A dinuclear triazenido-copper complex: A new molecular electro-catalyst for generating hydrogen from acetic acid or water

Article abstract of DOI:10.1016/j.molcata.2014.04.034

The reaction of 1,3-bis[(4-chloro)benzene]triazene (HL) and CuCl ₂·2H₂O affords a dinuclear triazenido-copper(II) complex [Cu₂L₄(NCCH₃)] (1), a new molecular electrocatalyst, which has been determined

Full text of DOI:10.1016/j.molcata.2014.04.034

Accepted Manuscript
Title: A dinuclear triazenido-copper complex: a new
molecular electro-catalyst for generating hydrogen from acetic
acid or water
Author: Jie-Ping Cao Ting Fang Zhuo-Qiang Wang Yan-Wei
Ren Shuzhong Zhan
PII:
S1381-1169(14)00172-1
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.molcata.2014.04.034
MOLCAA 9096
To appear in:
Journal of Molecular Catalysis A: Chemical
Received date:
Revised date:
Accepted date:
9-1-2014
23-4-2014
26-4-2014
Please cite this article as: J.-P. Cao, T. Fang, Z.-Q. Wang, Y.-W. Ren, S. Zhan, A
dinuclear triazenido-copper complex: a new molecular electro-catalyst for generating
hydrogen from acetic acid or water, Journal of Molecular Catalysis A: Chemical (2014),
http://dx.doi.org/10.1016/j.molcata.2014.04.034
This is a PDF file of an unedited manuscript that has been accepted for publication.
As a service to our customers we are providing this early version of the manuscript.
The manuscript will undergo copyediting, typesetting, and review of the resulting proof
before it is published in its final form. Please note that during the production process
errors may be discovered which could affect the content, and all legal disclaimers that
apply to the journal pertain.

A dinuclear triazenido-copper complex: a new molecular
electro-catalyst for generating hydrogen from acetic acid or water
Jie-Ping Cao, Ting Fang, Zhuo-Qiang Wang, Yan-Wei Ren*, Shuzhong Zhan*
College of Chemistry and Chemical Engineering, South China University of
Technology, Guangzhou 510640, China.
__________________________________________
ABSTRACT
The reaction of 1,3-bis[(4-chloro)benzene]triazene (HL) and CuCl2·2H2O affords a
dinuclear triazenido-copper(II) complex [Cu2L4(NCCH3)] (1), a new molecular
electrocatalyst, which has been determined by X-ray crystallography. Electrochemical
studies show that complex 1 can catalyze hydrogen evolution from acetic acid or
water. Turnover frequency (TOF) reaches a maximum of 43 (in DMF) and 174 (in
phosphate buffer, pH 6) moles of hydrogen per mole of catalyst per hour, respectively.
Sustained proton reduction catalysis occurs at glassy carbon (GC) electrode to give H2
over a 72 h electrolysis period and no observable decomposition of the catalyst.
Keywords: Triazenido-copper(II) complex; molecular electro-catalyst; hydrogen
evolution
__________________________________________
1. Introduction
The ability to store and recover secure, clean, sustainable energy is arguably the
most important scientific and technical challenge facing humanity in the 21st century.
Chemical fuels are ideal in this context since chemical bonds have high energy density.
And hydrogen has attracted considerable attention as an alternative chemical fuel
vector. In this regard, on a mid- to long-term basis, it is urgent to provide an appealing
approach toward generating and storing hydrogen in more sustainable manner [1-3].
___________________
* Corresponding authors. Fax: +86 20 87112053.
E-mail address: shzhzhan@scut.edu.cn (S.-z. Zhan).
1
Page 1 of 26

Hydrogenase enzymes [4,5] can efficiently catalyze both the production and the
oxidation of hydrogen using earth-abundant metals (nickel and iron). Structural
studies have established that the FeFe-hydrogenases function by combing cofactors
that couple acid-base and redox reactions mediated by the diiron dithilate core and its
cofactors (Fig. 1) [4,6,7]. The structural studies have led to the suggestion that a basic
nitrogen or oxygen atom may be incorporated into the backbone of the dithiolate
ligand [4,8-10]. The amine [11] or oxygen [12] cofactor is proposed to facilitate the
cleavage/formation of the hydrogen-hydrogen bond and the transfer of protons to and
from the distal metal centre [12]. 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 [4,5,13]. These considerations have led to
the development of molecular catalysts employing more abundant metals, and several
complexes that contain nickel, iron or cobalt [4,14-22] have been developed as
electrocatalysts for the production of hydrogen. However, an important fundamental
question for the enzymes that remains to be answered is the precise role of the
proposed pendant base in hydrogen oxidation and production. And most of the diiron
dithiolate complexes have been electrochemically investigated by using organic acids
in organic solvents. With this mind, our group has tried to probe new electro-catalysts
through the synthesis of dinuclear complexes with triazenido ligands [23, 24]. The
central nitrogen of the triazenido ligand 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. To the best of our knowledge,
there is as yet no report on the use of complexes of the first-row transition metals with
triazenido ligands to electrocatalyze hydrogen evolution. Here we present the
synthesis, structure and properties of a new dinuclear triazenido-copper(II) complex
[Cu2L4(NCCH3)] 1, that can catalyze hydrogen evolution from acetic acid or water.
2. Experimental
2.1 Materials and physical measurements
2
Page 2 of 26

All commercially available reagents were used without further purification.
Cyclic voltammograms were obtained on a CHI-600E electrochemical analyzer under
oxygen-free conditions using a three-electrode cell in which a glassy carbon electrode
GC, 1 mm in diameter) was the working electrode, a saturated Ag/AgNO3 electrode
was the reference electrode, and platinum wire was the auxiliary electrode. A
ferrocenium/ ferrocene couple was used as an internal standard and E1/2 of the
ferrocenium/ferrocene (FeCP2+/0) couple under the experimental condition was 0.47 V.
0.1 M [(n-Bu)4N]ClO4 was used as the supporting electrolyte. Acetic acid was added
by syringe. 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 a Ag/AgNO3 reference
electrode. The auxiliary compartment was fitted with a Pt gauze electrode. The
working compartment was filled with 50 mL of 80 mM acetic acid in a 0.1 M
[(n-Bu4N)]ClO4 DMF solution, while the auxiliary compartment was filled with 35
mL of 0.1 M [(n-Bu4N)]ClO4 DMF solution, resulting in equal solution levels in both
compartments. Both compartments were sparged for 30 min with N2 and cyclic
voltammograms were recorded as controls. Complex 1 (0.34 M) was then added and
a cyclic voltammogram was recorded. CPE in aqueous media was also 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 a 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 phosphate buffer solution at different
pH values containing 0.1 M KCl of the supporting electrolyte, while the auxiliary
compartment was filled with 35 mL phosphate buffer. Both compartments were
sparged for 30 min with N2 and cyclic voltammograms were recorded as controls.
Complex 1 (0.34 M) was then added and a cyclic voltammogram was recorded.
After electrolysis under a nitrogen atmosphere, a 0.5 mL aliquot of the headspace was
removed and replaced with 0.5 mL of CH4. A sample of the headspace was injected
into the gas chromatograph (GC) (Fig. S11). GC experiments were carried out with an
Agilent Technologies 7890A gas chromatography instrument. UV-Vis spectra were
3
Page 3 of 26

recorded on a U-3900H spectrophotometer by using a cryostatted optically
semi-transparent thin-layer electro-synthetic (OSTTLE) cell.
2.2. Synthesis of 1,3-bis[(4-chloro)benzene]triazene (HL)
A sodium nitrite aqueous solution (15 %) was added dropwise to a mixed
solution of 4-chloroaniline (10 mmol) in water (5 mL) and 1 molL-1 of HCl (30 mL)
under stirring at 0℃. Later, an equal amount of 4-chloroaniline in ethanol was added
with continuous stirring. The resulting solution was neutralized with sodium acetate
(15%), affording yellow precipitate which was filtered, and dried under vacuum
(71%). Re-crystallization from ethyl acetate gave yellow crystals. The yield was 71%.
Calcd for C12H9Cl2N3: C, 54.16; H, 3.41; N, 15.79. Found: C, 54.65; H, 3.40; N,
15.37. 1H NMR (400 MHz, CDCl3) δ 8.37 (s, 1H), 7.36–7.20 (m, 8H). 13C NMR (100
MHz, CDCl3) δ 143.45, 130.73, 129.43, 119.11 (Fig. S1 and Fig. S2).
2.3. Synthesis of complex 1
To a solution of HL (0.28 g, 1 mmol) and triethylamine (0.10 g, 1 mmol) in
acetonitrile/tetrahydrofuran (20 mL, 1:1), CuCl2·2H2O (0.17 g, 1.0 mmol) was added
and the mixture was stirred for 15 min. The solution was allowed to evaporate,
affording black crystals, which were collected and dried in vacuo (0.74, 60.0 %).
Calcd for C50H35Cl8Cu2N13: C, 48.88; H, 2.87; N, 14.82. Found: C, 48.91; H, 2.88; N,
14.86. UV-vis [DMF, max/nm ( /L mol-1 cm-1)]: 242 (9.68×103), 348 (1.39×104).
2.4. Crystal structure determination
The X-ray analysis of 1 was carried out with a Bruker SMART CCD area detector
using graphite monochromated Mo-Kα Radiation ( = 0.71073 Å) at room
temperature. All empirical absorption corrections were applied by using the SADABS
program [25]. The structure was solved using direct methods and the corresponding
non-hydrogen atoms are 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
4
Page 4 of 26

least-squares refinement. All calculations were performed using the SHELXTL
computer program [26]. Crystallographic data for complex 1 are given in Table S1
and selected bond lengths are list in Table S2.
3. Results and discussion
3.1 Structure analysis
In the presence of Et3N, the reaction of ligand HL and CuCl2·2H2O affords
brown crystals of complex 1 in 60.0 % yield. As shown in Fig. 2, the structure of
complex 1 consists of two copper centers bridged by four triazenido ligands. Two
copper centers present the different coordination environments, Cu1 is
four-coordinated by four nitrogen atoms from four anionic ligands (L-) and Cu2 is
five-coordinated by four nitrogen atoms from L- and one nitrogen atom from
acetonitrile molecule. The Cu-N bond distances of triazenido ligands fall in the range
2.0256(19)-2.0777(19) Å, and the distance between copper and nitrogen atom of
acetonitrile is 2.165(4) Å. The Cu…Cu distance (2.5016(8) Å) is longer than that
found in Cu2(dpt)4 (2.441 Å) [27], but shorter than that in tetra-bridged copper(II)
dimer Cu2(OAc)4 (2.616 Å) [28].
3.2. Cyclic voltmmetry studies
Cyclic voltammogram of complex 1 in DMF exhibits two reversible couples at
-1.72 and 0.22 V versus Ag/AgNO3 (Fig. 3). We assign the reversible couples to the
metal-centred CuI/0 and CuI/II processes, respectively. As observed in Fig. 4, the
current response of the redox events at -1.72 V versus Ag/AgNO3 shows a linear
dependence on the square root of the scan rate, which is an indicative of a
diffusion-controlled process, with the electrochemically active species freely diffusing
in the solution. Such distinctive potential prompted possible usage of this complex as
an electrocatalyst for hydrogen evolution.
3.3. Catalytic hydrogen evolution in DMF
To determine possible electrocatalytic activity of this complex, cyclic
5
Page 5 of 26

voltammograms of complex 1 were recorded in the presence of acetic acid in DMF.
Fig. 5 shows a systematic in icat observed near –0.95 V with increasing acid
concentration from 7.2 to 35.0 mM. This rise in current can be attributed to the
catalytic generation of H2 from acetic acid [29]. Reduction potentials of complex 1
slightly move to more anodic values with the sequential increments of acid
concentration. Based on eq. (1) [29], this reduction occurred at overpotential of 220
mV.
E⊙HA=E⊙ +-(2.303RT/F)pKaHA (1)
H
A number of control experiments were carried out to verify that complex 1 is
responsible for the catalysis. In particular, acetic acid, the free ligand, CuCl2·2H2O,
and the mixture of the free ligand and CuCl2·2H2O were each measured under
identical conditions. As can be seen in Fig. S3-Fig. S6, the catalytic competency
achieved with 1 is not matched by just ligand, CuCl2·2H2O, or the mixture of the free
ligand and CuCl2·2H2O, as might arise from dissociation of the ligand, nor can it be
accomplished with the ligand bound to a redox-inactive metal. Thus, a combination of
the redox-active copper ion and the ligand is essential for catalytic activity.
Further evidence for the electro-catalytic activity was obtained by bulk
electrolysis of a DMF solution of complex 1 (0.34 μM) with acetic acid (80 mM). Fig.
6 shows the charge of bulk electrolysis of solution containing complex 1 in presence
of acetic acid, the charge significantly increased when the applied potential was set to
more negative, accompanying gas bubble appeared on the electrode (Fig. S7). When
an applied potential was -1.47 V versus Ag/AgNO3, the maximum charge reached 105
mC during 2 min of electrolysis. Controlled-potential electrolysis (CPE) experiment
under the same potential with a catalyst-free solution only gave a charge of 10 mC
(Fig. S8), showing that this complex does serve an effective hydrogen production
under such conditions. This study suggested that 1 is capable of catalyzing the
reduction of protons from acid. Assuming every catalyst molecule was distributed on
the electrode surface and every electron was used for the reduction of protons.
According to eq. (2) [30], we calculated TOF for the catalyst reaching a maximum of
6
Page 6 of 26

43 moles of hydrogen per mole of catalyst per hour (Fig. S9 and Fig. 7).
TOF = C / (F*n1*n2*t) (2)
Where, C is charge from catalyst solution during CPE minus charge from solution
without catalyst during CPE, F is Faraday's constant, n1 is mol of electrons required to
generate a mol of H2, n2 is mol of catalyst in solution, t is duration of electrolysis.
3.4. Catalytic hydrogen evolution in aqueous media
Inspired by DuBois group’s work [31, 32], 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 phosphate buffers at
different pH values. As shown in Fig. 8, at pH 7.0, in the absence of complex 1, a
catalytic current was not apparent until a potential of -1.45 V versus Ag/AgCl was
attained. With addition of complex 1, the onset of catalytic current was observed at
about -1.25 V versus Ag/AgCl, and the current strength increased significantly with
increasing concentrations of complex 1 from 0.00 to 12.8 M. When concentration of
complex 1 reached 12.8 μM, the potential (-1.12 V) and current strength did not
change at all. The potential moved positive about 330 mV compared to that in the
absence of complex 1. This study suggested that 1 is also can catalyze the reduction of
protons from water. Based on Eq. (1) [29], complex 1 shows an overpotential of 776
mV (pH 4.0), 628 mV (pH 6.0), 639 mV (pH 7.0) and 779 mV (pH 8.0), respectively.
Catalytic hydrogen production can also be achieved with complex 1 in buffer.
Fig. 9 shows the total charge of bulk electrolysis of complex 1 at different pH values.
H2 production can be observed at all the pH values studied (4 to 8). When an applied
potential was -1.47 V versus Ag/AgCl at pH 6.0, the maximum charge was only 49
mC during 2 min of electrolysis in absence of complex 1 (Fig. S10). Under the same
conditions, the charge reached 435 mC with addition of complex 1, accompanying the
formation of a large amount of gas bubbles, which was confirmed to be H2 by GC
analysis (Fig. S11). Based on eq. (2), TOF for electrocatalytic hydrogen production by
complex 1 is 69 (pH 4.0), 174 (pH 6.0), 82 (pH 7.0) and 35 (pH 8.0) moles of
7
Page 7 of 26

hydrogen per mole of catalyst per hour, respectively (Fig. 10). At pH 6, TOF reached
the maximum value of 174 moles of hydrogen per mole of catalyst per hour,
indicating the catalyst shows a better catalytic efficiency under a weak acidic
environment.
3.5. Investigation of the stability and durability of complex 1
We used UV-Vis spectra to investigate the stability of complex 1 by employing an
optically semi-transparent thin layer electrode. As shown in Fig. 11, the
electrochemical reduction of complex 1 at -1.47 V vs. Ag/AgCl results in one new
absorption appeared at 330 (pH 4) and 354 (pH 8) nm, respectively, over an
electrolysis period of 1 h. At pH 6 and pH 7, the intensity of the absorption band at
348 nm slightly decreases, but the peak position does not change, indicating the
catalyst is stable in a weak acid or weak base environment. This is also in accordance
with the results as revealed by CPE that complex 1 shows a better catalytic efficiency
under a weak acidic environment. The durability of the catalyst was assessed in an
extended CPE experiment performed in a 1.0 M buffer at pH 6. To ensure a rapid
turnover rate during the electrolysis, a potential of -1.47 V vs Ag/AgCl was employed
for the measurement. As depicted in Fig. 12, the catalyst affords a robust and
essentially linear charge build-up over time, with no substantial loss in activity over
the course of 72 h.
4. Conclusion
In summary, we have successfully constructed one copper-triazenido complex 1.
Electrochemical studies show that 1 can not only generate dihydrogen from acetic
acid but also from water. This discovery has established a new chemical paradigm for
creating reduction catalysts that is highly active and robust in both organic and
aqueous media. Ongoing efforts are focused on modifying triazenido ligands and
related complexes to further facilitate new functional studies, especially emphasis on
chemistry relevant to sustainable energy cycles.
Supporting Information
8
Page 8 of 26

CCDC 978386 contains the supplementary crystallographic data for this paper.
This
data
can
be
obtained
free
or
of
from
charge
via
http://www.ccdc.cam.ac.uk/conts/retrieving.html,
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.
Acknowledgements
This work was supported by the Research Foundation for Returned 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.
References
[1] J. A. Turner, Science 305 (2004) 972-974.
[2] T. R. Cook, D. K. Dogutan, S. Y. Reece, Y. Surendranath, T. S. Teets, D. G.
Nocera, Chem. Rev. 110 (2010) 6474-6502.
[3] P. D. Tran, V. Artero, M. Fontecave, Energ. Environ. Sci. 3 (2010) 727-747.
[4] J. C. Fontecilla-Camps, A. Volbeda, C. Cavazza, Y. Nicolet, Chem. Rev. 107
(2007) 4273-4303.
[5] P. M. Vignais, B. Billoud, Chem. Rev. 107 (2007) 4206-4272.
[6] C. J. Pickett, C. Tard, Chem. Rev. 109 (2009) 2245-2274.
[7] M.Y. Darensbourg, W. Wegand, Eur. J. Inorg. Chem. (2011) 994-1004.
[8] C. Greco, M. Bruschi, L. De Gioia, U. Ryde, Inorg. Chem. 46 (2007) 5911-5921.
[9] A.S. Pandey, T.V. Harris, L.J. Giles, J.W. Peters, R.K. Szilagyi, J. Am. Chem. Soc.
130 (2008) 4533-4540.
[10] A. Silakov, B. Wenk, E. Reijerse, W. Lubitz, Phys. Chem. Chem. Phys. 11 (2009)
6592-6599.
[11] S. Ezzaher, J.F. Capon, F. Gloaguen, F.Y. Pétillon, P. Schollhammer, J. Talarmin,
Inorg. Chem. 46 (2007) 9863-9872.
9
Page 9 of 26

[12] T.B. Rauchfuss, B.E. Barton, M.T. Olsen, J. Am. Chem. Soc. 130 (2008)
16834-16835.
[13] T. Goris, A.F. Wait, M. Saggu, J. Fritsch, N. Heidary, M. Stein, I. Zebger, F.
Lendzian, F.A. Armstrong, B. Friedrich, O. Lenz, Nat. Chem. Biol. 7 (2011)
310-318.
[14] T.B. Rauchfuss, B.E. Barton, M.T. Olsen. J. Am. Chem. Soc. 130 (2008)
16834-16835.
[15] Ö.F. Erdem, L. Schwartz, M. Stein, A. Silakov, S. Kaur-Ghumaan, P. Huang, S.
Ott, E.J. Reijerse, W. Lubitz, Angew. Chem. Int. Ed. 50 (2011) 1439-1443.
[16] S. Ghosh, G. Hogarth, N. Hollingsworth, K. B. Holt, S. E. Kabirc, B. E.
Sancheza, Chem. Commun. 50 (2014) 945-947.
[17] S. Canaguier, V. Fourmond, C. U. Perotto, J. Fize, J. Pe′caut, M. Fontecave, M. J.
Field, V. Artero, Chem. Commun. 49 (2013) 5004-5006.
[18] S. Mandal, S. Shikano, Y. Yamada, Y. Lee, W. Nam, A. Llobet, S. Fukuzumi, J.
Am. Chem. Soc. 135 (2013) 15294–15297.
[19] J.M. Camara, T.B. Rauchfuss, Nat. Chem. 4 (2012) 26-30.
[20] N. Wang, M. Wang, Y. Wang, D. Zheng, H. Han, M. S. G. Ahlquis., L. Sun, J. Am.
Chem. Soc. 135 (2013) 13688–13691.
[21] M. E. Carroll, B. E. Barton, T. B. Rauchfuss, P. J. Carroll, J. Am. Chem. Soc. 134
(2012) 18843–18852.
[22] L.-C. Song, Z.-J. Xie, M.-M. Wang,G.-Y. Zhao, H.-B. Song, Inorg. Chem. 51
(2012) 7466–7468.
[23] J. Chu, X. Xie, S. Yang, S.z. Zhan, Inorg. Chim. Acta 410 (2014) 191–194.
[24] J. Chu, Q. Lv, J. Cao, X. Xie, S.z. Zhan, Inorg. Chem. Commun. 36 (2013)
245–248.
[25] G.M. Sheldrick, SADABS, Program for Empirical Absorption Correction of
Area Detector Data; University of Göttingen: Göttingen, Germany, (1996).
10
Page 10 of 26

[26] G.M. Sheldrick, SHELXS 97, Program for Crystal Structure Refinement,
University of Göttingen, Göttingen , Germany, (1997).
[27] M. Corbett, B. F. Hoskins, N. J. McLeod, B. P. O'Day, Aust. J. Chem. 28 (1975)
2377-2392.
[28] P. de Meester, S. R. Fletcher, A. C. Skapski, Dalton Trans. (1973) 2575-2578.
[29] C.J. Chang, H.I. Karunadasa, J.R. Long, Nature 464 (2010) 1329-1333.
[30] H.I. Karunadasa, E. Montalvo, Y. Sun, Science 335 (2012) 698-702.
[31] J.A.S. Roberts, U.J. Kilgore, D.H. Pool, A.M. Appel, M.P. Stewart, M.R. DuBois,
W.G. Dougherty, W.S. Kassel, R.M. Bullock, D.L. DuBois, J. Am. Chem. Soc.
133 (2011) 5861-5872.
[32] M.L. Helm, M.P. Stewart, R.M. Bullock, M.R. DuBois, D.L. DuBois, Science
333 (2011) 863-866.
11
Page 11 of 26

> Reaction of 1,3-bis[(4-chloro)benzene]triazene (HL) and CuCl2﹒2H2O affords a
complex, [Cu2L4(NCCH3)] 1;
> Structure of 1 is determined by single crystal X-ray diffraction analysis;
> Complex 1 is capable of generating hydrogen from acetic acid or water.
> TOF reaches a maximum of 43 (DMF) and 198 (buffer, pH 6) moles/mole /h,
respectively.
26
Page 12 of 26

*Graphical Abstract (for review)
10
0
-10
-20
-30
-40
-50
-60
-70
Cl
Cl
Cl
Cl
N
HOAc
N
N
N
N
N
0
NCCH3
Cu Cu
7.2 mM
N
N
10.8 mM
17.5 mM
35.0 mM
N
N
N
N
Cl
Cl
Cl
Cl
-2.1
-1.8
-1.5
-1.2
-0.9
-0.6
-0.3
0.0
Potential (V vs Ag/AgNO3)
Page 13 of 26

Figure
Fig. 1. Structure of active site of the FeFe-hydrogenase and its model.
Page 14 of 26

Figure
Fig. 2. Molecular structure of complex 1.
Page 15 of 26

Figure
9
6
3
0
-3
-6
-9
-12
-15
-2.1 -1.8 -1.5 -1.2 -0.9 -0.6 -0.3 0.0
0.3
0.6
Potential (V vs Ag/AgNO3)
Fig. 3. 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.
Page 16 of 26

Figure
25
20
15
10
5
-9
-12
-15
-18
-21
-24
-27
-30
0
-5
-10
-15
-20
-25
-30
-35
-2.1 -1.8 -1.5 -1.2 -0.9 -0.6 -0.3 0.0 0.3 0.6
0.2
0.3
0.4
0.5
0.6
0.7
Potential (V vs Ag/AgNO3)
1 /2 1 /2 -1 /2
( v )
ν
s
Fig. 4. Scan rate dependence of precatalytic waves for a 0.34 M solution of complex
1 (0.1 M [n-Bu4N]ClO4), at scan rates from 50 to 400 mV/s.
Page 17 of 26

Figure
10
0
-10
-20
-30
-40
-50
-60
-70
HOAc
0
7.2 mM
10.8 mM
17.5 mM
35.0 mM
-2.1
-1.8
-1.5
-1.2
-0.9
-0.6
-0.3
0.0
Potential (V vs Ag/AgNO3)
Fig. 5. Cyclic voltammograms of complex 1 (0.34 M) in the absence of acid (red
trace), with varying concentrations of acid in DMF. Conditions: r. t, 0.1 M
[n-Bu4N]ClO4 as supporting electrolyte, scan rate = 100 mV/s, GC working electrode
(1 mm diameter), Pt wire counter electrode, Ag/AgNO3 reference electrode, Fc
internal standard.
Page 18 of 26

Figure
100
80
60
40
20
0
Applied potential
-1.20 V
-1.25 V
-1.30 V
-1.35 V
-1.39 V
-1.43 V
-1.47 V
0
20
40
60
80
100
120
Time (s)
Fig. 6. Charge buildup versus time from electrolysis of a 0.34 M complex 1 in
DMF (0.1 M [n-Bu4N]ClO4) under various applied potentials. All data have been
deducted blank.
Page 19 of 26

Figure
45
40
35
30
25
20
15
10
5
-1.20
-1.25
-1.30
-1.35
-1.40
-1.45
-1.50
Applied potential (V vs Ag/AgNO )
3
Fig. 7. Turnover frequency (mol H2/mol catalysts/h) for electro-catalytic hydrogen
production by complex 1 (0.34 M) under a series of applied potentials.
Page 20 of 26

Figure
800
700
600
500
400
300
200
100
0
500
400
300
200
100
0
pH 6.0
pH 4.0
0
0.8 M
1.6 M
3.2 M
6.4 M
12.8 M
0
0.8 M
1.6 M
3.2M
6.4 M
12.8 M
0.0 -0.2 -0.4 -0.6 -0.8 -1.0 -1.2 -1.4 -1.6 -1.8 -2.0
0.0 -0.2 -0.4 -0.6 -0.8 -1.0 -1.2 -1.4 -1.6 -1.8 -2.0
Potential (V vs Ag/AgCl)
Potential (V vs Ag/AgCl)
400
400
pH 7.0
pH 8.0
300
200
100
0
300
200
100
0
0
0
0.8 M
1.6 M
3.2 M
6.4 M
12.8 M
0.8 M
1.6 M
3.2M
6.4 M
12.8 M
0.0 -0.2 -0.4 -0.6 -0.8 -1.0 -1.2 -1.4 -1.6 -1.8 -2.0
0.0 -0.2 -0.4 -0.6 -0.8 -1.0 -1.2 -1.4 -1.6 -1.8 -2.0
Potential (V vs Ag/AgCl)
Potential (V vs Ag/AgCl)
Fig. 8. Catalytic CV curves of complex 1 at different pH values
Page 21 of 26

Figure
450
400
350
300
250
200
150
100
50
160
140
120
100
80
pH 4.0
pH 6.0
Applied potential
Applied potential
-1.15 V
-1.20 V
-1.25 V
-1.30 V
-1.35 V
-1.39 V
-1.43 V
-1.47 V
-1.15 V
-1.20 V
-1.25 V
-1.30 V
-1.35 V
-1.39 V
-1.43 V
-1.47 V
60
40
20
0
0
0
20
40
60
80
100
120
0
20
40
60
80
100
120
Time (s)
Time (s)
90
80
70
60
50
40
30
20
10
0
pH 8.0
200
175
150
125
100
75
pH 7.0
Applied potential
Applied potential
-1.15 V
-1.20 V
-1.25 V
-1.30 V
-1.35 V
-1.39 V
-1.43 V
-1.47 V
-1.15 V
-1.20 V
-1.25 V
-1.30 V
-1.35 V
-1.39 V
-1.43 V
-1.47 V
50
25
0
0
20
40
60
80
100
120
0
20
40
60
80
100
120
Time/s
Time (s)
Fig. 9. Charge buildup versus time at various applied potentials in a 1.0 M phosphate
buffer at different pH values
Page 22 of 26

Figure
pH 4
70
60
50
40
30
20
10
0
-1.10 -1.15 -1.20 -1.25 -1.30 -1.35 -1.40 -1.45 -1.50
Applied potential (V vs Ag/AgCl)
pH 6
180
160
140
120
100
80
60
40
20
0
-1.10 -1.15 -1.20 -1.25 -1.30 -1.35 -1.40 -1.45 -1.50
Applied potential (V vs Ag/AgCl)
Page 23 of 26

90
80
70
60
50
40
30
20
10
pH 7
-1.10 -1.15 -1.20 -1.25 -1.30 -1.35 -1.40 -1.45 -1.50
Applied potential (V vs Ag/AgCl)
pH 8
35
30
25
20
15
10
5
-1.10 -1.15 -1.20 -1.25 -1.30 -1.35 -1.40 -1.45 -1.50
Applied potential (V vs Ag/AgCl)
Fig. 10. TOF at different pH under a series of applied potentials
Page 24 of 26

Figure
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
Complex 1
pH 8.0 1h
pH 7.0 1h
pH 6.0 1h
pH 4.0 1h
250
300
350
400
450
500
550
600
 (nm)
Fig.11. UV-Vis spectra of complex 1 during the electrochemical reduction at -1.47 V
vs. Ag/AgCl .
Page 25 of 26

Figure
2000
1500
1000
500
0
0
50000
100000
150000
200000
250000
300000
Time (s)
Fig. 12. Extended controlled potential electrolysis of 0.34 M complex 1, showing charge
buildup versus time with an applied potential of -1.47 V. Conditions: 1.0 M buffer solution, pH 6,
GC working electrode (1.25 cm2), Ag/AgCl in saturated KCl reference electrode, Pt wire counter
electrode, 72 h.
Page 26 of 26