the first reversible redox process
for complex 1b changed to the irreversible event in the presence
of acetic acid (Fig. 4d), indicating that the diradical species
2
+
[
CoL1] is first reduced at ꢁ1074 mV to give a cobalt radical spe-
Åꢁ + Åꢁ +
cies [CoL1 ] . Further reduction of [CoL1 ] affords a cobalt dirad-
2
(Åꢁ) 0
ical species [CoL1
] , then which may be protonated at the

6
44
X. Song et al. / Polyhedron 81 (2014) 639–645
ZnL12
(Åꢁ) 0
]
could suffer decomposition in acidic conditions.
[
Accordingly, the catalytic hydrogen evolution could not proceed
after two-electron reduction for the complex 1b.
3.4. Photochemical activity
The photocatalytic H
using [Ir(ppy) (bpy)]PF
2
production activity of 1a was tested by
as photosensitizer and triethylamine
2
6
(
TEA) as electron donor. The photocatalytic reactions were carried
out in CH CN–H O mixed solvent upon irradiation with a 300 W Xe
2
lamp (k > 400 nm). A series of factors such as pH value, CH CN–H O
3
2
3
ratios and the concentration of each component were studied to
obtain the optimal photocatalysis condition (Figs. S1–S5). As a
result, an optimized H
.1 mM [Ir(ppy) (bpy)](PF
O (1/1, v/v) at pH 10 is able to release a total amount of 17
(TON = 34) after 2 h irradiation (Fig. 5). Control experiments
(bpy)](PF ), or 1a showed no or only residual
production, indicating that all three components are
essential for the photochemical H evolution system.
In addition to 1a, the catalytic performance of 2a was also
investigated. The complex 2a displays lower H evolution activity
than complex 1a, with a TON of 7. For comparison, 1b, Co(H O)
BF were also tested for the photocatalytic H evolution in com-
bination with [Ir(ppy) (bpy)](PF ) under the same conditions
Fig. 5). The results show that only negligible amounts of H were
2
evolution system containing 0.1 mM 1a,
), and 10 vol% TEA in 5 mL CH CN–
mol
0
H
H
2
6
3
2 6
Fig. 6. Hydrogen production from the systems containing [Ir(ppy) (bpy)](PF )
ꢁ4
ꢁ4
2
l
(
1 ꢃ 10 M) and a cobalt complex (1 ꢃ 10 M) in a 10% TEA acetonitrile aqueous
2
solution (1/1, v/v) at pH 10, irradiation time 2 h.
without TEA, [Ir(ppy)
amounts of H
2
6
2
and the electron donor TEA, displayed lower efficiencies as com-
pared with the [FeFe]-hydrogenase mimic-based and the cobalt
complex-based photocatalytic systems in terms of turnover num-
bers of hydrogen evolution [27,32].
2
2
2
6
(
4
)
2
2
4. Conclusions
2
6
(
2
detected in both cases, suggesting that the active species in the
current system is the complex 1a rather than only the metal cen-
ter. The mechanism studies have shown the reductive quenching
Two Co(II) and one Zn(II) complexes of tripodal iminopyridine
type ligands were synthesized and characterized by spectroscopic
methods as well as X-ray crystallography. The cobalt complex 1a
was active for electrocatalytic proton reduction from weak acid
in acetonitrile. The electrocatalytic mechanism was also discussed.
Additionally, the complex 1a was an efficient catalyst for water
of the excited state of [Ir(ppy)
2
(bpy)](PF
(bpy)] (E = ꢁ1.81 V ersus Fc /Fc for [Ir(ppy)
2
bpy)] /[Ir(ppy) (bpy)]) [19,31], which can reduce the 1a to the
6
) could give a reduced
0
+
species [Ir(ppy)
(
2
v
2
+
2
(Åꢁ) 0
intermediate [CoL1
] . The intermediate could be protonated
2 6
reduction when using [Ir(ppy) (bpy)]PF as photosensitizer and
Å– Å +
to form [CoL1 H ] species, which finally generate H
2
.
TEA as electron donor. In contrast, the introduction of electron-
withdrawing ester group in the ligand led to its corresponding
cobalt complex 2a less active than complex 1a under the same
conditions.
The performance of complex 1a as a catalyst for photocatalytic
hydrogen evolution was also evaluated in comparison to other
hexacoordinated cobalt catalysts, such as [Co(bpy)
Co(dmg(BF)2/3 ](BF ), in association with [Ir(ppy) (bpy)](PF
under the same experimental conditions. As shown in Fig. 6, the
TONs of hydrogen evolution are 26 and 50 for [Co(bpy) ]Cl and
Co(dmg(BF)2/3 ](BF ), respectively. These results indicate that
3 2
]Cl and
[
)
3
4
2
6
)
Acknowledgements
3
2
[
)
3
4
The authors thank the National Natural Science Foundation
China (grant numbers 21101153, 21231003 and 21203195) for
financial support.
the photocatalytic activity of 1a could equal to that of other coord-
inatively saturated cobalt complexes. However, it was found that
these photocatalytic H
2
production systems, comprising the photo-
sensitizer [Ir(ppy) (bpy)](PF
2
6
), the hexacoordinated cobalt catalyst
Appendix A. Supplementary data
CCDC 988650, 988651 and 988652 contains the supplementary
crystallographic data for 1a, 2a and 1b. These data can be obtained
free of charge via http://www.ccdc.cam.ac.uk/conts/retriev-
ing.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.2014.07.031.
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