Tetrahedral Cu(I) complexes as electrocatalysts for the reduction of protons to dihydrogen gas

Article abstract of DOI:10.1002/ejic.202100295

Four copper(I) complexes [Cu(MeCN)₄]PF₆ (1), [Cu(dppe)₂]PF₆ (2), [Cu(bpy)₂]PF₆ (3) and [Cu(bpy)(dppe)]PF₆ (4) have been used as electrocatalysts in the generation of dihydrogen f

Full text of DOI:10.1002/ejic.202100295

European Journal of Inorganic Chemistry
Accepted Article
Title: Tetrahedral Cu(I) complexes as electrocatalysts for the reduction
of protons to dihydrogen gas
Authors: Wen Liang James Loke, Chen Hu, and Wai Yip Fan
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To be cited as: Eur. J. Inorg. Chem. 10.1002/ejic.202100295
Link to VoR: https://doi.org/10.1002/ejic.202100295
01/2020

10.1002/ejic.202100295
European Journal of Inorganic Chemistry
Tetrahedral Cu(I) complexes as electrocatalysts for the reduction of
protons to dihydrogen gas
Wen Liang James Loke[a], Chen Hu[a], and Wai Yip Fan*[a]
[a] Mr. W. L. J. Loke, Dr. C. Hu and Prof. W. Y. Fan
Department of Chemistry
National University of Singapore
3 Science Drive 3, Singapore 117543
E-mail: chmfanwy@nus.edu.sg
ABSTRACT: Four copper(I) complexes [Cu(MeCN)4]PF6 (1), [Cu(dppe)2]PF6 (2), [Cu(bpy)2]PF6 (3) and [Cu(bpy)(dppe)]PF6 (4)
have been used as electrocatalysts in the generation of dihydrogen from proton reduction. Low overpotentials of 0.4V have been
measured using cyclic voltammetry when 1 and 3 were used as the catalysts in the presence of acetic acid as the proton source in
acetonitrile. In contrast, a higher overpotential is observed for 4 while 2 is unable to catalyse the process. A mechanism for proton
reduction has been proposed.
INTRODUCTION
studied where reduction of trifluoroacetic acid was observed
in dimethylformamide at an overpotential of 0.72V.
Interestingly instead of the Cu(I) center, the pq ligand is
proposed to be more heavily involved in the protonation and
electrochemical reduction steps24.
The production of hydrogen from water, a cheap and abundant
source, has been regarded as an ideal energy source to replace
fossil fuels1-3. The proton reduction to hydrogen reaction
which forms half of water splitting has thus become a focus of
active research4-5. Although platinum is a highly efficient
catalyst in water-splitting6, its high cost prohibits large-scale
hydrogen production. Therefore the search for lower-cost
catalysts is still ongoing in order to achieve economical
As most catalysts go through a reduction process during the
catalytic cycle, we reason that Cu(II) compounds are likely to
reduce to Cu(I) at some stage of the cycle. This has led us to
prepare some Cu(I) complexes as the starting material so that
their role in the hydrogen evolution process can be explored
and evaluated.
industrialization7-9
.
Many studies have focused on the [FeFe] and [NiFe]
hydrogenases10-12, a class of biological enzyme capable of
releasing hydrogen from water reversibly and efficiently.
These have resulted in the development of many mimics or
In this work, we have used the commercially-available
[Cu(MeCN)4]PF6 (1) as a precursor to synthesize three more
Cu(I) complexes containing phosphine and nitrogen-centred
ligands such as [Cu(dppe)2]PF6 (2), [Cu(bpy)2]PF6 (3) and a
mixed copper N2P2 complex, [Cu(bpy)(dppe)]PF6 (4). Cyclic
voltammetry is then used to evaluate the efficiency of these
four complexes as proton reduction electrocatalysts. How the
various ligands affect the catalysis will be discussed and a
proton reduction mechanism is proposed based on our
experimental data.
model catalysts containing nickel and iron cores13-14
.
On the other hand, several non-biomimetic approaches which
utilized different metal cores such as cobalt (Co)15-18, copper
(Cu)19-21 and molybdenum (Mo)22-23 have also been reported
to display comparable catalytic activity as the hydrogenase
models.
Surprisingly, copper-containing compounds have been one of
the least-developed hydrogen evolution catalysts with the first
molecular catalyst [(bztpen)Cu(II)](BF4)2 (bztpen = N-benzyl-
N,N’,N’-tris(pyridin-2-ylmethyl)ethylenediamine) reported in
201419. Since then, a few other copper(II) complexes
containing nitrogen (N) and oxygen (O) donors20-21 such as
Na2[Cu(opba)] (opba: o-phenylenebis(oxamato)) and
RESULTS AND DISCUSSIONS
Preparation and Characterization of Complexes. The
synthetic method for the complexes was modified from the
process outlined by Min et al.25 Complexes 2-4 were readily
synthesized by direct coordination of the desired ligand to the
CuI center of 1. For each synthesis, two bidentate ligands
displaced the weakly bounded acetonitrile ligands, forming a
four-coordinated copper(I) complex. The structures of the
complexes are shown in Scheme 1.
[CuL1](ClO4)2
(L1:
1,3-bis{[(1-methyl-1H-imidazol-2-
yl)methyl]amino}propan-2-ol).
However, there have been much fewer reports of Cu (I)
compounds acting as proton reduction catalysts or catalytic
precursors. The use of [Cu(pq)2][BF4] (pq : quinoxaline) as
both photocatalyst and electrocatalyst has only recently been
1
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10.1002/ejic.202100295
European Journal of Inorganic Chemistry
Scheme 1. Molecular structures of complexes 1-4.
As coordinatively-saturated Cu(I) complexes, the products are
relatively stable in the solid state and do not undergo
disproportionation to Cu(0) and Cu(II) readily, a reaction
common for a number of Cu(I) compounds26-27. However, a
nitrogen atmosphere is still required during synthesis in order
to prevent oxidation of Cu(I) and possibly the ligands.
The structures of 2-4 were identified through their ESI-MS,
IR and NMR (1H and/or 31P) spectra (Supporting Information,
S1-S11). Melting point determination were also carried out.
In our experiment, we were able to isolate 3 as a reddish-
brown solid. However, upon dissolution in chloroform for
over two hours, the solution slowly changed from dark red to
pale yellow, suggesting decomposition of 3. Broad signals
were thus observed in its NMR spectrum; this observation was
also noted by Andrés-Tomé et al.28 It is thus essential to use a
freshly-prepared solution of 3 immediately for subsequent
electrochemical studies.
Cyclic Voltammetry Studies. The cyclic voltammograms of
the complexes 1-4 in acetonitrile were first recorded in the
absence of acid using a glassy carbon working electrode
(Figure 1). All solutions were freshly prepared and used
immediately.
Complex 1 shows a reduction (cathodic) peak at around Epc
=
Figure 1. Cyclic voltammograms recorded of acetonitrile
solutions containing 1 mM of the Cu(I) complex, 0.1 M
Bu4N+PF6- and 1 mM Fc at a scan rate of 100 mV/s. (a) 1 (b)
2 (c) 3 (d) 4.
-1.05 V (note that all potentials quoted herein are referenced
against the ferrocene couple Fc+/Fc in MeCN). The molecular
reduction peaks for 2, 3 and 4 were observed at Epc = -1.40 V,
Epc = -1.19 V and Epc = -1.47 V respectively. By comparison
with the literature CV profiles29 and the standard reduction
potentials, the aforementioned reduction peaks have been
assigned to the CuI/Cu0 couple of the respective complexes. A
sharp oxidation (anodic) peak at -0.52 V was seen in the
voltammograms of 1 and 3 together with another oxidation
peak which appeared close to 0 V and superimposed on the
Fc/Fc+ couple (indicated by the vertical dashed arrows). The
oxidation peak at -0.52 V is attributed to the reverse process
where elemental Cu(0) is oxidised to Cu(I) ions while the peak
at 0 V belongs to the CuI/CuII couple.
For 3 and 4, we notice a second reduction wave occurring at a
more negative potential of around -2.0 V. This wave most
likely arises from the reduction of the bpy ligand30, which is
the common ligand for both complexes.
The assignment of the peaks has indicated the weakly-bound
coordinating nature of acetonitrile (in 1) and bpy ligands (in
3). They have a tendency to dissociate from the copper centre
after its initial reduction, hence leaving the generated
elemental copper to be oxidized when the scan direction is
reversed. The CV profiles also implied the relative stability
2
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10.1002/ejic.202100295
European Journal of Inorganic Chemistry
for complexes containing dppe ligands (2 and 4), where
increasing acetic acid concentration from 0 mM to 25 mM. (a)
elemental copper oxidation at -0.52 V was not observed.
1 (b) 2 (c) 3 (d) 4.
The CV profiles of the complexes recorded in increasing
amount of acetic acid were investigated next (Figure 2). For 1,
3 and 4, strong cathodic peaks appeared during the scan and
continued to grow with increasing acid concentration. The
acid-concentration dependence strongly suggests that
catalytic reduction of protons is indeed occurring while the
appearance of a peak shows that the process is diffusion-
limited. However an acid-dependent wave was not observed
for 2 (Figure 2b).
Apart from the appearance of the catalytic wave, there was a
change in the oxidation peaks at -0.52 V and at 0 V with
varying acetic acid concentrations (See Supporting
Information Figure S12 for an enlarged version of Figure 2).
For 1 and 3, the current at -0.52 V decreases with an increasing
amount of acid. In contrast the oxidation peak at 0 V grows in
intensity. It would appear that Cu(0) generation is less
favoured compared to Cu(I) under high acid conditions. These
peaks were also observed to a smaller extent for 4 but did not
appear for 2. The change in the peak intensities of these
oxidation processes will provide some useful information for
understanding the mechanism later.
To evaluate the activity of the copper catalysts, we have
determined their overpotential necessary for catalysis, which
can be expressed by the following equation31
Overpotential necessary for catalysis = �ꢀꢁꢂꢃ/2 – ꢀHꢄA
�
where Ecat/2 is the catalytic half-wave potential and
2.303ꢅꢆ
ꢀHꢄA = ꢀHꢄ+ − �
� pꢈa,HA
ꢇ
Using the values ꢀHo = -0.028 V29 and pꢈa,AcOH = 22.332, the
+
thermodynamic potential ꢀAocOH is calculated to be -1.35 V.
With the catalytic half-wave potential determined using the
method of Appel and Helm33, the overpotential necessary for
catalysis of the complexes (Table 1) were computed by the
following,
Overpotential necessary for catalysis
= �ꢀꢁꢂꢃ/2 + 1.35� (V)
a
Table 1. Electrochemical data for complexes 1-4. Based on
peak current value at 15 mM acetic acid and 1 mM of complex.
bValues obtained from bulk electrolysis experiments. N.D. =
not determined.
Complex
1
2
3
4
Reduction Peak Potential for
CuI/Cu0 (V)
-1.05 -1.40 -1.19 -1.47
-1.77 N.D. -1.78 -1.94
0.43 N.D. 0.44 0.60
Catalytic Half-wave
Potentiala, Ecat/2 (V)
Overpotential necessary for
catalysisa (V)
Turnover Number, TONb
19.7
± 3.9
15.3 12.8
± 2.4 ± 2.9
N.D.
The use of 1 or 3 as the catalyst resulted in the lowest
overpotential (0.4 V) for proton reduction, which compares
favourably with some of the best mimics of hydrogenases7,8.
In addition, their molecular reduction peaks occur at a more
positive potential relative to the thermodynamic potential (-
Figure 2. Cyclic voltammograms recorded of acetonitrile
solutions containing 1 mM of the Cu(I) complex, 0.1 M
Bu4N+PF6- and 1 mM Fc at a scan rate of 100 mV/s with
3
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10.1002/ejic.202100295
European Journal of Inorganic Chemistry
1.35 V) of acetic acid reduction in acetonitrile. Somehow the
ease of molecular reduction may have facilitated the
subsequent proton reduction process to take place at a much
lower overpotential. Complexes 1 and 3 also have very similar
overpotentials. This observation suggests that a common
intermediate species may have been generated upon
dissociation of the acetonitrile and bpy ligands in 1 and 3
respectively.
show any peak shift or significant intensity changes despite
adding excess acid (Supporting Information, Figures S18-
S21). Therefore initial protonation of the complexes does not
occur or it is too slow to be considered the first step in the
mechanism.
In contrast, 2 did not exhibit catalytic behaviour while 4 gave
rise to a higher overpotential compare to 1 and 3. The
inactivity of 2 could be due to the tightly-bound dppe ligand
preventing the interaction of the Cu core with acetic acid or
protons. The stability of 2 is also supported by its partial
electrochemical reversibility depicted in Figure 2. While
complex 4 also contains dppe, dissociation of the bpy ligand
would still be able to provide a vacant site for proton
interaction.
We have conducted control experiments in the absence of the
copper complexes and noted only small contributions from the
direct reduction of acetic acid at the electrode surface
(Supporting Information, Figure S13). Thus the current
generated at the respective Ecat/2 potential of each catalyst was
mostly derived from the proton reduction process driven by
the catalyst itself.
Figure 3. Proposed mechanism for proton reduction catalysed
by the Cu(I) complexes. L represents the ligand surrounding
the copper centre. The steps are numbered according to the
sequence (1 to 4) and whether they are electrochemical (E) or
chemical protonation (C).
Bulk electrolysis experiments have been conducted for 1, 3
and 4 for several hours in large excess (1000 equiv.) of acetic
acid. Dihydrogen gas was successfully detected in the
headspace using mass spectrometry.
From the
chronoamperometry profiles (Supporting Information,
Figures S14-S16), the TON (Turnover Number) for each
catalyst has been estimated and shown in Table 1.
The results above lead us to propose the molecular reduction
peak observed in the CV scan to be the first electrochemical
step. It is interesting to speculate on the nature of the Cu(0)
species generated upon reduction. Cu(0) complexes are rarely
stable; most of the known complexes34-38 are highly air- and
moisture-sensitive and difficult to isolate. It is highly unlikely
that such a stable Cu(0) complex can be generated under our
experimental conditions. As mentioned, the similarity in the
overpotential values exhibited by 1 and 3 suggests that their
electrochemical reduction probably lead to ligand dissociation
and subsequent deposition of elemental copper on the
electrode surface. Furthermore the observation of a significant
Cu(0) to to Cu(I) oxidation peak at -0.52V for 1 and 3 supports
the hypothesis. In contrast, the phosphine ligands of 2 and 4
may have retained its coordination to Cu0 on the surface
sufficiently long to disrupt the catalytic cycle.
Rinse test has also been carried out to shed more light into the
proton reduction mechanism (Supporting Information, Figure
S17). For each test, the electrodes were first immersed into a
solution containing the catalyst and a fixed equivalent of acid.
After ten cycles of CV scans, the electrodes were removed
from the solution and rinsed with acetonitrile. They were re-
immersed into a fresh acetonitrile solution containing only
acetic acid (without any copper catalyst) and CV scans were
recorded thereafter. It was found that none of the new CV
profiles registered proton reduction signals. This set of
experiments indicates that there is no significant amount of
catalyst deposited on the electrode surface. However it still
does not rule out contributions from highly-active
heterogeneous species which requires only a small amount for
catalysis.
The second step of the mechanism is very likely a protonation
step (C) as a subsequent electrochemical step would have
generated copper with an oxidation state of -1. As noted
earlier, we have observed a decrease in the Cu0/CuI oxidation
peak at -0.52 V with increasing acid concentration (see Figure
3). This observation could be explained if the increase in the
acid concentration accelerates the protonation of Cu(0) on the
surface leading to the formation of a cationic Cu(II)-H hydride
species. If this formation is followed by a one-electron
reduction as the third step, a neutral Cu(I) hydride species will
be produced. However it is also likely that the second and third
Proposed Mechanism. The proton reduction mechanism
consists of two electrochemical (E) and two chemical (C) or
protonation steps undertaken by the catalyst. We are interested
in determining the sequence of these steps for 1 and 3 as they
have been shown to be the more efficient catalysts (Figure 3).
UV-visible absorption experiments were carried out in order
to verify whether these catalysts undergo protonation, which
will lead to a change in absorbance, prior to electrochemical
reduction. For each complex dissolved in acetonitrile, the UV
spectrum before and after the addition of acetic acid did not
4
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10.1002/ejic.202100295
European Journal of Inorganic Chemistry
steps are combined to form a proton-coupled electron transfer
step39, a process common in electrochemical reactions. For
example, slow protonation in step 2 will hamper the catalytic
process but once coupled with an exergonic electron transfer,
catalysis can proceed at a more facile rate.
Riedel-de Haën. Solvents such as dichloromethane (DCM)
were purchased from Fisher Chemical, while acetonitrile
(MeCN) was purchased from Fulltime and diethyl ether (Et2O)
was purchased from J.T.Baker. Deuterated solvents such as
chloroform-d (CDCl3) were purchased from Cambridge
Isotope Laboratories. All chemicals and solvents purchased
were used directly without purification.
Some evidence supporting the presence of an active but
relatively long-lived copper intermediate is presented when
the CV experiments at varying scan rates (Figure 4) were
performed. The CV recorded (black line) for the precursor
complex with 15 equivalents of acetic acid showed curve
crossing occurring at a low scan rate of 10 mV s-1. The
presence of curve crossing indicates the generation of
intermediate copper species with redox couples more positive
than the reactant redox couple. In our experiments, this
intermediate would correspond to the proposed Cu(I) hydride
complex. As the current did not return to near-zero on the
anodic scan, some catalytic activity for the species is still
observed even though the local concentration of substrate was
reduced during the forward scan catalysis. Curve crossing
may also suggest an ECE mechanism40, which is consistent
with the reaction pathways so far.
General Procedures. All synthetic procedures were carried
out under inert N2 conditions. Melting points were determined
using OptiMelt MPA100 Melting Point Apparatus.
Electrospray ionization mass spectrometry (ESI-MS) was
conducted using a Finnigan MAT LCQ spectrometer. Fourier
Transform Infrared (FT-IR) spectra were acquired with solid
samples in KBr disk using Bruker Alpha FT-IR spectrometer.
1H NMR and 31P NMR were recorded using Bruker ACF500
NMR spectrometer at room temperature.
Electrochemical Experiments. All electrochemical
experiments were conducted at 298 (±2) K. (a) Cyclic
voltammetry experiments were conducted with a computer-
controlled potentiostat (Princeton Applied Research
Potentiostat Model 263A) using a three-electrode system. The
working electrode was a 3 mm-diameter planar glassy carbon
(GC) disk, used together with a platinum counter electrode
and a silver wire miniature reference electrode connected to
the test solution via a salt bridge (containing 0.5 M Bu4NPF6
in MeCN). Prior to each scan, the solutions used for
voltammetric analysis were purged with high purity nitrogen
gas, and the working electrodes were prepared by polishing
with an alumina oxide (grain size 0.3 μm) slurry on a Buehler
Ultrapad polishing cloth, rinsing with ultrapure water and
acetone, and then drying. The test solutions were comprised
of 1 mM analyte and 0.1 M supporting electrolyte (Bu4N+PF6-)
in MeCN. Accurate potentials were obtained by using
ferrocene (Fc) as an internal standard. (b) Bulk electrolysis
was conducted with sample solutions containing 1 mM
analyte, 1 M acetic acid in 0.1 M Bu4N+PF6- in MeCN. The
sample solutions were electrolyzed at the respective catalytic
peak potential. The chronoamperometry curve was generated.
The gaseous content of the reaction vessel was removed with
a syringe of 1 cm3 volume and injected into a mass
spectrometer tuned to m/z = 2, corresponding to the detection
of H2.
Figure 4. Cyclic voltammograms recorded of acetonitrile
solutions containing 1 mM of 1, 0.1 M Bu4NPF6, 1 mM Fc
and 15 mM AcOH at different scan rates. Curve crossing
(black line) is observed when the scan rate is lowered to 10
mV s-1.
In the fourth step, the Cu(I) hydride species couples with a
proton to release dihydrogen gas and regenerates the Cu(I)
cation complex. Therefore we propose an ECEC mechanism
for complexes 1 and 3 acting as proton reduction catalysts. We
believed that this mechanism may also be applicable for 2 and
4 where the phosphine ligands have deactivated the Cu(0)
species on the electrode surface and thus impeded the catalytic
cycle. Further work is being planned to understand the roles
played by strong donor ligands in Cu(I)-catalysed proton
reduction processes.
Syntheses of the Copper Complexes. [Cu(MeCN)4]PF6
(74.4 mg, 0.2 mmol) was dissolved in DCM (1 mL) under
nitrogen flow. The corresponding ligand was dissolved in
DCM (1 mL) and the resultant solution was added dropwise
to the copper solution. An immediate color change from the
original colorless solution was observed. The mixture was
allowed to stir for 30 minutes. The solid product was
precipitated by the addition of diethyl ether to the reaction
mixture, and was collected via suction filtration.
EXPERIMENTAL SECTION
Chemicals.
Tetrakis(acetonitrile)copper(I)
hexafluorophosphate [Cu(MeCN)4]PF6, 2,2’-bipyridyl (bpy),
[Cu(dppe)2]PF6 (2). dppe (159.2 mg, 0.4 mmol) was used as
the ligand. White solid was obtained following the general
procedure outlined above. Yield: 132.2 mg, 66% (based on
[Cu(MeCN)4]PF6). mp 233 °C. Anal. Calcd for C52H48P5F6Cu:
1,2-bis(diphenylphosphino)ethane
(dppe),
tetrabutylammonium hexafluorophosphate Bu4N+PF6- were
purchased from Aldrich. Ferrocene (Fc) was purchased from
5
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10.1002/ejic.202100295
European Journal of Inorganic Chemistry
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was obtained following the general procedure outlined above.
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235 °C. Anal. Calcd for C36H32N2P3F6Cu: C, 56.65; H, 4.24;
N, 3.67. Found: C, 57.08; H, 4.24; N, 3.43. ESI-MS (DCM, +,
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bpy), 8.17 (dd, 2H, bpy), 8.32 (d, 2H, bpy), 8.56 (d, 2H, bpy).
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dppe).
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ASSOCIATED CONTENT
Supporting Information. This material is available free of
charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail for W.Y.F.: chmfanwy@nus.edu.sg.
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ACKNOWLEDGMENT
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We acknowledge the National University of Singapore for
participating through a research grant 143-000-B49-114.
Keywords: Proton reduction • copper • electrochemistry •
mechanism • green chemistry
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10.1002/ejic.202100295
European Journal of Inorganic Chemistry
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10.1002/ejic.202100295
European Journal of Inorganic Chemistry
For Table of Contents Only
Table of Contents Synopsis:
Cu (I) complexes have been demonstrated to be efficient proton reduction electrocatalysts in acetonitrile. An overpotential
+
-
value approaching 0.4V has been achieved for Cu(CH3CN)4 PF6 using acetic acid as the proton source. Adsorbed Cu atoms
generated from the reduction of Cu(I) are believed to play a vital role in the catalytic cycle.
8
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