Synthesis and characterization of a molecular electrocatalyst based on an iron(III) complex supported by an amine-bis(phenolate) ligand for proton or water reduction

Article abstract of DOI:10.1007/s11243-015-9945-y

Abstract Reaction of FeCl₃ with N,N-bis[methyl(2-hydroxy-3-tert-butyl-5-methoxyphenyl)]-N'N'-dimethylethylenediamine (H₂L) produces an iron(III) complex [LFeCl] 1, which has been characterized by X-ray crystallography. Electrochemical studies show that 1 can electrocatalyze hydrogen evolution, both from acetic acid with a turnover frequency (TOF) of 11 mol of hydrogen per mole of pre-catalyst per hour at an overpotential of 470 mV (in DMF), and from water with a TOF of 205 mol of hydrogen per mole of pre-catalyst per hour at an overpotential of 987.6 mV (in buffer, pH 7.0).

Full text of DOI:10.1007/s11243-015-9945-y

Transition Met Chem
DOI 10.1007/s11243-015-9945-y
Synthesis and characterization of a molecular electrocatalyst
based on an iron(III) complex supported by an amine-
bis(phenolate) ligand for proton or water reduction
1
1
1
•
Zhuo-Qiang Wang
1
Shu-Zhong Zhan
•
Ling-Zhi Tang
1
Jian-Shan Ye
•
Yun-Xiao Zhang
•
Received: 19 March 2015 / Accepted: 5 May 2015
Ó Springer International Publishing Switzerland 2015
Abstract Reaction of FeCl3 with N,N-bis[methyl(2-hy-
0
take of dihydrogen [4]. However, enzymes are difficult to
adapt for commercial applications, and their stability is
often limited outside of their native environment [5].
Electrolysis of water is the simplest way to produce hy-
drogen. To increase the reaction rate and lower the
overpotential, it is necessary to use an efficient hydrogen
evolution reaction (HER) electrocatalyst. Many research
groups, including ours, have focused on the development
of molecular catalysts employing the more abundant
metals, and several complexes of nickel [6], cobalt [7–9],
copper [10–12], and molybdenum [13–17] have been
developed as electrocatalysts for the reduction of acetic
0
droxy-3-tert-butyl-5-methoxyphenyl)]-N N -dimethylethylene-
diamine (H L) produces an iron(III) complex [LFeCl] 1, which
2
has been characterized by X-ray crystallography. Electro-
chemical studies show that 1 can electrocatalyze hydrogen
evolution, both from acetic acid with a turnover frequency
(TOF) of 11 mol of hydrogen per mole of pre-catalyst per hour
at an overpotential of 470 mV (in DMF), and from water with a
TOF of 205 mol of hydrogen per mole of pre-catalyst per hour
at an overpotential of 987.6 mV (in buffer, pH 7.0).
acid or water to form H . In general, iron(III) complexes
2
are employed as electrocatalysts for water oxidation [18–
Introduction
2
0], and there are few reports on water reduction by the
iron(III) complexes [21]. Herein, we present the synthesis
and characterization of an iron(III) complex, [LFeCl] 1, as
well as its electrocatalytic properties for hydrogen evo-
lution from both acetic acid and water.
Hydrogen is one of the most ideal energy carriers and a
possible alternative to fossil fuels in the future, because of
its numerous advantages such as recyclability and pollu-
tion-free use [1]. Water splitting is an important method
for hydrogen production in high purity and large quanti-
ties [2]. Effective proton reduction to form H has been a
2
Experimental
subject of intense study, and significant effort has been
made to design metal complexes for proton reduction [3].
In nature, hydrogenase enzymes are very efficient cata-
lysts for both the reductive generation and oxidative up-
Materials and measurements
All commercially available reagents were used as received
without further purification. The amine-bis(phenolate) li-
gand, H L, was prepared using a literature procedure by
2
Electronic supplementary material The online version of this
article (doi:10.1007/s11243-015-9945-y) contains supplementary
material, which is available to authorized users.
involving a Mannich condensation [22]. Elemental ana-
lyses for C, H, and N were obtained on a Perkin–Elmer
analyzer model 240. Cyclic voltammograms were obtained
&
Shu-Zhong Zhan
shzhzhan@scut.edu.cn
on a CHI-660E electrochemical analyzer under N using a
2
three-electrode cell in which a glassy carbon electrode GC
(1.0 mm in diameter) was the working electrode, a
saturated Ag/AgCl or Ag/AgNO3 electrode was the
1
College of Chemistry and Chemical Engineering, South
China University of Technology, Guangzhou 510640, China
123

Transition Met Chem
reference electrode, and a platinum wire was the auxiliary
electrode. In organic media, the ferrocene/ferrocenium
Results and discussion
General characterization
(
[
1?) couple was used as an internal standard and 0.10 M
(n-Bu) N]ClO was used as the supporting electrolyte.
4
4
Controlled-potential electrolysis (CPE) in aqueous media
was conducted using an airtight glass double-compartment
cell separated by a glass frit. The working compartment
was fitted with a glassy carbon plate and an Ag/AgCl
reference electrode. The auxiliary compartment was fitted
with a Pt gauze electrode. The working compartment was
charged with 50 mL of 0.25 M phosphate buffer solution,
while the auxiliary compartment contained 35 mL of
phosphate buffer solution. After addition of the iron
complex, the cyclic voltammogram was recorded. After
electrolysis, a 0.5-mL aliquot of the headspace was re-
In the presence of triethylamine, the reaction of FeCl and
3
H L affords blue crystals of the complex [LFeCl] 1 in
2
88 % yield (Scheme 1). This is in agreement with results
from MS measurements which exhibited an ion at a mass-
to-charge ratio (m/z) of 526.25 (Fig. S1), with the mass and
isotope distribution pattern corresponding to that of
?
[Fe(III)(L)] (calculated m/z of 526.44).
As shown in Fig. 1, in complex 1, the iron ion is bonded
-
to two oxygen atoms, two nitrogen atoms, and one Cl ion,
which defines the trigonal plane of the bipyramid. The cen-
tral nitrogen atoms of the ligand and the chloride ions occupy
˚
the apical sites. Fe–N distances are 2.273(2) and 2.178(2) A,
moved and replaced with 0.5 mL of CH . The headspace
4
˚
respectively. The Fe(1)–O(3) distance [1.846(2) A] is same
sample was injected into the gas chromatograph. GC ex-
periments were carried out with an Agilent Technologies
as that of Fe(1)–O(4). The Fe(1)–Cl(7) bond distance is
˚
2.3125(9) A.
7
890A gas chromatography instrument. ESI–MS ex-
periments were performed on a Bruker Daltonics Esquire
000 spectrometer by introducing samples directly into the
3
Electrochemical studies
ESI source using a syringe pump.
When bare platinum or glassy carbon was used as the
working electrode, in the absence of complex 1, the proton
reduction was not apparent until a potential of -0.88 or
Synthesis of complex 1
To a solution of H L (0.235 g, 0.5 mmol) and triethy-
2
-1.62 V versus Ag/AgNO , respectively (Fig. S2). Upon
3
lamine (0.100 g, 1 mmol) in methanol (10 ml), was added
a solution of anhydrous FeCl3 (0.082 g, 0.5 mmol) in
methanol and the mixture was stirred for 10 min. The so-
lution was allowed to slowly evaporate, affording dark-
blue crystals, which were collected and dried in vacuo
addition of complex 1, the onset of catalytic current was
observed at about -0.25 V versus Ag/AgNO3.
As shown in Fig. 2a, the cyclic voltammogram (CV) of
a DMF solution of complex 1 shows a reversible redox
peak at -0.30 V and an irreversible wave at -1.68 V,
III
II
II
I
(
0.245 g, 88 %). The elemental analysis results (Found C
which are assigned to the couples of Fe /Fe and Fe /Fe ,
respectively. No significant change was found after 10
scans, indicating that the catalyst is stable under these
conditions (Fig. S3). Both current responses of the redox
events at -0.30 and -1.68 V show linear dependence on
the square root of the scan rate (Fig. S4), indicative of a
6
0.1; H 7.5; N 5.0 %. C H N O FeCl requires C 59.8; H
28 42 2 4
7
.5; N 5.0 %) were in agreement with the formula of the
sample used for X-ray analysis.
X-ray crystallography
The X-ray analysis of complex 1 was carried out with a
Bruker Smart Apex II DUO area detector using graphite-
tBu
tBu
˚
OH
monochromated Mo-Ka radiation (k = 0.71073 A) at
tBu
HO
OH
NH2 2CH O
2
room temperature. All empirical absorption corrections
were applied by using the SADABS program [23]. The
structure was solved using direct methods, and the corre-
sponding non-hydrogen atoms were refined anisotropically.
All the hydrogen atoms of the ligands were placed in cal-
culated 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-97 computer
program [24]. Crystallographic data for complex 1 are
given in Table S1, and selected bond lengths are listed in
Table S2.
2
H2L
+
N
N
MeO
OMe
OMe
N
Et N
3
FeCl3
Cl
tBu O
N
O
Fe
tBu
N
MeO
OMe
[
LFeCl]1
Scheme 1 Synthesis of complex 1
1
23

Transition Met Chem
3
2
1
0
1
(
a)
*
-
-2.0
-1.5
-1.0
-0.5
0.0
0.5
Potential (V vs Ag/AgNO )
3
˚
Fig. 1 Molecular structure (ORTEP) of 1. Selected bond lengths (A):
Fe(1)–O(3), 1.846(2); Fe(1)–N(2), 2.273(2); Fe(1)–O(4), 1.846(2);
Fe(1)–N(4), 2.178(2); Fe(1)–Cl(7), 2.3125(9)
(
b)
*
0
0
0
mM
diffusion-controlled process, with the electrochemically
active species freely diffusing in solution.
.67 mM
1.33 mM
-
-
10
20
2.67 mM
5.33 mM
8.33 mM
13.33 mM
Catalytic hydrogen evolution in DMF
23.33 mM
3.33 mM
50.00 mM
6.67 mM
From Fig. 2b, it can be seen that the catalytic current near
3
-
1.68 V increased with increasing proton concentration
acetic acid concentration increased from 0.0 to
6.67 mM). This indicates that hydrogen evolution elec-
6
(
6
-30
-2
-1
0
trocatalyzed by 1 requires the reduction of Fe(II) to
Fe(I) and protonation, with the catalytic onset shifted to
more positive potential (from -1.58 V to -1.20 V). Based
Potential (V vs Ag/AgNO )
3
II
I
Fig. 2 a Cyclic voltammogram of 1.0 mM complex 1 in 0.10 M of
n-Bu N]ClO DMF solution at a glassy carbon electrode and a scan
rate of 100 mV/s. b Cyclic voltammograms of a 1.0-mM solution of
complex 1, with varying concentrations of acetic acid in DMF.
on these observations, only the Fe /Fe couple is involved
in proton reduction, which is similar to a previous report
[
4
4
[
21]. Further mechanistic studies are in progress.
Conditions: room temperature, 0.10 M [n-Bu
electrolyte, scan rate: 100 mV/s, GC working electrode (1 mm
4 4
N]ClO as supporting
Several control experiments were then carried out to
confirm that complex 1 was indeed responsible for the
catalytic reaction. In particular, the free ligand, FeCl , and
diameter), Pt counter electrode, Ag/AgNO
internal standard (*)
3
reference electrode. Fc
3
the mixture of the free ligand and FeCl were each inves-
3
tigated under identical experimental conditions. As can be
seen in Figs. S5–S7, the catalytic competency achieved
with complex 1 is not matched by either the free ligand or
to more negative. When the applied potential was -1.50 V
versus Ag/AgNO , the maximum charge reached 68 mC
3
FeCl , nor can it be accomplished with the ligand bound to
3
during 2 min of electrolysis. A controlled-potential elec-
trolysis (CPE) experiment under the same potential with a
catalyst-free solution gave a charge of only 17 mC (Fig.
S8b), showing that this complex does indeed serve as an
effective hydrogen producer under such conditions. As-
suming every pre-catalyst molecule was distributed only on
the electrode surface and every electron was used for the
reduction of protons, according to Eqs. (1) [6] and [25], we
calculated the TOF for the pre-catalyst as reaching a max-
imum of 11 moles of hydrogen per mole of pre-catalyst per
hour at an overpotential of 470 mV (Fig. S9).
a redox-inactive metal. Thus, the combination of the redox-
active Fe center and the ligand is essential for catalytic
activity.
Further evidence for the electrocatalytic activity was
obtained by bulk electrolysis of a DMF solution of complex
1
(0.714 lM) with acetic acid (40.0 mM) at variable applied
potential using a glassy carbon plate electrode in a double-
compartment cell. Fig. S8a shows the total charge of bulk
electrolysis of complex 1 in the presence of acid; the charge
significantly increased when the applied potential was made
123

Transition Met Chem
TOF ¼ DC= ðF Ã n1 Ã n2 Ã tÞ
Overpotential ¼ Applied potential À E
ð1Þ
ð2Þ
mediated by complex 1 [9]. Figure 3b exhibits a systematic
increase in icat with decreasing pH from 8.0 to 5.6. Further
evidence for the electrocatalytic activity of complex 1 was
obtained by bulk electrolysis of an aqueous solution of 1
(0.714 lM) with buffer (0.25 M, pH 7.0) at variable po-
tential using a GC electrode in a double-compartment cell.
When the applied potential was -1.60 V versus Ag/AgCl,
the maximum charge was only 66 mC for 2 min of elec-
trolysis in the absence of complex 1 (Fig. 4a). Under the
same conditions, the charge reached 1006 mC during 2 min
of electrolysis in the presence of 1 (Fig. 4b), accompanied
ꢀ
HA
¼
Applied potential
À
Á
ꢀ
þ
À EH À ð2:303RT=FÞpKaHA
where DC is the charge from the pre-catalyst solution
during CPE minus the charge from solution without pre-
catalyst during CPE; F is Faraday’s constant, n is the
1
number of moles of electrons required to generate one mole
of H , n is the number of moles of pre-catalyst in solution,
2
2
and t is the duration of electrolysis.
by evolution of a gas, which was confirmed as H by gas
2
chromatography. According to Fig. S10a, *0.445 mL of
H was produced over an electrolysis period of 2 h with a
Catalytic water reduction experiments
2
Faradaic efficiency of 93.5 % for H (Fig. S10-b). Ac-
2
We further explored the electrochemical behavior of com-
plex 1 in buffered aqueous solutions, where pH = 5.6–8.0
which is the range associated with catalytic water reduction.
The background CV in the absence of 1 exhibits no catalytic
cording to Eqs. (1) and (3) [7, 13], we calculated the TOF
for the pre-catalyst as reaching a maximum of 205 mol of
hydrogen per mole of pre-catalyst per hour at an overpo-
tential of 987.6 mV (Fig. S11), where
current (Fig. 3a), suggesting that water reduction to H is
2
(
a)
(a)
0.07
blank
0
.08
complex 1
0
0
0
0
0
0
0
.06
.05
.04
.03
.02
.01
.00
0.00
-
-
-
0.08
0.16
0.24
-2
-1
0
1
2
0
20
40
60
80
100
120
Potential (V vs Ag/AgCl)
Time (s)
(
b)
(
b) 1.0
0
20
40
60
Applied potential Overpotential
-
-
-
-
0.8 V
0.9 V
1.0 V
1.1 V
187.6 mV
287.6 mV
387.6 mV
487.6 mV
587.6 mV
687.6 mV
787.6 mV
887.6 mV
987.6 mV
0
0
.8
.6
pH
-
-1.2 V
1.3 V
5
6
6
7
.6
.2
.6
.0
-
-1.4 V
-1.5 V
-
1.6 V
-
-
7.5
.0
0.4
8
0
0
.2
.0
-2.0
-1.5
-1.0
-0.5
0
20
40
60
80
100
120
Potential (V vs Ag/AgCl)
Time (s)
Fig. 3 a CVs of buffers with and without complex 1 (pH 7.0, scan rate:
00 mV/s). b Cyclic voltammograms of 0.30 mM complex 1, showing
the variation in catalytic current with pH (scan rate: 100 mV/s)
Fig. 4 a Charge buildup of 0.10 M buffer (pH 7.0) at -1.60 V vs
Ag/AgCl. b Charge buildup of complex 1 (0.714 lM) versus a series
of applied potentials at pH 7.0. All data have been deducted blank
1
1
23

Transition Met Chem
Acknowledgments This work was supported by the National Sci-
ence Foundation of China (Nos. 20971045 and 21271073).
Overpotential ¼ Applied potential À EðpHÞ
¼
Applied potential À ðÀ0:059pHÞ
ð3Þ
To prove that complex 1 acts as a homogeneous pre-
electrocatalyst, we measured the dependence of the cat-
alytic current on the concentration of 1. As shown in Fig.
S12, the catalytic current was indeed dependent on the
complex concentration. In addition, several other pieces of
evidence suggest that this iron complex is a homogeneous
pre-catalyst, as follows. (1) There is no evidence for a
heterogeneous electrocatalytic deposit. For example, the
electrode was rinsed with water and electrolysis at -1.50
versus Ag/AgCl was run for an additional 2 min in 0.10 M
phosphate buffer at pH 7.0 with no pre-catalyst present in
solution. During this period, ca. 67 mC of charge was
passed, a similar magnitude as observed for electrolyses
conducted with freshly polished electrodes. (2) No discol-
oration of the electrodes was observed during cyclic
voltammetry or bulk electrolysis. In order to determine
whether complex 1 retains its activity over longer time
periods, a 72-h controlled-potential electrolysis at -1.45 V
versus Ag/AgCl was conducted in stirred 0.25 M phos-
phate buffer solution (pH 7.0), containing 1 (7.14 lM). On
completion of this experiment, the pH has increased by
four units (from 7.0 to 11), consistent with accumulation of
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