Synthesis and studies of a molecular molybdenum-Schiff base electrocatalyst for generating hydrogen from organic acid or water

Article abstract of DOI:10.1016/j.inoche.2014.11.009

The reaction of N,N-dimethylethylenediamino-N,N-bis(2,4-dimethylphenol) (H₂L) (H₂L) and MoCl₅ gives a Mo(VI) complex [MoL(O)₂] 1, which has been determined by X-ray crystallography. Electrochemical studies show that complex 1 can catalyze hydrogen evolution from acetic acid, trifluoroacetic acid and water with a turnover frequency (TOF) of 25.72 (acetic acid), 101.15 (trifluoroacetic acid) and 676 (buffer, pH 7.0) moles of hydrogen per mole of catalyst per hour, respectively.

Full text of DOI:10.1016/j.inoche.2014.11.009

Inorganic Chemistry Communications 51 (2015) 66–70
Contents lists available at ScienceDirect
Inorganic Chemistry Communications
journal homepage: www.elsevier.com/locate/inoche
Synthesis and studies of a molecular molybdenum–Schiff base
electrocatalyst for generating hydrogen from organic acid or water
⁎
Ting Fang, Hai-Xia Lu, Jia-Xing Zhao, Shu-Zhong Zhan
College of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou 510640, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 2 August 2014
Received in revised form 25 October 2014
Accepted 10 November 2014
Available online 12 November 2014
The reaction of N,N-dimethylethylenediamino-N,N-bis(2,4-dimethylphenol) (H2L) (H2L) and MoCl5 gives a
Mo(VI) complex [MoL(O)2] 1, which has been determined by X-ray crystallography. Electrochemical studies
show that complex 1 can catalyze hydrogen evolution from acetic acid, trifluoroacetic acid and water with a
turnover frequency (TOF) of 25.72 (acetic acid), 101.15 (trifluoroacetic acid) and 676 (buffer, pH 7.0) moles of
hydrogen per mole of catalyst per hour, respectively.
© 2014 Elsevier B.V. All rights reserved.
Keywords:
Molybdenum(VI) complex
Molecular structure
Molecular electrocatalyst
Hydrogen evolution
Hydrogen, when derived from carbon-neutral processes, is an at-
tractive clean fuel candidate for renewable energy storage and trans-
port [1–3]. In nature, hydrogenase enzymes [4–6] can efficiently
catalyze both the production and the oxidation of hydrogen using
earth-abundant metals (such as nickel and iron). However, enzymes
are difficult to obtain in sufficient amounts to adapt for commercial
applications and their stability is often limited outside of their native
environment.
These considerations have led to the development of molecular
catalysts employing more abundant metals, and several complexes
that contain nickel, iron, copper and cobalt which have been developed
as electrocatalysts for the production of hydrogen [7–21].
A recent report from Chang and co-workers described a highly active
molecular molybdenum electrocatalyst, [(Py5Me2)MoO]2+ (Py5Me2 =
2,6-bis(1,1-bis(2-pyridyl)ethyl)pyridine, a neutral pentadentate li-
gand) that reduces water to H2 at neutral pH in aqueous buffer [22]. It
has been shown that the donor type and electronic properties of the
ligands play vital roles in determining the structure and reactivity of the
corresponding metal complexes. Identification of the factors that control
the oxidation/reduction site in these complexes, determining of their
redox potentials and characterization of their electronic structures are
critical for the design of more effective molecular electrocatalysts
for H2 production. With this in mind, we chose tetradentate ligands,
such as H2L, a potential deprotonated ligand to react with MoCl5 to
construct the corresponding Mo complex, and explore its electrocat-
alytic properties. In this paper, we present the synthesis, structure
and properties of a new molybdenum(VI) complex [MoL(O2)] 1, as
well as its catalytic function for hydrogen evolution from acetic
acid or water thereof.
The reaction of ligand, H2L ([23], Figs. S1–S2) and MoCl5 affords
yellow crystals of complex 1 (Scheme 1, [24]), which is air stable in
the solid state or liquid state, solvable in DMF, CH2Cl2, and CH3CN.
The IR spectrum of complex 1 displays two strong νMo_O bands at
960 and 913 cm−1 (Fig. S3), characteristic for symmetric and asym-
metric vibrational modes, respectively, of the cis-[MoO2]2+ fragment
[25,26].
Crystallographic data for complex 1 are given in Table S1 and select-
ed bond lengths are listed in Table S2. As shown in Fig. 1, X-ray structure
of complex 1 reveals a six-coordinate Mo atom in a distorted octahedral
surrounding, with a fac coordination of the ligand. The molybdenum
oxo groups show the expected mutual cis configuration. The Mo_O
bond lengths (Mo–O3, 1.7010(15) Å; Mo–O4, 1.6999(16) Å) are in the
expected range of cis-dioxo MoVI complexes [27,28].
As shown in Fig. 2, cyclic voltammogram of complex 1 exhibits two
reversible couples at −1.17 V and −1.32 V versus Ag/AgNO3, which can
be assigned to MoVI/V and MoV/IV, respectively. The current responses of
the redox events at −1.17 V and −1.32 V show linear dependence on
the square root of the scan rate (Fig. S4), which is an indicative of a
diffusion-controlled process, with the electrochemically active species
freely diffusing in the solution.
To determine possible electrocatalytic activity of this complex, cyclic
voltammograms of complex 1 were recorded in the presence of acetic
acid. Fig. 3-a shows a systematic increase in icat observed near −1.32 V
with increasing acetic acid concentration from 0.0 to 45 mM. The second
redox wave depicted in Fig. 3-a is dependent of acid concentration, indi-
cating that this one electron-transfer step is devoted to proton reduction.
⁎
Corresponding author.
E-mail address: shzhzhan@scut.edu.cn (S.-Z. Zhan).
http://dx.doi.org/10.1016/j.inoche.2014.11.009
1387-7003/© 2014 Elsevier B.V. All rights reserved.

T. Fang et al. / Inorganic Chemistry Communications 51 (2015) 66–70
67
2.0
1.5
*
1.0
0.5
0.0
-0.5
-1.0
-1.5
-2.0
-2.5
-2.0
-1.5
-1.0
-0.5
0.0
0.5
1.0
Potential (V vs Ag/AgNO )
3
Scheme 1. Schematic representation of the synthesis of complex [LMo(O)2].
Fig. 2. Cyclic voltammogram of complex 1 in 0.1 M of [n-Bu4N]ClO4 DMF solution at a
glassy carbon electrode and a scan rate of 100 mV/s. Ferrocene internal standard (*).
The onset of this catalytic current is clearly influenced by the acid
concentration, the applied potential declines with increasing acid
concentration. Cyclic voltammogram of background in the absence of
1 exhibits no catalytic current at the potential of the couple of MoV/IV
(Fig. S5), suggesting that proton reduction to H2 occurs with 1 and a
Mo(IV) intermediate is responsible for the reductive proton to generate
H2 [22], with catalytic onset shift to more positive potential (from
−0.93 to −0.67 V). On the basis of literature precedent [22,29] and
above analyses, we postulate the catalytic cycle depicted in Scheme 2
for the generation of hydrogen from acid mediated by 1. Two-electron
reduction of [MoVIL(O)2] gives [MoIVL(O)2]2−. Addition of proton pro-
duces the reactive intermediate [MoIVL(OH)2] and then affords H2 and
gives rise to a cycle in which [MoIVL(OH)2] precedes the formation of
[MoVIL(O)2] 1.
electrolysis of a DMF solution of complex 1 (16.3 μM) with acetic acid
(2.70 mM) or trifluoroacetic acid (1.20 mM) at variable applied poten-
tial. Fig. 4 shows the total charge of bulk electrolysis of complex 1
in the presence of acid, the charge significantly increased when the applied
potential was set to more negative. According to Eq. (1) [22], we calculated
TOF for the catalyst reaching a maximum of 25.72 (acetic acid) and 101.15
(trifluoroacetic acid) moles of hydrogen per mole of catalyst per hour (Eq.
(2) in Supplementary material 4 and Fig. S6). The value (25.72) is lower
than that of [LMoVI(O)2] (L = 2-pyridylamino-N,N-bis(2-methylene-4,6-
difluorophenol) ion) (50.6 mol of hydrogen per mole of catalyst per
hour), a similar type of complex [29]. This is consistent with an evident in-
crease in the catalytic activity when electron-withdrawing groups are pres-
ent at the phenol para-position of the ligand [30].
TOF ¼ ΔC= ðF ꢀ n1 ꢀ n2 ꢀ tÞ
ð1Þ
We also used trifluoroacetic acid as proton source to test the catalyt-
ic function of complex 1. Fig. 3-b also exhibits a systematic increase in
icat observed near −1.32 V with increasing trifluoroacetic acid concen-
tration from 0.67 to 20.0 mM. This rise in current also can be attributed
to the catalytic generation of H2 from trifluoroacetic acetic acid with cat-
alytic onset shift to more positive potentials (from −1.10 to −0.35 V).
Further evidence for the electro-catalytic activity was obtained by bulk
We also tried to explore the catalytic hydrogen evolution in aqueous
media, a much more attractive medium for the sustainable generation
of hydrogen. CVs were conducted in 0.25 M phosphate buffers at differ-
ent pH values. As shown in Fig. S7-(a), in the absence of complex 1, a
catalytic current was not apparent until a potential of −1.50 V versus
Fig. 1. Molecular structure of 1.

68
T. Fang et al. / Inorganic Chemistry Communications 51 (2015) 66–70
Acknowledgments
This work was supported by the National Science Foundation of
China (No. 20971045, 21271073), and the Student Research Program
(SRP) of South China University of Technology (No. B15-B7050170).
Appendix A. Supplementary material
CCDC 1005596 contains the Supplementary crystallographic data for
this paper. This 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 associ-
ated with this article can be found, in the online version, at http://dx.doi.
org/10.1016/j.inoche.2014.11.009.
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solid formed. The solvents were decanted, and the remaining solid residue was
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We have successfully prepared a new molybdenum(VI) complex
1, that can catalyze hydrogen evolution not only from acetic acid or
trifluoroacetic acid but also from water. Ongoing efforts are focused on
modifying Schiff base ligands and generating the related water-soluble
complexes to further facilitate new functional studies.

T. Fang et al. / Inorganic Chemistry Communications 51 (2015) 66–70
69
Scheme 2. The possible catalytic mechanism by [MoL(O)2] 1.
Anal. calcd for C22H32N2O2: C, 74.16; H, 8.99; N, 7.86%. Found C, 74.33; H, 8.88; N, 7.
81%. 1H NMR (400 MHz, CDCl3) δ 6.84 (s, 2H), 6.66 (s, 2H), 3.57 (s, 4H), 2.55 (s, 4H),
2.30 (s,6H), 2.19 (s, 12H) (Figure S1). 13C NMR (100 MHz, CDCl3) δ 152.723, 131.
285, 128.424, 127.481, 125.507, 121.674, 77.163, 56.509, 56.171, 49.191, 45.093,
20.496, 16.273 (Figure S2).
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obtained from the filtrate which was allowed to stand at room temperature for
several days, collected by filtration, and dried in vacuo (0.12 g, 48%). The elemental
analysis results (found C, 54.67; H, 6.01; N, 5.78. C32H42N2O4Mo requires C, 54.84; H,
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Fig. 5. Charge buildup of complex 1 (0.82 μM) versus overpotentials (mV vs SHE) at pH 7.0.
All data have been deducted blank.

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