Synthesis and electro-catalytic properties of a dinuclear triazenido-palladium complex

Article abstract of DOI:10.1016/j.inoche.2013.09.007

The reaction of 1,3-bis[(4-chloro)benzene]triazene (HL) and [Pd(CH ₃CN)₄]Cl₂ gave a dinuclear triazenido complex Pd₂L₄ 1, which has been characterized by ¹H NMR spectrum and X-ray crystallo

Full text of DOI:10.1016/j.inoche.2013.09.007

Inorganic Chemistry Communications 36 (2013) 245–248
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Inorganic Chemistry Communications
journal homepage: www.elsevier.com/locate/inoche
Synthesis and electro-catalytic properties of a dinuclear
triazenido-palladium complex
Jing Chu a, Qi-ying Lv a, Jie-ping Cao a, Xiao-hua Xie b, Shuzhong Zhan a,
⁎
a
College of Chemistry & 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:
The reaction of 1,3-bis[(4-chloro)benzene]triazene (HL) and [Pd(CH3CN)4]Cl2 gave a dinuclear triazenido
complex Pd2L4 1, which has been characterized by 1H NMR spectrum and X-ray crystallography. Electrochemical
studies showed that complex 1 is capable of generating hydrogen from acetic acid in DMF with icat/ip ~ 7.5.
© 2013 Elsevier B.V. All rights reserved.
Received 13 June 2013
Accepted 1 September 2013
Available online 8 September 2013
Keywords:
Triazenido-palladium complex
Crystal structure
Molecular electro-catalyst
Hydrogen evolution
The production of hydrogen from proton sources is a major goal of
contemporary energy science [1,2]. Hydrogenase enzymes [3,4] can
efficiently catalyze both the production and the oxidation of hydrogen
using earth-abundant metals (nickel and iron). The electrocatalytic
mechanism of enzymes in hydrogen-producing system has been inves-
tigated [5,6], but an important fundamental question for the enzymes
that remains to be answered is the precise role that the proposed
pendant base would play in hydrogen oxidation and production.
These considerations have led to the development of molecular cat-
alysts employing more abundant metals, and several complexes
that contain nickel [7–10], cobalt [11–16], iron [17–20], or molybde-
num [21,22] have been developed as electrocatalysts for the produc-
tion of hydrogen. However, to the best of our knowledge, there is as
yet no report on the use of palladium complexes to electrocatalyze
hydrogen evolution. With this mind, our group is trying to probe
the possible mechanism through the synthesis of dinuclear palladi-
um complex with triazenido ligands, which have been successfully
used in transition metal chemistry [23,24], and to explore the
possibility of electro-catalysis for hydrogen evolution. The central
nitrogen of the triazenido imparts greater basicity on the [N⋯N⋯N]−
relative to the neutral nitrogen, making the triazenido more electron
donating, amenable to binding to H+ and hydrogen production. Here
we present the synthesis and structure of a new dinuclear triazenido-
palladium complex [Pd2L4] 1 (L = 1,3-bis[(4-chloro)benzene]triazene
ion), as well as its electro-catalysis for the production of hydrogen from
acetic acid in DMF (DMF = dimethylformamide) thereof.
In the presence of Et3N, the reaction of 1,3-bis[(4-chloro)benzene]
triazene (HL) and [Pd(CH3CN)4]Cl2 affords red crystals of complex 1 in
41% yield [25,26] (Figs. S1–S2). The X-ray crystal structure of complex
1 has been obtained. Table S1 lists the details of the crystal parameters,
data collection and refinement for 1. The selected bond distances and
angles are listed in Table S2.
The formation of complex 1 also was characterized by 1H NMR and
PXRD spectra. Resonances at 7.417, 7.395, 7.188 and 7.166 ppm in the
1H NMR spectrum of complex 1 are similar to 7.525, 7.503, 7.422 and
7.400 ppm in ligand (L), indicating that complex 1 is diamagnetic (see
Figs. S1 and S2). As shown in Fig. S3 (PXRD spectrum of complex 1),
the experimental results are similar to the single crystal X-ray dif-
fraction data.
As shown in Fig. 1, the two palladium atoms are bridged by four
triazenido units, resulting in a typical tetragonal paddlewheel-type
structure, among which two metal centers connected by two triazenido
ligands in an μ, η1, η1-fashion, with approximately linear two coordinate
geometries [N(1)\Pd(1)\(N3)#1: 169.8°, N(4)\Pd(1)\(N6):170.4°]
with the deviation caused by the metal atom moving away from each
other. Similar to those of Pd2L4 found in literature [27–29], the configu-
ration of 1 has essentially D4h symmetry. The bond distances of Pd and N
of triazenido ligands fall in the range 2.041(3) to 2.071(3) Å, which are
similar to those found in other dipalladium complexes 2.023(2) to
2.09(1) Å [27]. The array of nitrogen atoms about each palladium atom
is almost planar and the nitrogen–nitrogen atom distance shows that
the two N\N bonds of each ligand group are equivalent (average
1.296(4) Å). This suggests delocalization of the π electrons over the
triazenido moiety. The average N\N\N angle is close to 119.85° in struc-
ture. The Pd\Pd bond length of 1 (2.5862(4) Å) is slightly longer than
that found in Pd2(DPhBz)4 (DPhBz = N,N′-diphenylbenzamidinate)
⁎
Corresponding author. Fax: +86 20 87112053.
E-mail address: shzhzhan@scut.edu.cn (S. Zhan).
1387-7003/$ – see front matter © 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.inoche.2013.09.007

246
J. Chu et al. / Inorganic Chemistry Communications 36 (2013) 245–248
Fig. 1. Molecular structure of complex 1. Hydrogen atoms have been omitted.
(2.576(1) Å) [27] and Pd2(DPhTA)4 (DPhTA = 1,3-diphenyltriazene)
(2.5626(7) Å) [28], but is shorter than that in α-Pd2(DAniF)4 (DAniF =
N,N′-di-p-anisylformamidinate) (2.6486(8) Å) [29] and Pd2(DTolF)4
(DTolF = N,N′-di-p-tolylformamidinate) (2.622(3) Å) [27].
maxima of the low-energy band of complex 1 (338 nm) is blue-shifted
to a shorter-wavelength region.
The cyclic voltammogram of 1 in DMF exhibits a reversible couple
at −0.77 and two irreversible reductions at −1.62 and −1.78 V vs.
Ag/AgCl (Fig. 3a), respectively. The reversible couple and the second re-
ductive wave can be assigned to the PdII/I and PdI/0 reduction processes.
The third reduction at −1.78 V is tentatively assigned to the ligand
reduction. Plots of ip vs the square root of the scan rate are linear
for these two waves (Fig. S4), indicating that the electron-transfer re-
ductions are diffusion-controlled. Such distinctive potential prompted
possible usage of this complex as an electrocatalyst for hydrogen evolu-
tion reaction. To determine possible electrocatalytic activity of this
complex, cyclic voltammograms of complex 1 were recorded in the
presence of acetic acid (pKa = 13.5 in DMF) in DMF [30]. In order to
test the catalytic ability of complex 1, we also investigated CVs at
different acetic acid concentrations. Fig. 3b shows a systematic in
icat observed near −1.62 V with increasing acid concentration from
0.1 to 8 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 concentra-
tion, and the reduction potentials of complex 1 slightly move to
more cathodic values. As shown in the Fig. 3b inset, the peak
current varies linearly with the square root of R (R = [HOAc]/[1]),
indicating that the rate-determining in the catalytic reaction is
the first-order in acid [31]. An overall catalytic rate constant
k ~ 1.21 × 103 M−1 s−1 for complex 1 is estimated. This value
can compare well with those calculated by the same procedure for
The electronic spectra of HL and complex 1 were recorded in
dichloromethane solutions as shown in Fig. 2. HL exhibits three bands at
240 (ε = 2.22 × 104 L·mol·cm−1), 298 (ε = 1.34 × 104 L·mol·cm−1
)
and 357 nm (ε = 2.97 × 104 L·mol·cm−1). Complex 1 shows two
bands at 253 (ε = 1.58 × 105 L·mol·cm−1) and 338 nm (ε = 1.36 ×
105 L·mol·cm−1). Compared with that of HL (357 nm), the absorption
Fig. 2. Electronic spectra of HL and complex 1 in CH2Cl2.

J. Chu et al. / Inorganic Chemistry Communications 36 (2013) 245–248
247
Fig. 3. (a) Cyclic voltammogram of 1 (0.314 mM) in the absence of acid in DMF (0.1 M n-Bu4NClO4) with scan rate 100 mV/s. (b) Cyclic voltammograms of 1 (0.314 mM) with varying
acetic acid in DMF (0.1 M n-Bu4NClO4). Scan rate v = 100 mV/s. Inset: Dependence of icat/ip on ([HOAc]/[1])1/2
.
cobaloxime [32], [Fe2(S2C2H4)(CO)3(dppv)(NO)]BF4 or [Fe2(S2C3H6)
(CO)3(dppv)(NO)]BF4 catalysts [31]. When the concentration ratio
of complex 1 and HOAc is increased to 25, the resulting icat/ip value of
complex 1 ~ 7.5 is higher than that of reported diiron model com-
plexes, such as [Fe(S2C3H6)-(CO)3(dppv)(NO)]+ and (μ-SC6H4-2-(CO)
S-μ)[Fe2(CO)6] (both icat/ip b 2) [31,33,34].
maximum of 2.6 mol of hydrogen per mole of catalyst per min (Fig. 4,
inset) [35].
In summary, we have prepared a new paddlewheel-type dipalladium
complex that is active for the reduction of proton to hydrogen from weak
acid in organic solvent. The electro-catalytic mechanism of complex 1 is
under investigation.
Bulk electrolysis of complex 1 (6.28 μM) with HOAc (70 mM) in
DMF at variable potential resulted in evolution of the total charge during
2 min. Catalytic reduction of acetic acid was significantly faster when
the applied potential was set to more negative (Fig. 4). Assuming
every electron is used for the reduction of proton, the TOF reaches a
Acknowledgments
This work was supported by the Research Foundation for
Returned Chinese Scholars Overseas of Chinese Education Ministry
Fig. 4. Charge buildup versus time at various applied potentials in DMF (0.1 M n-Bu4NClO4). Inset: TOF versus applied potential for a 6.28 μM solution of 1 in DMF. The background solvent
activity has been subtracted.

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J. Chu et al. / Inorganic Chemistry Communications 36 (2013) 245–248
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Appendix A. Supplementary material
CCDC 896097 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 Crystallo-
graphic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax:
+44 1223 336 033; or e-mail: deposit@ccdc.cam.ac.uk. Supplementary
data to this article can be found online at http://dx.doi.org/10.1016/j.
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from solution without catalyst during CPE) / (Faraday's constant × mol of electrons
required to generate a mol of H2 × mol of catalyst in solution × duration of electrolysis
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