Synthesis and electro-catalytic properties of a dinuclear palladium(I) 1,3-bis[(2-chloro)benzene]triazenide complex

Article abstract of DOI:10.1016/j.ica.2013.10.026

The reaction of 1,3-bis[(2-chloro)benzene]triazene (HL) and [Pd(CH ₃CN)₄]Cl₂ gave a new dinuclear triazenido complex [Pd₂L₂] 1 (L = 1,3-bis[(2-chloro)benzene]triazene ion). Electrochemcial studies sho

Full text of DOI:10.1016/j.ica.2013.10.026

Inorganica Chimica Acta 410 (2014) 191–194
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Inorganica Chimica Acta
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Synthesis and electro-catalytic properties of a dinuclear palladium(I)
1,3-bis[(2-chloro)benzene]triazenide complex
a,
Jing Chu a, Xiao-hua Xie b, Shao-rong Yang a, , Shu-zhong Zhan
⇑
⇑
a College of Chemsitry & Chemical Engineering, South China University of Technology, Guangzhou 510640, China
b College of Chinese Language and Culture of Jinan University, 510610 Guangzhou, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 19 July 2013
Received in revised form 9 September 2013
Accepted 27 October 2013
Available online 9 November 2013
The reaction of 1,3-bis[(2-chloro)benzene]triazene (HL) and [Pd(CH3CN)4]Cl2 gave a new dinuclear tria-
zenido complex [Pd2L2] 1 (L = 1,3-bis[(2-chloro)benzene]triazene ion). Electrochemcial studies showed
that complex 1 is capable of generating hydrogen from acetic acid in CH2Cl2.
Ó 2013 Elsevier B.V. All rights reserved.
Keywords:
Triazenido-palladium complex
Crystal structure
Molecular electro-catalyst
Hydrogen evolution
1. Introduction
2. Experimental
The molecular hydrogen derived from non-carbon sources has
emerged as a potential fuel for sustainable energy cycles that min-
imize carbon dioxide emissions [1,2]. Hydrogenase enzymes [3,4]
efficiently catalyze both the production and the oxidation of
hydrogen using earth-abundant metals (nickel and iron). The elec-
trocatalytic mechanism of enzymes in hydrogen-producing sys-
tems has been investigated [5], but enzymes are difficult to
obtain in sufficient amounts to adapt for commerical applications
and their stability is often limited outside of their native environ-
ment [6]. These considerations have led to the development of
molecular catalysts employing more abundant metals, and several
complexes that contain nickel [7–10], cobalt [11–16], iron [17–20],
or molybdenum [21,22] have been developed as electrocatalysts
for the production of hydrogen. In contrast, there are few molecu-
lar complexes of the second-row transition metals that perform
hydrogen evolution have been studied for electrocatalytic activity
[21,22]. To the best of our knowledge, there is as yet no report
on the use of palladium complexes to electrocatalyze hydrogen
evolution. Here we present the synthesis, structure and properties
of a new dinuclear triazenido-palladium(I) complex [Pd2L2] 1, as
well as its electro-catalytic properties for the production of hydro-
gen from acetic acid in CH2Cl2 thereof.
All manipulations were carried out under air. Solvents and
chemicals were of analytical grade purity from commercial sources
and were used without further purification.
2.1. Physical measurements
1H NMR spectra were recorded on a Bruker AM 500 instrument
at room temperature and were measured in Chloroform-d using
tetramethylsilane (TMS) as an internal reference. IR spectra were
carried out on a Bruker FT-IR 1730. UV–Vis absorportions were
measured on a Hitachi U-3010. All electrochemical measurements
were undertaken under nitrogen in CH2Cl2 solution with 0.1 M [n-
Bu4N][ClO4] as a supporting electrolyte. Cyclic voltammograms
(CV) were recorded on an Ingsens1030 Electrochemical Worksta-
tion (Ingsens Instruments (Guangzhou) Co., Ltd., China) using a
standard three-electrode cell with an auxiliary platinum wire elec-
trode (CE), a glassy carbon electrode (GCE, 3 mm in diameter) and
an Ag/AgCl (1 M AgClO4 non-aqueous) reference electrode in
CH2Cl2. Acetic acid was added by syringe.
2.2. Synthesis
2.2.1. Synthesis of HL
2-Chloroaniline (10 mmol) in water (5 mL) was mixed
with1 mol LÀ1 HCl (30 mL), and a sodium nitrite aqueous solution
(15%) was added dropwise with stirring at 0 °C. Later, an equal
amount of 2-chloroaniline in ethanol was added with continuous
⇑
Corresponding authors. Fax: +86 20 87112053.
E-mail address: shzhzhan@scut.edu.cn (S.-z. Zhan).
0020-1693/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.ica.2013.10.026

192
J. Chu et al. / Inorganica Chimica Acta 410 (2014) 191–194
Table 2
stirring. The resulting solution was neutralized with sodium ace-
tate (15%). The yellow product precipitated and then was filtered.
Finally, the product was purified by crystallization from ethyl ace-
tate to obtain a yellow crystalline solid. The yield was 75%. Anal.
Calc. for C12H9Cl2N3: C, 54.16; H, 3.41; N, 15.79. Found: C, 54.65;
H, 3.40; N, 15.37%. 1H NMR (CDCl3) d 10.14 (s, 1H), 7.66 (d,
J = 2.0 Hz, 2H), 7.39 (d, J = 2.0 Hz, 2H), 7.26 (t, J = 2.0 Hz, 2H), 7.10
Selected bond distances (Å) and angles for complex 1.
Pd(1)–Pd(1–2)
Pd(1)–N(3–2)
N(1)–N(2)
N(1)–N(2)–N(3)
N(1)–Pd(1)–Pd(1–2)
N(2)–N(1)–Pd(1)
2.4228(3)
Pd(1)–N(1)
Pd(1)–Cl(2–2)
N(2)–N(3)
N(1)–Pd(1)–N(3–2)
N(3–2)–Pd1–Pd(1–2) 87.671(52)
2.0311(19)
2.4939(6)
1.3044(28)
173.497(80)
2.0125(20)
1.2969(24)
114.937(4)
85.828(51)
126.358(48) N(2)–N(3)–Pd(1–2)
125.187(51)
104.272(52)
(t, J = 2.0 Hz, 2H). UV–Vis [CH2Cl2, kmax/nm (
e
/L molÀ1 cmÀ1)]:
Cl(2–2)–Pd(1)–N(3–2) 82.231(52)
Cl(2–2)–Pd(1)–N(1)
241 (1.65 Â 104), 355 (1.91 Â 104).
3. Results and discussion
2.2.2. Synthesis of complex 1
3.1. Synthesis and characterization
To a solution, containing 1,3-bis[(2-chloro)benzene]triazene
(HL) (0.28 g, 1 mmol) and triethylamine (0.10 g, 1 mmol) in aceto-
nitrile/tetrahydrofuran (20 mL, 1:1), [Pd(CH3CN)4]Cl2 (0.34 g,
1.0 mmol) was added and the mixture was stirred for 15 min.
The solution was allowed to slowly evaporate, affording deep red
crystals, which were collected and dried in vacuo (46.8%). Anal.
Calc. for C24H16Pd2Cl4N6: C, 39.27; H, 2.20; N, 11.45. Found: C,
38.78; H, 2.17; N, 11.21%. 1H NMR (CDCl3) d 7.31 (d, J = 2.0 Hz,
4H), 7.27 (d, J = 2.0 Hz, 4H), 7.14 (t, J = 2.0 Hz, 4H), 7.00 (t,
In the presence of Et3N, the reaction of 1,3-bis[(2-chloro)ben-
zene]triazene (HL) and [Pd(CH3CN)4]Cl2 affords red crystals of
complex 1 in 46.8% yield. During the course of the synthesis of 1,
the Pd(II) was reduced by one electron to Pd(I) ion. At this time,
we do not have experimental data concerning this process. It is
well-known, however, that the reduction of Pd(II) starting materi-
als to Pd(I) is common [25]. Palladium(I), which has the d9 config-
uration, is expected to exhibit paramagnetic behavior. Complex 1,
however, is diamagnetic and amenable to NMR analysis (Figs. S1
and S2). It is assumed that the d9 Pd(I) centers are spin-coupled
through a Pd–Pd bond, as observed in other dinuclear Pd(I)
complexes [25].
J = 2.0 Hz, 4H). UV–Vis [CH2Cl2, kmax/nm (e
/L molÀ1 cmÀ1)]: 229
(8.68 Â 104), 300 (8.83 Â 104), 378 (6.67 Â 104), 485 (0.71 Â 104).
2.3. X-ray crystallography
3.2. Crystal structures
Data were collected with a Bruker SMART CCD area detector
using graphite monochromated Mo Ka radiation (0.71073 Å) at
room temperature. All empirical absorption corrections were ap-
plied by using the SADABS program [23]. The structures were solved
using direct methods 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 full-matrix least-squares refinement. All calcula-
tions were performed using the SHELXTL computer program [24]. Ta-
ble 1 lists details of the crystal parameters, data collection and
refinement for complex 1. Table 2 lists the selected bond distances
and angles for complex 1.
Single crystal X-ray diffraction analysis reveals the solid state
structure of the complex
1 to be a di-palladium complex
[Pd2(L)2]. The molecular structure of the complex [Pd2(L)2] is de-
picted in Fig. 1. The palladium complex [Pd2(L)2] consists of two
metal centers bridged by two triazenido ligands in an
l, , -
g1 g1
fashion, with approximately linear two coordinate geometries
[N(1)–Pd1–(N3–2): 173.497o] with the deviation caused by the
metal atom moving away from each other.
Complex 1 is formed by the two N3-ligands and the {M2} unit
(M = Pd). The Pd–N bond distances in [Pd2(L)2] are within the ex-
pected ranges for terminal M(I)–N bonds [Pd–N: 2.0311(19) and
2.0125(20) Å] [25]. Similarly the N–N bond lengths within the tria-
zenido ligands [N(1)–N(2): 1.2969(24) Å, N(2)–N(3): 1.3044(28) Å
are also comparable to those observed in related systems [25].
The Pd–Pd distances of 2.4228(3) Å in complex 1 is consistent with
Table 1
Crystal data and structure refinements for complex 1.
Parameter
Complex 1
C24H16Pd2Cl4N6
744.26
0.71073
Empirical formula
Formula weight
k (Å)
Crystal system
Space group
a (Å)
monoclinic
P2(1)/c
8.4645
b (Å)
10.5335
c (Å)
14.0155
a
(°)
90.000
b (°)
93.451
c
(°)
90.000
1247.4
2
2.923
739.9
2.19–27.57
8275/2860
2860/0/163
1.032
V (Å3)
Z
Dc (Mg mÀ3
F(000)
h (°)
Reflections collected/unique
Data/restraints/parameters
Goodness-of-fit (GOF) on F2
)
Final R indices [I > 2
R indices (all data)
r(I)]
R1 = 0.0241, wR2 = 0.0598
R1 = 0.0329, wR2 = 0.0379
Fig. 1. Molecular structure of complex 1.

J. Chu et al. / Inorganica Chimica Acta 410 (2014) 191–194
193
Pd–Pd bond. The distance is similar to those Pd–Pd bonds reported
for dinuclear Pd(I)–Pd(I) complexes with triazenido ligands
(2.4309(3), 2.4202(3), and 2.4158(4) Å) [25].
5
4
3
2
1
3.3. Electronic spectra
The electronic spectra of HL and complex 1 were recorded in
dichloromethane solution (Fig. S3). HL exhibits two bands at 241
and 355 nm, respectively. Complex 1 shows four bands at 229,
300, 378 and 485 nm. Compared with that of HL (355 nm), the
absorption maxima of the low-energy band of complex
1
(378 nm) are red-shifted to a longer- wavelength region. This is as-
signed to spin–orbit coupling from the heavy atom effect of the
palladium(I) ion, as well as a Pd ? L MLCT transition upon depro-
tonated LÀ.
The energy band around 485 is assignable to a
r ?
rà transi-
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
tions. It is known that Pd(I) easily cleave their metal–metal bond.
In order to confirm this assignment, a photochemical reaction
was carried out in the system containing complex 1 and CCl4. Com-
plex 1 and carbon tetrachloride were mixed in CH2Cl2 and were
irradiated by a 100-W high-pressure mercury lamp. The dynamic
changes were monitored by UV–Vis absorption spectroscopy. The
band around 485 nm decreased during the photolysis (Fig. S4),
indicating the cleavage of the metal–metal bond [26].
SR1/2(V1/2S-1/2
)
Fig. 3. Scan rate dependence of precatalytic wave for
complex 1 in CH2Cl2 with 0.1 M [n-Bu4N][ClO4], at scan rates from 30 to 450 mV/s
on a glassy carbon electrode (1 mm).
a 0.20 mM solution of
To determine possible electrocatalytic activity of this complex,
cyclic voltammograms of complex 1 were recorded in the presence
of acetic acid in CH2Cl2. To test the catalytic ability of complex 1,
we obtained CVs at different acetic acid concentrations. Fig. 4
shows a systematic in icat observed near À0.95 V with increasing
acid concentration from 0.87 to 5.22 mM. The CVs in the presence
of increasing acid indicate hydrogen evolution, with catalytic onset
shift to more positive potentials. The height of the reduction peak
in each CV shows a further increase with the sequential increments
of acid concentration. The reduction potentials of complex 1
slightly move to more cathodic values with increasing acid concen-
tration, and the catalysis is dependent on the acid concentration.
In order to further confirm the electrocatalytic acitivity of com-
plex 1, bulk electrolysis of complex 1 was carried out. The onset
potential À1.03 V vs. Fc+/0 (E1/2 = 0.63 V) of acetic acid in CH2Cl2
was obtained according to the CVs which are in the presence and
absence of acetic acid (Fig. S5). Bulk electrolysis of complex 1
(0.45 mM) with acetic acid (23 mM) at potential from À0.84 to
3.4. Electrochemical studies
Cyclic voltammogram of complex 1 was measured at a glassy
carbon electrode in CH2Cl2 in the presence of [n-Bu4N][ClO4] as
supporting electrolyte (Fig. 2). All potentials in this study were gi-
ven versus the Fc+/0 couple internal standard.
The cyclic voltammogram of complex 1 in CH2Cl2 exhibits two
reversible couples at À0.92 and 1.25 V vs. Fc+/0 (E1/2 = 0.65 V). We
assign the reversible couples to the metal-centred PdI/0 and PdI/II
process, respectively. As observed in Fig. 3, the current response
of the redox events at À0.95 V shows a linear dependence on the
square root of the scan rate, which is a indicative of a diffusion-
controlled process, with the electrochemically active species freely
diffusing in the solution. Such distinctive potential prompted pos-
sible usage of this complex as an electrocatalyst for hydrogen evo-
lution reaction.
4
3
30
C
HOAc
0mM
0.87mM
2.18mM
2
20
1
3.48mM
4.35mM
5.22mM
0
10
-1
-2
-3
0
1.5
1.0
0.5
0.0
-0.5
-1.0
-1.5
1.5
1.0
0.5
0.0
-0.5
-1.0
-1.5
Potential (V vs Fc+/0
)
Potential(V vs Fc+/0
)
Fig. 2. Cyclic voltammogram of a 0.20 mM solution of complex 1. Conditions: room
temperature, 0.1 M [n-Bu4N][ClO4] as supporting electrolyte, scan rate = 100 mV/s,
GC working electrode (1 mm diameter), Pt counter electrode, Ag/Ag+ reference
electrode, Fc internal standard.
Fig. 4. Cyclic voltammogram of a 0.12 mM solution of complex 1, at increasing
concentrations of acetic acid. Conditions: r. t, 0.1 M [n-Bu4N][ClO4] as supporting
electrolyte, scan rate = 100 mV/s, GC working electrode (1 mm diameter), Pt
counter electrode, Ag/Ag+ reference electrode, Fc internal standard.

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J. Chu et al. / Inorganica Chimica Acta 410 (2014) 191–194
Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/
0.6
0.5
0.4
0.3
0.2
0.1
0.0
Applied potential
-0.84V
data_request/cif. Supplementary data associated with this article
can be found, in the online version, at http://dx.doi.org/10.1016/
j.ica.2013.10.026.
-0.87V
-0.90V
-0.93V
-0.96V
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-0.99V
-1.02V
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À1.02 V was conducted. The charge significantly increased when
the applied potential was set to more negative (Fig. 5).
In summary, we have prepared a new dipalladium(I) complex
that is active for reduction proton to hydrogen from weak acid in
organic solvent. The electro-catalytic mechanism of complex 1 is
under investigation.
Acknowledgements
This work was supported by the Research Foundation for Re-
turned Chinese Scholars Overseas of Chinese Education Ministry
(No. B7050170), the National Science Foundation of China (No.
20971045, 21271073), and the Student Research Program (SRP)
of South China University of Technology.
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CCDC 937392 contains the supplementary crystallographic data
for this paper. These data can be obtained free of charge from The