Sulfur substitution in a Ni(cyclam) derivative results in lower overpotential for CO2 reduction and enhanced proton reduction

Article abstract of DOI:10.1039/c8dt04740e

The replacement of the opposing nitrogen atoms in 1,4,8,11-tetraazacyclotetradecane (cyclam) with two sulfur atoms in 1,8-dithia-4,11-diazacyclotetradecane (dithiacyclam) enables the electrochemical reduction of protons and CO₂via the correspon

Full text of DOI:10.1039/c8dt04740e

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Sulfur substitution in a Ni(cyclam) derivative
results in lower overpotential for CO2 reduction
Cite this: DOI: 10.1039/c8dt04740e
and enhanced proton reduction†
P. Gerschel,a K. Warm,b E. R. Farquhar,c U. Englert, d M. L. Reback,a
a,e
D. Siegmund, e K. Ray *b and U.-P. Apfel
*
The replacement of the opposing nitrogen atoms in 1,4,8,11-tetraazacyclotetradecane (cyclam) with two
sulfur atoms in 1,8-dithia-4,11-diazacyclotetradecane (dithiacyclam) enables the electrochemical reduction
of protons and CO2 via the corresponding nickel(II) complex at more positive potentials. In addition, a
10-fold enhancement in the proton reduction rate of [Ni(dithiacyclam)]2+ relative to [Ni(cylcam)]2+ was
observed. The study provides vital insight into Nature’s choice of employing predominantly sulfur based
ligand platforms in achieving biological proton and CO2 reductions.
Received 30th November 2018,
Accepted 21st December 2018
DOI: 10.1039/c8dt04740e
rsc.li/dalton
Among the numerous methods to activate small molecules,
e.g. H2 and CO2, biomimetic electrocatalysis has been proven
Introduction
As part of the global industrialization era and its concomitant to be one of the most promising and sustainable strategies for
demand of fossil energies, the concentration of carbon dioxide the storage and release of energy in chemical bonds.9,10
(CO2) in the atmosphere has dramatically risen to values over Thereby the electrocatalyst serves to stabilize intermediary
400 parts per million.1,2 CO2 is one of the major greenhouse species, hence enabling a facilitated reduction at milder poten-
gases and at such high concentrations it is detrimental to the tials.11 However, finding suitable electrocatalysts with low over-
environment. Therefore, it is of crucial importance to establish potentials for the reduction of CO2 as well as protons remains
a “green” future utilizing the abundant CO2 in the atmosphere a challenge.12,13 Notably, one crucial problem in the electro-
as a feedstock for higher energy products and to establish chemical reduction of CO2 is to suppress the formation of H2
post-fossil energy sources like hydrogen.3,4 In order to generate which is usually favored due to the inherent mass transport
useful C1 building blocks such as carbon monoxide, formal- problem of CO2 and its low solubility as compared to
dehyde, methanol and methane, one has to overcome the protons.14 In order to allow for new potential industrially-rele-
characteristic high energy demands associated with the con- vant electrocatalysts to arise, an understanding of the chem-
version of CO2.5–7 Specifically, this tremendous energy istry of such catalysts on a molecular level is of utmost impor-
demand originates from the initial one electron reduction step tance to facilitate the CO2 reduction and hamper the H2
of the thermodynamically and kinetically inert linear CO2 formation.15
molecule to reorganize into the highly reactive bent CO2
radical anion.8
Among the spectrum of homogeneous electrocatalysts that
were reported to be highly selective for CO2 reduction, a series
of macrocyclic tetra-aza Co and Ni complexes was shown to be
particularly promising.16 The most potent electrocatalyst for
aRuhr-Universität Bochum, Anorganische Chemie I, Universitätsstraße 150,
44801 Bochum, Germany. E-mail: ulf.apfel@rub.de
bHumboldt-Universität zu Berlin, Institut für Chemie, Brook-Taylor-Straße 2,
12489 Berlin, Germany. E-mail: kallol.ray@chemie.hu-berlin.de
cCWRU Center for Synchrotron Biosciences, NSLS-II, Brookhaven National
Laboratory, Upton, NY 11973, USA
the selective CO2 reduction in this series was [Ni(cyclam)]2+
1
(cyclam = 1,4,8,11-tetraaza-cyclotetradecane) (Scheme 1).8,16,17
dRWTH Aachen, Institut für Anorganische Chemie, Landoltweg 1, 52056 Aachen,
Germany
eFraunhofer UMSICHT, Osterfelder Straße 3, 46047 Oberhausen, Germany
†Electronic supplementary information (ESI) available: Synthesis and character-
ization of compounds, UV/vis spectra, SEM images, EDX spectra, gas chromato-
grams, CO & H2 quantification via GC and X-ray crystallographic analysis. CCDC
1539266. For ESI and crystallographic data in CIF or other electronic format see
DOI: 10.1039/c8dt04740e
Scheme 1 Schematic overview of [Ni(cyclam)]2+ 1 and different N- and
C-substituted derivatives.16–19
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At an applied potential of ca. −1.0 V vs. NHE at pH 4 in water
and utilizing a mercury working electrode, 1 allowed for a
selective reduction of CO2 to CO with a reaction rate of 32 h−1
and high faradaic efficiencies of 96%. Furthermore, an excellent
stability of compound 1 was reported allowing for more than
8000 turnovers over 8 hours without any notable decompo-
sition.18 Although the reduced intermediate [NiI(cyclam)]+
species readily adsorbs on the Hg electrode and it was suggested
that this adsorbate is responsible for the high activity of complex
1, it is noteworthy that the CO2 reduction is still observed when
using glassy carbon working electrodes that did not reveal any
interaction with the complexes.11,19
While the electrocatalytic reduction processes are
sufficiently well investigated and different variations of substi-
tution patterns on the cyclam N-donor and C-framework
atoms are reported (Scheme 1),19–22 incorporation of alterna-
tive donor atoms and their influence on the H+ vs. CO2
reduction are not yet investigated. Inspired by the sulfur-rich
donor sets that nature has established in its powerful and
efficient reductive enzymatic machineries, e.g. CO-dehydrogen-
ase23 or [FeFe]- and [NiFe]-hydrogenases,24,25 we became inter-
ested in investigating the CO2 and proton reduction abilities of
the nickel-cyclam complex upon partial replacement of its
nitrogen atoms with sulfur. The potential advantage of using
N,S-donor ligands was recently shown by Kojima et al., where a
NiII complex bearing an S2N2-type ligand (NiII-N2S2, Scheme 2)
allowed for photocatalytic reduction of CO2 with a TON of 713
and a high selectivity >99% for the formation of CO.29
Scheme 3 Synthesis of [Ni(dithiacyclam)(NCCH3)2](ClO4)2 3.
tion of the N2S2-donor ligand 2 with Ni(ClO4)2·6H2O in aceto-
nitrile subsequently afforded a dark purple crystalline solid in
87% yield (Scheme 3). The metalation of 2 was confirmed by
electrospray-ionization mass spectrometry (ESI-MS), revealing
two characteristic peaks at m/z = 292 and 391 for the
[Ni(dithiacyclam)]+ and the [Ni(dithiacyclam)(ClO4)]+ fragments.
Moreover, the presence of two characteristic CuN stretching
band at 2284 and 2309 cm−1 in the IR spectrum of 3 indicated
the additional binding of acetonitrile at the nickel center.
According to single crystal X-ray analysis, the molecular compo-
sition of 3 can be best described as [Ni(dithiacyclam)(NCCH3)2]
(ClO4)2 (Fig. 1 and Scheme 3).
Similar to the previously reported molecular structure of
the [Ni(cyclam)(NCCH3)2]2+ complex 1 that revealed the exist-
ence of two acetonitrile ligands in the axial positions,31 the
NiII center in 3 is octahedrally surrounded by the dithiacyclam
ligand and two additional acetonitrile molecules (Fig. 1).
However, contrary to 1 and in line with the molecular structure
of NiII-N2S2, the nitrogen atoms of the dithiacyclam and the
acetonitrile ligands occupy equatorial positions with averaged
Ni–Ndithiacyclam and Ni–NMeCN distances of 2.13 and 2.07 Å,
respectively. The sulfur donor atoms occupy the axial coordi-
nation sites and show averaged Ni–Sdithiacyclam distances of
2.39 Å, as defined by the local symmetry of the metal ion.
Furthermore, a detailed analysis of the N/S–Ni–N/S bond angles
of neighboring donor atoms showed angles between 83.5° and
97.4° revealing an overall distorted octahedral geometry for 3.
While the sulfur-containing cyclam derivatives shown in
Scheme 2 and their corresponding first-row transition metal
complexes have been reported,26–28 we herein outline for the
first time their potential to act as electrocatalysts for the
reduction of CO2 to valuable C1 building blocks.
Results and discussion
Synthesis and characterization
Solution behavior
Additional X-ray absorption near-edge spectroscopic studies
(XANES) on a frozen acetonitrile solution of 3 at the Ni K-edge
exhibits an edge inflection energy of ca. 8341.2 eV, with a
In order to better understand the importance of sulfur within
the many known metalloenzymes and to allow for comparison
with the electrocatalytic system 1, we synthesized the sulfur-
containing cyclam derivative 1,8-dithia-4,11-diazacyclotetrade-
cane (dithiacyclam) 2 (Scheme 3 and Scheme S1†).30 The reac-
Fig. 1 Structural motif of [Ni(dithiacyclam)(NCCH3)2](ClO4)2 3 drawn as
balls and sticks (hydrogen atoms and ClO4− counter ions are omitted for
clarity). Color code: C grey, N blue, S yellow and Ni turquoise.
Scheme 2 Schematic overview of sulfur-containing cyclam derivatives
(left) and the photocatalyst NiII-N2S2 (right).26,29
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Fig. 3 UV/vis/NIR titration of 3 with water. The full black line corres-
ponds to 3 in acetonitrile and the full red line to 3 in water.
Fig. 2 X-ray absorption near-edge spectroscopic studies (XANES) on a
frozen acetonitrile solution of compound 1 (red) and 3 (black).
shoulder along the rising edge at 8336.3 eV corresponding to a
1s → 4p shakedown transition, and a broad 1s → 3d pre-edge
peak at 8331.4 eV (Fig. 2). The data is well in accordance
with the observed distorted octahedral structure of this
complex in the solid state. The XANES spectrum of complex 1
shows a 0.6 eV blue shift of the edge inflection energy to
8340.6 eV, relative to 3, thereby supporting a more reduced Ni
center in the latter. However, the 1s → 3d pre-edge peak in 1 is
observed at identical energy and comparable intensity relative
to 3. Thus, the six coordinate geometry of 3 is also conserved
in solution. Nevertheless, the 1s → 4p shakedown transition
shoulder is much less intense in 1 relative to 3, with a more
symmetric Ni(N6) core in 1 as compared to the asymmetric
Ni(S2N4) core in 3 and is in good agreement to the structural
observations.
equilibrium constant of the ligand exchange an UV/vis/NIR
titration was carried out (Fig. 3). While complex 3 in aceto-
nitrile absorbs light at 552 and 851 nm, the absorption in
water is shifted to higher wavelengths at 591 and 962 nm gen-
erating two isosbestic points at 587 and 910 nm.
Subsequently, the equilibrium constant was determined as K =
4.3 × 104 and suggests that in water the formation of
[Ni(dithiacyclam)(H2O)2](ClO4)2 is favored. However, no appar-
ent UV/vis/NIR changes are visible for 1 up to the addition of
600 equivalents H2O (Fig. S1†). This behavior is in line with
previous reports showing no apparent electronic effects of
water or acetonitrile ligands in axial position of 1.31,32 The
observed solvent exchange clearly suggests that, contrary to 1,
the electrochemical properties of 3 as well as any catalytic
performance will strongly depend on the solvent used.
Solvent dependency
Electrochemistry
Complex 3 is stable under argon, while under atmospheric
conditions the color of the solid rapidly changes from purple
to blue, suggesting a ligand exchange by reaction with air.
Dissolving complex 3 in water also gives the same purple to
blue transition yielding a blue solid after concentration. The
obtained blue crystals were of poor quality thus not allowing
for structural determination, although the ESI-MS revealed a
similar pattern observed for 3 suggesting only the exchange of
the acetonitrile ligands. Notably, if the blue solid was redis-
solved in dry acetonitrile, the color reverted back to purple and
pure 3 was obtained. This experimental finding supports the
assumption of a reversible ligand exchange between aceto-
nitrile and water (Scheme 4). Furthermore, to determine the
Complex 3 (1 mM) was characterized by cyclic voltammetry
(CV) in dry acetonitrile with 0.1 M [nBu4N]+PF6− as the support-
ing electrolyte. The CV of 3 reveals two distinct irreversible
1e− reduction waves at −0.72 and −1.21 V vs. NHE for the
NiII → NiI and NiI → Ni0 transitions (Fig. 4), which is in line
with previous reports on NiII-N2S2.29 In addition, a controlled
potential coulometry held at −1.8 V vs. NHE revealed a total
charge consumption equivalent to a two-electron reduction
(Fig. S2†). The generated Ni0 species was found to adsorb on
the surface of the working electrode where it gets oxidized to
NiII at 0.80 V during the back scan of the CV, indicated by its
strong non-diffusion-controlled oxidation signal. If the scan is
reversed after the first reduction wave the stripping signal is
avoided, which suggests an adsorption of the Ni0 species. In
addition, the NiII/I couple stays irreversible with increasing
scan rate (Fig. S3†) suggesting a fast chemical reaction/struc-
tural reorganization after the electrochemical reduction. In
contrast, 1 shows only a single reversible NiII + e− ↔ NiI
reduction at a more cathodic potential (−1.16 V) under the
same experimental conditions. Based on these results, it can
be concluded that the incorporation of sulfur into the cyclam
Scheme 4 Schematic overview of the ligand exchange of 3 with water.
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and 1 under similar conditions. Upon addition of CO2 to an
acetonitrile/water (4 : 1) solution of 3, the shape of the catalytic
current changed drastically in comparison to the measure-
ments without CO2 (Fig. 5). Here, the catalytic current is
shifted to more cathodic potentials and the H2 formation
seems to be hampered by the addition of CO2. Furthermore, a
diffusion limitation, indicated by a wave shape, can be
observed at −1.30 V, suggesting a preferable CO2 reduction fol-
lowed by an additional catalytic current below −1.50 V for the
evolution of hydrogen. Notably, 3 enables a facilitated CO2
reduction at a potential ∼200 mV more anodic compared to
complex 1. In an effort to verify the CO2 reduction and to
identify the nickel carbonyl species generated during the CO2
reduction, IR spectroelectrochemical (SEC-IR) experiments of 1
and 3 were carried out in an electrochemical cell with a three-
electrode setup (WE: glassy carbon electrode, RE: Ag wire, CE:
Pt wire) attached to the ATR unit of an IR spectrometer. All
measurements were performed under catalytic conditions in a
CO2 saturated acetonitrile/water mixture (4 : 1) containing
Fig. 4 Cyclic voltammograms of 1 mM 1 (red) and 3 (black) in aceto-
nitrile, with 0.1 M [nBu4N]+PF6− at 100 mV s−1
.
framework enables a facilitated reduction of the complex and
allows for the formation of a Ni0 species that is not observed
for complex 1. Compared to our measurements in pure aceto-
nitrile, cyclic voltammograms of 3 in mixtures of acetonitrile/
water (4 : 1) revealed a significantly altered behavior. Herein
only the first reduction wave, above assigned as NiII → NiI
reduction, is observed at −0.79 V concomitant with a re-
oxidation wave at −0.16 V (Fig. 5 – black dashed line). Notably,
while the reduction to Ni0 is not visible under such conditions,
a strong catalytic current corresponding to water reduction can
be observed and indicates a high affinity for H2 evolution,
which is confirmed by gas chromatographic analysis (section:
solvent dependency of the HER activity). Similar solvent-
dependent electrochemical changes were not observed for 1.
20 mM of the NiII complex 1 or 3 with 0.1 M [nBu4N]+PF6 as
−
the electrolyte. The working electrode was placed 200 µm
above the ATR crystal and the potential was held for 800 s
before the IR scan was initiated. The SEC-IR 3D-spectrum of 3
shows the generation of a low-valent Ni-CO species with a dis-
tinct IR band at 2065 cm−1 for a broad potential range below
−1.0 V, confirming the catalytic reduction of CO2 (Fig. 6A). The
frequency observed for the Ni-CO species is in the same range
as the stretching frequency observed for Ni(CO)4 (2056 cm−1),
suggesting the formation of a Ni0-CO species.33,34 However,
based on these experiments no statement about the exact
nature of the reduced species can be made. Furthermore, this
band is not observed when the experiments were performed in
pure acetonitrile (non-catalytic conditions) or pure water (only H2
formation). [Ni(cyclam)]2+ (1) also shows catalytic CO2 reduction,
but within a more narrow potential range between −1.1 to −1.4 V
with a characteristic IR band at 1944 cm−1, which corresponds to
a NiI-CO species (Fig. 6B).11 Notably, at more cathodic potentials
the NiI-CO species disappears and H2 formation is dominant,
which is in line with previous literature reports on complex 1.19
Contrary to the reported experiments in pure acetonitrile (non-
catalytic conditions),11 an additional band in the IR spectrum
can be observed at 1780 cm−1 for 1 in CO2 saturated acetonitrile/
water mixtures (4 : 1) indicating the formation of formic acid/
formate, which is in line with earlier reports for the formation of
formic acid by 1 upon CO2 reduction.35 The most striking result
of the presented measurements is the small potential window
(from ca. −1.1 V to ca. −1.4 V) necessary for achieving the NiI-CO
species and for CO2 reduction when using complex 1. In contrast,
more positive potentials (<−1.0 V) are required for 3, conse-
quently revealing an improved catalytic performance towards CO2
reduction of 3 compared to 1.
Electrocatalytic CO2 reduction
[Ni(cyclam)]2+ 1 is reported to show a high selectivity towards
CO2 reduction over H2 formation in acetonitrile/water (4 : 1)
mixtures.18,19 To see if the dithiacyclam 2 ligand provided any
benefit, we looked at the CO2 reduction ability of complexes 3
Homogenous vs. heterogenous nature of the catalytic CO2
reducing species
Fig. 5 Cyclic voltammograms of 1 mM 1 (red) and 3 (black) with 0.1 M
[nBu4N]+PF6 in acetonitrile/water (4 : 1) at 100 mV s−1. The solutions are
−
Admittedly, based on these experiments, no information about
the nature of the catalytic active species (homogeneous
either purged with argon (dashed lines) or saturated with CO2 (solid lines).
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Fig. 6 Spectroelectrochemical-IR spectra of a 20 mM solution of complex 3 (A) or complex 1 (B) in acetonitrile/water (4 : 1), saturated with CO2,
recorded during chronoamperometry at different potentials.
complex vs. deposited material on the electrode surface) and
its corresponding electronic state is obtained. Thus, an initial
rinse-test was performed. A 1 mM solution of complex 3 in
acetonitrile was subjected to controlled potential coulometry
(CPC). While holding a reducing potential during CPC at
either the NiII → NiI or NiI → Ni0 couple, an enrichment of the
NiI or Ni0 oxidation state will occur and will eventually affect
the catalysis. The electrochemical cell was then refurbished
with fresh CO2 saturated acetonitrile/water (4 : 1) electrolyte
without manipulation of the working electrode followed by
linear sweep (LSV) experiments to provide insight into the
nature of the active species that is present. The results of these
experiments show that the LSV after generation of the Ni0
species is similar to the LSV in presence of complex 3 under
Fig. 7 Linear sweep voltammograms measured in the presence of CO2
catalytic conditions (Fig. 7). Hence, a deposited Ni0 species
−
in acetonitrile/water (4 : 1), with 0.1 M [nBu4N]+PF6 at 100 mV s−1. LSV
seems to significantly contribute to the overall CO2 reduction
of a solution without 3 (dashed black); LSV of a solution with 1 mM 3
as a catalytic active species (Fig. S4†). If the active species was
in solution, the LSV would have been identical to those
measured in acetonitrile/water (4 : 1) with CO2 in the absence
of any complex 3. In addition, this experiment shed some light
on the nature of the electronic state of the active species.
Additionally, to elucidate if the deposited Ni0 species is
solely elemental nickel, a rinse-test was performed with
[Ni(NCCH3)6](BF4)2. Although we observed nickel deposition
upon reduction at the same experimental conditions as shown
before, the deposited species showed no activity for the CO2
reduction (Fig. S5†).
(black); LSV of a solution without 3, after previous CPC of a solution
with 1 mM 3 held at −0.72 V (1st reduction wave, red); LSV of a solution
without 3, after previous CPC of a 1 mM solution of 3 held at −1.21 V
(2nd reduction wave, blue).
compartment cell where the working and reference electrodes
are separated from the counter electrode via a Nafion mem-
brane. During CPC at −1.5 V vs. NHE for 32 h, the quantity of
produced CO and H2 was measured after 1 h and subsequently
after every 2 h via an online GC (Table S1 and Fig. S6†). At the
beginning of the experiment, CO was observed as the main
product of the electrolysis with 70% CO and 30% H2. Notably,
in course of the experiment, H2 became the main product and
Catalyst performance and nature of the active CO2 reduction
catalyst
Additional long-term electrolysis was performed in order to after 6 h an equilibrium-ratio of ∼24% CO and ∼76% H2 was
determine the formed products, the turnover numbers (TONs), reached. During the electrolysis, the FE continuously increased
turnover frequencies (TOFs) and faradaic efficiencies (FEs) for and converged to a maximum of 97% after 30 h. The steady
the catalytic reduction of CO2. For the long-term measure- increase of the FE can be attributed to the continuous co-depo-
ment, a 1 mM solution of 3 in CO2 saturated acetonitrile/water sition of the catalytic active species throughout the experi-
(4 : 1) with 0.1 M [nBu4N]+PF6− as electrolyte was used in a two- ment. Based on the GC data after 30 h a TON of 23 with a TOF
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of 2.1 × 10−4 s−1 and a faradaic efficiency of 23% for the CO
generation was obtained and a TON of 740 with a TOF of 6.9 ×
10−3 s−1 and a faradaic efficiency of 74% for the hydrogen evol-
ution was obtained.
Electrocatalytic H2 evolution
While both complexes were capable of electrochemically redu-
cing CO2 at moderate potentials, the observed electrocatalytic
behavior of 3 in the presence of water (Fig. 5) highlights the
persistent problem of competing proton and CO2 reductions.
Thus, we likewise pointed our attention towards the use of
complex 1 and 3 as potential electrocatalyst for the hydrogen
evolution reaction (HER). Titration experiments were carried
out in the presence of equimolar amounts of acetic acid to a
1 mM solution of 3 in acetonitrile. Upon addition of 1–10
equivalents acetic acid, a continuous increase of the current
for both reduction waves is observed (Fig. 8). This increase
indicates that the H2 formation already starts from the NiI
state of complex 3. Both reduction waves are, however, shifted
to more anodic potentials upon the addition of 1 equiv. acid
indicating a protonation of complex 3. Adding further equiva-
lents of acetic acid leads to a shift of the onset values towards
more anodic potentials and finally resulting in an onset poten-
tial of −0.61 V at −0.1 mA in the presence of an excess acid
(>1000 equiv.). It is worth mentioning that for the same excess
amounts of acetic acid without 3, H2 formation starts at poten-
tials below −1.15 V. Hence, complex 3 significantly lowers the
overpotential by 540 mV for HER.
Fig. 9 Linear sweep voltammograms measured in the presence of
−
acetic acid (HOAc) in acetonitrile, with 0.1
M
[nBu4N]+PF6 at
100 mV s−1. LSV of a solution without 3 (dashed black); LSV of a solution
with 1 mM 3 (black); LSV of a solution without 3, after previous CPC of a
solution with 1 mM 3 held at −0.72 V (1st reduction wave, red); LSV of
a solution without 3, after previous CPC of a 1 mM solution of 3 held
at −1.21 V (2nd reduction wave, blue).
spectra. During CPC of a solution of 3 (5 µM) in acetonitrile
with 0.1 M [nBu4N]+PF6− for 15 h at −1.1 V vs. NHE, a light yel-
lowish tan colored solid was observed to be deposited on the
glassy carbon working electrode (Fig. S7 and S8†). Subsequent
EDX analysis of the deposited species revealed not only the
presence of nickel but also a significant amount of sulfur and
nitrogen with a ratio ranging from a formal Ni1.3S1N1.2 to
Ni3.6S1N2.3 composition (Fig. S9 and Table S2†). Thus, it seems
that nickel is not solely present as a pure metal but rather as a
hitherto unknown amorphous, heterogeneous film. Moreover,
these experimental findings are in line with earlier reports
which showed that [Ni(tetrathiacyclam)]2+ (tetrathiacyclam =
1,4,8,11-tetrathia-cyclotetradecane) also generates a deposited
Ni species on the working electrode upon reduction.28
Nature of the catalytic HER species
In order to determine the nature of the catalytic active species,
again a rinse test was performed. The results of these experi-
ments show that both LSVs either after CPC for the generation
of NiI and after CPC for the generation of Ni0 are similar to the
LSV in presence of 3 under catalytic HER conditions (Fig. 9).
Consequently, 3 seems to be a precursor for the generation of
a deposited catalytic active NiI species for HER. The deposited
Ni species was further confirmed by SEM images and EDX
Solvent dependency of the HER activity
While the previous experiments were performed in aceto-
nitrile, the apparent reaction of 3 with water (Scheme 4) drew
our attention to the influence of the solvent on the catalytic
hydrogen evolution. Electrochemical measurements revealed
that 3 enables H2 evolution in a mixture of acetonitrile/water
(4 : 1) at −1.00 V and in pure water at more cathodic potentials
(−1.23 V), but with a significantly enhanced current vs. poten-
tial slope (Fig. 10A). Further addition of acetic acid showed the
expected shift of the catalytic current towards more anodic
potentials revealing an onset potential of −0.73 V in case of
acetonitrile/water (4 : 1) and −0.58 V in case of water (Fig. 10A).
Catalyst precursor 3 showed the best performance for H2 evol-
ution in water (−0.58 V), followed by acetonitrile (−0.61 V) and
a mixture of acetonitrile/water (4 : 1) (−0.73 V). These results
were further supported by GC quantification of the generated
H2 gas during CPC at −1.1 V in different solvents in the pres-
ence of acetic acid (Fig. 10B). Compound 3 in water shows by
Fig. 8 Cyclic voltammograms of 1 mM 3 with increasing equivalents of
acetic acid (HOAc) in acetonitrile, with 0.1 M [nBu4N]+PF6− at 100 mV s−1
.
far the highest production of H2 with 4.86 mmol g−1 h−1
,
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Fig. 10 (A) Cyclic voltammograms of 1 mM 3 in acetonitrile (black), acetonitrile/water (4 : 1) (blue) and water (red) in absence (dashed lines) and
presence (solid lines) of acetic acid (HOAc) at 100 mV s−1. (B) Solvent-dependent GC quantification of the amounts of H2 generated by 1 and 3
during controlled potential coulometry for 3 minutes at −1.1 V vs. NHE in the presence of acetic acid.
10 times higher compared to the production of H2 in aceto- and enables the CO2 reduction at a more positive potential
nitrile (0.47 mmol g−1 h−1) and 40 times higher than in aceto- and an 18-fold increase in the H2 evolution capability of
nitrile/water (4 : 1) (0.12 mmol g−1 h−1). While the amounts of [Ni(dithiacyclam)]2+ 3 relative to [Ni(cyclam)]2+ 1. Based on CV
H2 generated by 1 in acetonitrile (0.08 mmol g−1 h−1) and studies, the improved catalytic performance of 3 can be attrib-
acetonitrile/water (4 : 1) (0.18 mmol g−1 h−1) are comparable to uted to the access of a thermodynamically facilitated low-
with compound 3, an 18-times lower H2 amount for 1 is found valent deposited catalytic active NiI species, which is hitherto
in water (0.27 mmol g−1 h−1). Overall, complex 3 serves as a present as an unknown amorphous, heterogeneous film on
catalyst precursor to a heterogeneous electrocatalyst for the H2 the electrode surface. The results reported herein thus clearly
evolution reaction that is significantly more active compared indicate the importance of sulfur due to tuning of the coordi-
to 1. The strong solvent dependency of the HER furthermore nation environment and with this, the catalytic and electronic
suggests that the catalytic activity most likely stems from a properties of the active sites.
molecular catalyst adsorbed on the surface of the glassy
carbon electrode. Since 3 showed the best catalytic perform-
ance in water, this system was chosen for a long-term measure-
ment in order to determine the TON, TOF and FE. For the
long-term measurement, a 0.5 mM solution of 3 with 350 mM
All reactions were performed under a dry Ar or N2 atmosphere
acetic acid and 1 M KCl as electrolyte was used in a two-
compartment cell, where the working and reference electrodes
are separated from the counter electrode via a Nafion mem-
brane. During CPC at −0.9 V vs. NHE for 20 h, the quantity of
produced H2 was measured every 30 minutes via an online GC
Experimental
General techniques
using standard Schlenk techniques or by working in a glove-
box. Starting materials and chemicals were obtained from
commercial suppliers and used without further purification.
Prior to their use, all solvents were dried and degassed accord-
ing to standard methods. 1,8-Dithia-4,11-diazacyclotetrade-
(Fig. S10†). Notably, the H2 production decreases with time
and via linear convergence, a catalytic collapse after roughly
cane (dithiacyclam) 2 was synthesized according to a literature-
known procedure with minor alterations.30 A synthetic descrip-
40 h is anticipated (Fig. S11†). Based on the GC data a TON of
613 with a TOF of 4.3 × 10−3 s−1 and a faradaic efficiency of
tion is provided within the SI part. Mass spectra were obtained
with a Bruker Daltonics Esquire 6000 instrument. UV/Vis/NIR
75% for the proton reduction catalyzed by 3 at −0.9 V vs. NHE
spectra were recorded with a JASCO V-670 at 25 °C and are
in water was obtained.
reported in [nm]. IR spectra were recorded with a Bruker
Tensor 27 FT-IR attached with a Pike Miracle ATR unit and are
reported in [cm−1]. CHN-analyses were measured with an
Conclusions
Elementar vario MICRO cube.
Caution! Perchlorate salts of metal complexes with organic
ligands are potentially explosive. They should be handled with
care and prepared only in small quantities.
In conclusion, we herein describe the synthesis and molecular
structure of [Ni(dithiacyclam)(NCCH3)2]2+ 3 and compare its
performance for the electrocatalytic reduction of protons and
CO2 relative to the intensively investigated [Ni(cyclam)]2+
complex 1. The replacement of the two nitrogen atoms oppo-
Synthesis of [Ni(dithiacyclam)(NCCH3)2](ClO4)2 (3)
site to each other in cyclam with sulfur atoms to form dithiacy- Dithiacyclam (200 mg, 0.84 mmol) was suspended in 10 mL
clam, induces a significantly altered electrochemical behavior toluene and Ni(ClO4)2·6H2O (312 mg, 0.84 mmol) dissolved in
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10 mL acetonitrile was added. First, the color of the reaction attached to a Bruker Tensor 27 FT-IR spectrometer with a Pike
mixture changed to purple and subsequently a purple precipi- Miracle ATR unit. A PalmSens3 was used as potentiostat with a
tate was formed. The mixture was stirred for 24 h. Afterwards, standard three-electrode setup (WE: glassy carbon electrode,
the mixture was filtered, the remaining solid was washed with RE: Ag wire, CE: Pt wire). All measurements were carried out in
toluene and dried in vacuum to afford 3 (360 mg, 87%) as a CO2 saturated acetonitrile/water (4 : 1) mixtures containing
−
purple solid. IR: ˜ν = 3264, 2922, 2874, 2309, 2284, 1473, 1419, 20 mM of the NiII species and 0.1 M [nBu4N]+PF6 as electro-
1366, 1309, 1083 cm−1. UV/Vis/NIR (MeCN): λ = 352, 552, 851, lyte. Prior to each experiment, the electrochemical cell was
925 (shoulder) nm. ESI-MS: calcd for [C10H22N2NiS2]+: 292.1 degassed with Ar for 10 min and an Ar atmosphere was main-
and [C10H22ClN2NiO4S2]+: 391.0; found: 292 and 391. EA: calcd tained throughout the measurement. Furthermore, the
for [C14H28Cl2N4NiO8S2 + 0.2 CH3CN]: C, 29.70%; H, 4.95%; N, working electrode was prepared by successive polishing with
10.10%; found: C, 29.85%; H, 5.16%; N, 9.78%.
1.0 and 0.3 µm alumina pastes and subsequent sonication.
The WE was placed 200 µm above the ATR crystal and the
potential was held for 800 s before the IR scan was initiated.
Generation of [Ni(dithiacyclam)(H2O)2](ClO4)2
A sample of complex 3 was dissolved in 5 mL water. After The IR measurement (128 scans) was completed in roughly
5 min stirring at room temperature, the solvent was removed 260 s.
in vacuum to afford a dark blue solid. The solid was used
without further purification. UV/Vis/NIR (H2O): λ = 371, 591,
X-ray absorption spectroscopy (XAS)
847
(shoulder),
962
nm.
ESI-MS:
calcd
for
XAS measurements were carried out on beamline 9-3 of the
Stanford Synchrotron Radiation Lightsource (SLAC National
Accelerator Laboratory, Menlo Park, CA, USA). The SPEAR3
storage ring was operated at 3.0 GeV and 500 mA with top-off
injection. A cryogenically cooled Si(220) (φ = 0°) double crystal
monochromator was used for energy selection. A Rh-coated
collimating mirror upstream of the monochromator set to a
13 keV cutoff rejected higher order harmonics, while a Rh-coated
toroidal mirror downstream of the monochromator focused
the beam in both vertical and horizontal planes. The beamline
was operated in a fully-tuned configuration. Sample tempera-
tures were maintained at 10 K using an Oxford Instruments
CF1208 continuous flow liquid helium cryostat. A Ni metal foil
was used for internal energy calibration, with the first inflec-
tion point of the reference foil edge set to 8333.0 eV. XAS data
were collected as fluorescence spectra using a 100 pixel mono-
lithic germanium detector (Canberra). A Co filter of six absorp-
tion lengths and Soller slits were used to reduce scatter and
maintain detector linearity. Samples were monitored for
photoreduction during data collection, and new spots exposed
as required. Tandem Mossbauer/XAS cups with a sample
window of 6 mm × 10 mm were used to provide 2–3 indepen-
dent beam spots on each sample. Averaging and normalization
of the XAS data was performed using Athena, a graphical
implementation of the IFEFFIT package.
[C10H22ClN2NiO4S2]+: 391.0; found: 391. EA: calcd for
[C10H26Cl2N2NiS2O10 + 0.15 CH3CN]: C, 23.16%; H, 4.99%; N,
5.64%; found: C, 23.24%; H, 4.76%; N, 5.74%.
Electrochemistry
The electrochemical studies were performed using a GAMRY
Reference 600 or a PalmSens3 potentiostat in a standard three-
electrode setup. A glassy carbon electrode was used as working
electrode (WE), a Ag wire as pseudo-reference or a conc.
Ag/AgCl as reference electrode (RE) and a Pt wire as counter
electrode (CE). The working electrode was prepared by succes-
sive polishing with 1.0 and 0.3 µm alumina pastes and
subsequent sonication. Tetrabutylammonium hexafluoro-
phosphate ([nBu4N]+PF6−, 0.1 M) was used as electrolyte in all
electrochemical measurements either in anhydrous acetonitrile
or a mixture of acetonitrile/water (4 : 1). For measurements in
pure water KCl (1 M) was used as electrolyte. Prior to each experi-
ment, the electrochemical cell was degassed with Ar for 10 min
and an Ar atmosphere was maintained throughout the measure-
ment. All cyclic voltammograms were recorded at a scan rate of
100 mV s−1 and all pseudo-referenced potentials were referenced
against the ferrocene/ferrocenium couple (Fc/Fc+). Controlled
potential coulometry (CPC) were performed at defined potentials
under otherwise identical conditions as reported above.
Quantification of the reduction products36
Electron dispersive X-ray (EDX) spectroscopy
Quantification of the headspace gas composition within the
H-type cell was performed using an Agilent Technologies SEM images and EDX spectra of the deposited species were
7820A gas chromatograph equipped with a thermal conduc- obtained using a ZEISS Gemini 2 Merlin HR-FESEM. The
tivity (TCD) and a flame ionization detector (FID) as well as a sample was placed on a conductive carbon-based carrier inside
methanizer. Gas separation was performed using a two- the vacuum chamber of the instrument. The SEM images were
column separation system (HP-PLOT Q 30 m × 0.53 mm × recorded with an operating voltage of 20 keV and the EDX
40 µm column & HP-Molesieve 5 Å 30 m × 0.53 mm × 25 µm) spectra were recorded with an operation voltage in the 0 to
using argon as the carrier gas.
20 keV range. In order to identify the elements and obtain
their mass fractions, the spectra were correlated to a data base.
The spot size during the EDX measurements amounted up to
Spectroelectrochemistry (SEC)
Infrared spectroelectrochemistry (IR-SEC) measurements were 2.5 µm in diameter and depth with an operating voltage of
carried out on a SP-02 cell (Spectroelectrochemistry Partners) 20 keV.
Dalton Trans.
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Paper
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with Paratone N oil, mounted on a fiber loop and placed in a
cold N2 stream on the diffractometer. A Bruker D8 Venture
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The structure was solved by intrinsic phasing;37 a difference
Fourier synthesis revealed disorder for the cationic complex, a
counter anion and the solvent region. The final structure model
was refined on F2.37 Two conformations of the macrocyclic ligand
involve disorder of the ethylene versus propylene bridges. A per-
chlorate anion was treated as disordered over two edge-sharing
alternative orientations. The sum of the occupancies for three
mutually exclusive sites of the solvent water molecule was
restrained to unity. All hydrogen atoms were introduced at
their idealized positions and refined using a riding model.
Crystallographic data as well as refinement parameters are pre-
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Conflicts of interest
14 A. A. Peterson, ECS Trans., 2015, 66, 41–52.
15 F. Möller, S. Piontek, R. G. Miller and U.-P. Apfel, Chem. –
Eur. J., 2018, 24, 1471–1493.
There are no conflicts to declare.
16 J. Qiao, Y. Liu, F. Hong and J. Zhang, Chem. Soc. Rev., 2014,
43, 631–675.
Acknowledgements
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This work was supported by the Fonds of the Chemical
Industry (Liebig grant to U.-P. A. and Kekulé fellowship to
P. G.) and the Deutsche Forschungsgemeinschaft (Emmy
Noether grant to U.-P. A., AP242/2-1 and AP242/5-1). K.R.
thanks financial support from the DFG (Cluster of Excellence
“Unifying Concepts in Catalysis”; EXC 314-2, and the
Heisenberg-Program). XAS studies at SSRL BL 9-3 were made
possible by the US DOE Office of Science (Contract No.
DE-AC02-76SF00515) and US NIH (P41-GM-103393 to SSRL SMB
Program and P30-EB-009998 to CWRU Center for Synchrotron
Biosciences). This work was supported by the Fraunhofer
Internal Programs under Grant No. Attract 097-602175.
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