Electrochemical CO2 Reduction-The Effect of Chalcogenide Exchange in Ni-Isocyclam Complexes

Article abstract of DOI:10.1021/acs.organomet.0c00129

Among the numerous homogeneous electrochemical CO₂ reduction catalysts, [Ni(cyclam)]²⁺ is known as one of the most potent catalysts. Likewise, [Ni(isocyclam)]²⁺ was reported to enable electrochemical CO₂ conversion but has received significantly less attention. However, for both catalysts, a purposeful substitution of a single nitrogen donor group by chalcogen atoms was never reported. In this work, we report a series of isocyclam-based Ni complexes with {ON₃}, {SN₃}, {SeN₃}, and {N₄} moieties and investigated the influence of nitrogen/chalcogen substitution on electrochemical CO₂ reduction. While [Ni(isocyclam)]²⁺ showed the highest selectivity toward CO₂ reduction within this series with a Faradaic efficiency of 86% for the generation of CO at an overpotential of-1.20 V and acts as a homogeneous catalyst, the O-and S-containing Ni complexes revealed comparable catalytic activities at ca. 0.3 V milder overpotential but tend to form deposits on the electrode, acting as precursors for a heterogeneous catalysis. Moreover, the heterogeneous species generated from the O-and S-containing complexes enable a catalytic hydride transfer to acetonitrile, resulting in the generation of acetaldehyde. The incorporation of selenium, however, resulted in loss of CO₂ reduction activity, mainly leading to hydrogen generation that is also catalyzed by a heterogeneous electrodeposit.

Full text of DOI:10.1021/acs.organomet.0c00129

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Article
Electrochemical CO2 Reduction  The Effect of Chalcogenide
Exchange in Ni-Isocyclam Complexes
Philipp Gerschel, Beatrice Battistella, Daniel Siegmund, Kallol Ray,* and Ulf-Peter Apfel*
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ABSTRACT: Among the numerous homogeneous electrochem-
ical CO2 reduction catalysts, [Ni(cyclam)]2+ is known as one of
the most potent catalysts. Likewise, [Ni(isocyclam)]2+ was
reported to enable electrochemical CO2 conversion but has
received significantly less attention. However, for both catalysts, a
purposeful substitution of a single nitrogen donor group by
chalcogen atoms was never reported. In this work, we report a
series of isocyclam-based Ni complexes with {ON3}, {SN3},
{SeN3}, and {N4} moieties and investigated the influence of
nitrogen/chalcogen substitution on electrochemical CO2 reduc-
tion. While [Ni(isocyclam)]2+ showed the highest selectivity
toward CO2 reduction within this series with a Faradaic efficiency
of 86% for the generation of CO at an overpotential of −1.20 V
and acts as a homogeneous catalyst, the O- and S-containing Ni complexes revealed comparable catalytic activities at ca. 0.3 V milder
overpotential but tend to form deposits on the electrode, acting as precursors for a heterogeneous catalysis. Moreover, the
heterogeneous species generated from the O- and S-containing complexes enable a catalytic hydride transfer to acetonitrile, resulting
in the generation of acetaldehyde. The incorporation of selenium, however, resulted in loss of CO2 reduction activity, mainly leading
to hydrogen generation that is also catalyzed by a heterogeneous electrodeposit.
the metal environment can be made selectively and
purposefully in order to understand the various effects on
the overall performance.15 Furthermore, and in contrast to
heterogeneous reactions, a plethora of spectroscopic techni-
ques are available to investigate the individual reaction steps
and electronics.16
Among the large number of homogeneous CO2RR electro-
catalysts that have been reported,17−21 [Ni(cyclam)2+] (Ni-N4,
cyclam = 1,4,8,11-tetraazacyclotetradecane; Scheme 1) has
INTRODUCTION
■
As the atmospheric concentration of carbon dioxideone of
the major greenhouse gasesrises and thus adversely affects
the ongoing climate change, the utilization and storage of CO2
has received ever-increasing attention.1−3 While the storage of
CO2 in porous materials such as MOFs or in geological
formations entails the risk of unforeseen, uncontrolled, and
concentrated release of the harmful gas and thus can only be
regarded as an interim solution if at all allowed by legal
regulations,3,4 its utilization circumvents this hazard and allows
the establishment of closed carbon cycles with CO2 as a
valuable carbon source in the chemical industry.5,6 Con-
sequently, the development of an efficient and sustainable
pathway for the selective conversion of CO2 is required.
Herein, the electrocatalytic reduction of CO2 (CO2RR) by
applying renewable energy sources is envisioned as a promising
approach.7 Thereby, a suitable electrocatalyst lowers the
energy barrier required for the activation of CO2 and controls
the selectivity toward different products of the carbon dioxide
reduction reaction.8,9 However, suitable and industrially
applicable electrocatalysts for the CO2RR remain rare.10−13
Although industrially relevant electrocatalysts are preferably
of heterogeneous nature,14 homogeneous systems are key
toward an understanding of essential features and underlaying
mechanisms for an efficient CO2RR catalyst. Notably,
molecular systems offer the advantage that modifications to
Scheme 1. Schematic Overview of Ni-N4, Ni-LN and Ni-N2S2
Special Issue: Organometallic Chemistry for Enabling
Carbon Dioxide Utilization
Received: February 21, 2020
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been shown to be particularly suitable for this type of
reaction.22 At a moderate potential of −1.05 V vs NHE at pH
4.1 in water and in the presence of a mercury working
electrode, the surface-adsorbed complex catalyzed the selective
reduction of CO2 to CO with a reaction rate of 32 h−1 and a
high Faradaic efficiency of 96%. Furthermore, the system
reveals an excellent stability and remains unaltered for more
than 8 h, reflecting 8000 turnovers.8,22−25
Scheme 2. Mustard Gas Containing (I) and Bypassing (II)
Synthesis Routes for the Generation of Isocyclam-Based
Macrocycles
Along this line, investigations on [Ni(isocyclam)]2+ (Ni-LN,
isocyclam = 1,4,7,11-tetraazacyclotetradecane), which is the
simplest structural isomer of Ni-N4 (Scheme 1), revealed the
CO2 to CO conversion at a 0.16 V anodically shifted potential,
unfortunately concomitant with a lower activity in comparison
to Ni-N4.26 These changes were attributed to the altered
geometry of Ni-LN, which exhibits a configuration between
that of square-planar NiII and tetrahedral NiI. The structural
reorganization energies are thus low during catalysis, and
furthermore the overall energetic demands for the CO2
conversion are minimized.
According to literature reports, the presence of sulfur in
ligand frameworks can be advantageous for electrochemical
and photochemical CO2 reduction.27−31 Along this line, we
recently showed that N/S heteroatom replacement within the
cyclam framework enabled the electrochemical reduction of
CO2 by [Ni(dithiacyclam)]2+ (Ni-N2S2, dithiacyclam =
1,8-dithia-4,11-diazacyclotetradecane; Scheme 1) at 0.2 V
more positive potentials.32 This reduced energy demand,
however, comes at the price of a loss in selectivity. Here, CO2
is reduced and affords CO, but reduction of protons also
occurs simultaneously, yielding significant amounts of hydro-
gen.
ring closure procedures, typically a combination of tosylamides
(NHTs) and tosylates (OTs) together with Cs+ templates are
utilized.38−40 Consequently, TsN(CH2CH2CH2OTs)2 (1) and
E(CH2CH2NHTs)2 are the synthons used herein to assemble
the macrocycles LO, LS, and LSe (Scheme 3). Similar to this
procedure and in contrast with literature procedures,26,41
isocyclam LN was synthesized via the novel synthesis route
(Scheme 3).
Synthesis of the Synthons. Dipropanolamine (2) was
synthesized by condensation of 2 equiv of 3-amino-1-propanol
with 1 equiv of 3-chloro-1-propanol and obtained as a colorless
oil in 53% yield.42 Tosylation of 2 with 3 equiv of tosyl
chloride subsequently afforded 1 as a colorless solid in 83%
yield.43 In parallel, reaction of diethylenetriamine with 3 equiv
of tosyl chloride afforded 3-N as a colorless solid in 79%
yield.44 Furthermore, the sulfur-containing compound 4 was
synthesized from cystamine dihydrochloride by tosylation with
2 equiv of tosyl chloride. Compound 4 was obtained in 94%
yield as a colorless solid.45
Next, the disulfide 4 was reduced in situ and immediately
treated with 2 equiv of 2-chloroethylamine to give 5 as a
colorless solid in 94% yield. Additional tosylation of 5 with 1
equiv of tosyl chloride afforded 3-S as a colorless solid in 86%
yield.
Reaction of 2-bromoethylamine hydrochloride and 1 equiv
of tosyl chloride gave 6 as a colorless solid in 93% yield.46
Treatment of selenium with 2 equiv of KBH4 in ethanol
provided [KHSe] in situ and was reacted with 2 equiv of 6 to
give 3-Se as a colorless solid in 99% yield.47
An analogous procedure was subsequently attempted to
synthesize the homologous tellurium compound 3-Te.
Notably, the reaction worked only after addition of a surplus
amount of water.
Subsequently, the reaction of [KHTe] and compound 6
afforded the desired compound 3-Te as an orange solid in 82%
yield.
Ring Closure Reaction. With the dipropyl framework 1 and
the compounds 3-N, 3-S, 3-Se, and 3-Te in hand, ring closure
reactions were carried out.38 In the presence of cesium
carbonate, tosylate 1 was added to a solution of tosylamides
3-N, 3-S, 3-Se, or 3-Te, respectively. After workup, the
respective tosylated macrocycles 7-N, 7-S, 7-Se, and 7-Te were
obtained as colorless solids in moderate to good yields (64−
85%).
These independent studies on the influence of geometrical
changes as well as on the role of sulfur in the Ni-cyclam
framework pointed our attention toward the role of the various
chalcogenides in the Ni-isocyclam structure in the reduction of
CO2. Consequently, we aimed for the preparation and
investigation of chalcogenide (O, S, Se, Te)-containing
isocyclam-based nickel complexes and their electrochemical
properties.
RESULTS AND DISCUSSION
■
Syntheses of the Macrocycles. For the synthesis of
chalcogenide (O, S, Se, Te)-containing isocyclam derivatives,
we herein established a general template-assisted ring closure
procedure for coupling symmetrical, functionalized diethyl and
dipropyl frameworks via condensation reactions in high
dilution. At first, the reaction of mustard gas derivatives with
commercially available bis(3-aminopropyl)amine (Scheme 2,
(I)) was conducted. Despite the ease of the mustard gas
syntheses33 as well as the simplicity of the synthetic
transformations, we discarded this pathway due to the severe
toxicity of the intermediates34 and aimed at finding an
alternative, straightforward, and easily performed synthetic
protocol.
Thus, the synthesis was modified by inverting the functional
groups of the diethyl and dipropyl frameworks (Scheme 2
(II)). Here the amines of the diethyl framework are poor
leaving groups and render a mustard gas like internal
cyclization reaction unlikely. Likewise, the cyclization of the
dipropyl framework is inhibited. The overall synthesis scheme
thus reveals significantly reduced danger.
While the presented synthesis route is suitable for the N-, S-,
Se- and Te-containing macrocycles and avoids the use of
mustard gas derivatives, synthesis of the O-containing
The E(CH2CH2NH2)2 framework is easily accessible for a
variety of different heteroatoms.35−37 Moreover, for similar
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a
Scheme 3. Syntheses of Isocyclam and the Chalcogen-Containing Isocyclam Derivatives
a
Conditions: (a) H2O, reflux, 24 h;42 (b) DCM, NEt3, TsCl, 0 °C, 3 h → rt, 12 h;43 (c) (1) NaOH, H2O, (2) TsCl, DCM, rt, 12 h;45 (d) (1) PPh3,
THF/H2O, reflux, 48 h, (2) NaOH, 2-chloroethylamine, reflux, 12 h; (e) (1) NaOH, H2O, (2) TsCl, DCM, rt, 12 h; (f) (1) TsCl, DCM, 0 °C, (2)
NEt3, 0 °C, 1 h;46 (g) [E = Se] EtOH, reflux, 2 h;47 (h) [E = Te] EtOH/H2O, reflux, 2 h; (i) [E = Se] EtOH, reflux, 12 h;47 (j) [E = Te] EtOH/
H2O, reflux, 12 h; (k) Cs2CO3, DMF, rt, 3 days; (l) [E = NTs] concentrated H2SO4, 100 °C, 5 days; (m) [E = O, S] phenol, 33% HBr in HOAc,
reflux, 72 h; (n) [E = Se] (1) Na-naphthalenide, DME, −45 °C, (2) NaHCO3, −45 °C → rt, 12 h.
macrocycle 7-O by the same procedure proved to be laborious.
Here, a mustard gas derivative cannot be circumvented for the
synthesis of O(CH2CH2NTs)2 (Scheme 4). Macrocycle 7-O
was therefore synthesized following the literature procedure
depicted in Scheme 3.41
All compounds were analyzed by NMR spectroscopy and
electrospray-ionization mass spectrometry (ESI-MS) to
support their chemical nature (see the Supporting Informa-
tion). In addition, crystals of 7-S were obtained by
crystallization from acetonitrile (Figure 1) and further support
the successful ring closure experiments.
Deprotection. To afford the macrocycles LN, LO, LS, and
LSe, suitable deprotection protocols had to be established. To
enable deprotection of 7-N, the macrocycle was heated in
concentrated H2SO4, affording LN as a colorless solid in 87%
yield. The same procedure was not successful for compounds
Scheme 4. Synthesis Route for the Generation of
O(CH2CH2NHTs)2
Figure 1. Molecular structure of the sulfur-containing tosylated
macrocycle 7-S (30% ellipsoid probability, H atoms omitted for
clarity). Color code: C, black; N, blue; O, red; S, yellow.
C
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Scheme 5. Syntheses of the Complexes Ni-LN, Ni-LO, Ni-LS, and Ni-LSe
7-O and 7-S. Here, deprotection was carried out in HBr (30%
in glacial acetic acid) under reflux conditions and yielded 1-
oxa-4,8,12- triazacyclotetradecane LO and 1-thia-4,8,12-triaza-
cyclotetradecane LS as brown solids in 49% and 77% yields,
respectively.
Table 1. Characteristic IR and UV/Vis/NIR Spectroscopic
Features of Complexes Ni-LN, Ni-LO, Ni-LS, and Ni-LSe
UV/vis/NIR λ (nm)
IR ν
̃
(cm−1) solid
MeCN
335
499
714
349
538
626
356
563
826 (sh)
916
357
567
830 (sh)
926
H2O
Ni-LN
Ni-LO
Ni-LS
3275
2309
2278
3269
2310
2280
3258
2311
2282
349
464
720
359
571
672
366
593
This method was also applied to achieve deprotection of
compound 7-Se but resulted in decomposition of the
macrocycle. Consequently, deprotection of 7-Se was attempted
under milder conditions48 via reaction with an in situ generated
sodium naphthalenide suspension at −45 °C, affording 1-
selena-4,8,12-triazacyclotetradecane LSe as a brown solid in
78% yield. Upon detosylation, the 77Se NMR signal shifted
from 144.2 ppm (7-Se) to 44.9 ppm (LSe) (Figure S1).
Likewise, deprotection of 7-Te was attempted by the methods
described above. Hitherto, generation of 1-tellura-4,8,12-
triazacyclotetradecane was not feasible, as either the macro-
cycle decomposed along with precipitation of elemental
tellurium or a mixture of multiple compounds was observed
(Figure S2).
972
373
605
Ni-LSe
3256
2309
2282
978
All macrocycles LN, LO, LS, and LSe were analyzed by NMR
spectroscopy and mass spectrometry. Especially, 13C{1H}
NMR spectroscopy was found to be a valuable tool to
investigate trends among the four tosylated homologous
chalcogenide macrocycles 7-O, 7-S, 7-Se, and 7-Te (Figure
S3A). The chemical shifts of the carbon atoms neighboring the
chalcogens gradually decrease within the group from 72.7 ppm
(O), to 31.4 ppm (S), 22.6 ppm (Se), and 2.5 ppm (Te),
indicating the increase in electron density as well as the
electron-donating effect within this series. A similar trend is
likewise observed for the free macrocycles: 13C{1H} NMR
signals are in the order 68.9 ppm (LO), 31.8 ppm (LS), and
25.0 ppm (LSe) (Figure S3B).
Moreover, comparison of the N−H stretching frequencies
around 3265 cm−1 reveals a bathochromic shift from Ni-LO
over Ni-LS to Ni-LSe. This bathochromic shift can be explained
by an increase of electron density at the nickel centers due to
an increasing electron-donating character within the higher
chalcogen congeners (Table 1). Furthermore, bands at ∼3475
and 1643 cm−1 for Ni-LS and Ni-LSe, which grow in intensity
and also appear for Ni-LN and Ni-LO over time, suggest that
the complexes enable a water/acetonitrile ligand exchange, as
was reported earlier for the comparable [Ni-S2N2]2+
complex.32
Syntheses and Characterization of the Ni Complexes.
Isocyclam ligands LN, LO, LS, and LSe were further reacted with
equimolar amounts of [Ni(ClO4)2]·6H2O in acetonitrile in
order to generate their corresponding nickel complexes
(Scheme 5). Purification was performed via column
chromatography of the pure complexes with acetonitrile as
the eluent. The desired Ni complexes were obtained in 92%
([Ni(LN)(NCCH3)2](ClO4)2 (Ni-LN), red solid), 80% ([Ni-
(LO)(NCCH3)2](ClO4)2 (Ni-LO), blue solid), 90% ([Ni(LS)-
(NCCH3)2](ClO4)2 (Ni-LS), purple solid), and 50% yields
([Ni(LSe)(NCCH3)2](ClO4)2 (Ni-LSe), purple-green solid).
The successful complex formation was supported by ESI-
MS, revealing characteristic peaks at m/z 356.9 (Ni-LN), 358.2
(Ni-LO), 374.1 (Ni-LS), and 422.1 (Ni-LSe) for the
corresponding [Ni(LE)(ClO4)]+ fragments (Figure S4).
Furthermore, all complexes show the presence of two
characteristic CN stretching frequencies around 2280 and
2310 cm−1 in their IR spectra, indicating the additional binding
of acetonitrile at the nickel center (Table 1 and Figure S5).
UV/vis/NIR measurements were thus performed in both
water and acetonitrile to account for the acetonitrile/water
ligand exchange, as anticipated from IR measurements. These
measurements are furthermore important to give a more
realistic picture of the electronic and geometric structures of
the Ni complexes present during electrochemical studies,
which are performed in acetonitrile/water mixtures.
Notably, while all nickel complexes reveal a violet color in
acetonitrile, the complexes can be classified into two categories
with slightly different properties. Complexes Ni-LN and Ni-LO
reveal a similar band structure showing d−d transition bands at
499 and 714 nm as well as at 538 and 626 nm, respectively,
and complexes Ni-LS and Ni-LSe both reveal characteristic
bands at 563 and 826/916 nm as well as at 567 nm and 830/
926 nm, respectively (Figure 2 top, Figure S6, and Table 1).
This observation suggests that Ni-LN and Ni-LO as well as Ni-
LS and Ni-LSe have comparable structural features and
electronics. Therefore, the sulfur/selenium complexes differ
from their nitrogen and oxygen macrocyclic counterparts.
D
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Figure 2. UV/vis/NIR spectra of complexes Ni-LN, Ni-LO, Ni-LS, and
Ni-LSe in acetonitrile (top) and water (bottom).
In comparison, the UV/vis/NIR spectra in water reveal
different features. In particular, the UV/vis/NIR spectrum of
Ni-LN reveals the most striking changes, which were already
noticeable by a color change to red. In water, the absorption
band at 464 nm becomes the dominating spectroscopic feature
due to a severe change in the coordination environment
(Figure S6A).26,49 The metal center possesses a square-planar
coordination environment upon addition of water, in contrast
to the octahedral environment found in acetonitrile. The band
structures and violet color of complexes Ni-LO, Ni-LS, and Ni-
LSe remained unaffected from the solvent change (Figure
S6B−D). However, a bathochromic band shift of the d−d
bands is observed in all cases and likewise suggests small
electronic changes without major structural alterations (Figure
2 bottom).
Figure 3. Structural motifs of the complexes Ni-LN, Ni-LO, Ni-LS, and
−
Ni-LSe (30% ellipsoid probability, H atoms and ClO4 counterions
omitted for clarity).
while larger donor atoms with higher trans directing capability
such as S and Se force the complex into a cis configuration.
Electrochemical Studies. All isocyclam-based nickel
complexes Ni-LN, Ni-LO, Ni-LS, and Ni-LSe were characterized
by cyclic voltammetry (CV) in dry acetonitrile with 0.1 M
[nBu4N]PF6 as the supporting electrolyte (Figure 4).
Additional EPR studies of the nickel complexes Ni-LN, Ni-
LO, Ni-LS, and Ni-LSe in frozen acetonitrile, water, and
acetonitrile/water (4:1) mixtures revealed no observable EPR
signal pointing to the presence of NiII centers.50,51
Crystallization of compounds Ni-LN, Ni-LO, Ni-LS, and Ni-
LSe from acetonitrile yielded the compounds as violet single-
crystal materials. Although the X-ray data are partially of poor
quality and therefore do not provide reliable information on
bond lengths and angles (Table S1), they reveal the
connectivity and present a structural motif of the macrocyclic
complexes (Figure 3).
While the octahedral coordination environment including
two acetonitrile ligands and the macrocyclic ring are present in
all complexes, as was predicted from the UV/vis/NIR spectra
(Figure 2), the orientation of the macrocyclic ligand is
dependent on the nature of the implemented chalcogen atom.
In Ni-LN and Ni-LO the acetonitrile ligands bind in the axial
positions, and the macrocycle coordinates in a square-planar
fashion. In contrast to Ni-LN and Ni-LO and in line with the
altered UV/vis/NIR features, the structures of complexes Ni-
LS and Ni-LSe reveal the macrocycle in a folded binding mode
and a cis binding of the acetonitrile ligands. This finding
suggests that the altered donor character and the size of donor
atoms inside the macrocycle define the coordination environ-
ment. Herein, N and O preferentially form the trans isomer,
Figure 4. Cyclic voltammograms of 2 mM solutions of complexes Ni-
LN, Ni-LO, Ni-LS, and Ni-LSe in acetonitrile on a glassy-carbon
working electrode, with 0.1 M [nBu4N]PF6 at 100 mV s−1.
The CV of Ni-LN shows a NiII/NiI redox couple at −1.67 V
vs Fc/Fc+, which is in line with the literature.26 While the
change of the macrocyclic ligand from Ni-LN to Ni-LO does
not affect the cathodic peak potential significantly with a Epc
value of −1.70 V vs Fc/Fc+, the Epc value of the NiII/NiI couple
of Ni-LS shifts to −1.59 V vs Fc/Fc+, a comparably milder
potential in comparison to that in Ni-LN, which is presumably
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caused by the higher electron density of the softer heteroatom.
Furthermore, Ni-LS reveals a second oxidation signal at
−0.30 V vs Fc/Fc+, which can be attributed most likely to
an electrode surface deposited nickel species. The analysis of
Ni-LSe and the associated further increase in electron density
lead to an additional anodic shift of the cathodic NiII/NiI peak
potential to −1.52 V vs Fc/Fc+. Notably, similarly to Ni-LS,
Ni-LSe also shows an oxidative stripping signal. However, in
comparison to Ni-LS the intensity of the signal increases
drastically and shifts to −0.23 V vs Fc/Fc+. Furthermore, in
order to investigate the reversibility of the NiI/NiII redox
couples of the nickel complexes, scan-rate-dependent studies
were carried out (Figures S7−S10). When the peak currents
are plotted against the square root of the scan rate, the ratio of
the slopes for the anodic and cathodic peak currents can be
used to deduce the reversibility of the system. For an ideal
reversible system, the ratio of the slopes would be 1. The closer
the ratio of the slopes is to 0, the stronger is the irreversible
character of the system. Even though none of the nickel
complexes are completely reversible, Ni-LN reveals the highest
reversibility for the NiII/NiI couple with a ratio of 0.61,
followed by Ni-LS with a ratio of 0.46. On the other hand, Ni-
LO and Ni-LSe are fully irreversible, as no reoxidation to NiII
was observed even at increased scan rates.
Furthermore, the electrochemical properties of complexes
Ni-LN, Ni-LO, Ni-LS, and Ni-LSe were analyzed in an
acetonitrile/water mixture (4/1) (Figure 5 top), since the
electroreduction of CO2 requires a proton source and is usually
performed in this electrolyte. The potential of the NiII/NiI
redox couple of Ni-LN stays the same (−1.67 V vs Fc/Fc+) in
acetonitrile/water. Likewise, the cathodic peak potentials of
the NiII/NiI couples of the complexes Ni-LO, Ni-LS, and Ni-LSe
also remain mostly unaffected. Furthermore, the presence of
water causes a decrease in the oxidative stripping signal of Ni-
LS. Similarly, the voltammogram of Ni-LSe shows a more
defined shape in acetonitrile/water and the oxidation signal of
the deposited species at −0.24 V vs Fc/Fc+ also decreases in
intensity. In order to investigate if the oxidative signals for Ni-
LS and Ni-LSe are due to an electrodeposited surface species,
the potential of a 2 mM nickel complex solution was held at
the corresponding Epc value of the complex for 5 and 10
minutes, respectively. Notably, after these experiments the
oxidative signal at −0.24 V vs Fc/Fc+ increases with longer
electrolysis time and supports the presence of a surface-
deposited species (Figure S11). While additional scan-rate-
dependent reversibility studies (Figure S12−S15) showed a
decreased reversibility of the NiII/NiI redox couple in the
presence of water for Ni-LN (slope ratio 0.53) and Ni-LS
(slope ratio 0.26), Ni-LO (slope ratio 0.19) reveals a higher
reversibility in the presence of water. The irreversibility of Ni-
LSe remains unchanged in water/acetonitrile. Moreover, in the
presence of water, all complexes reveal an increased current,
which is most likely a result of the generation of the NiI species
with a concomitant catalytic hydrogen formation.
Figure 5. (top) Cyclic voltammograms of 2 mM solutions of
complexes Ni-LN, Ni-LO, Ni-LS ,and Ni-LSe on a glassy-carbon
working electrode with 0.1 M [nBu4N]PF6 in acetonitrile/water (4/1)
at 100 mV s−1 under an argon atmosphere. (bottom) Linear sweep
voltammograms of 2 mM solutions of complexes Ni-LN, Ni-LO, Ni-LS,
and Ni-LSe on a glassy-carbon working electrode with 0.1 M
[nBu4N]PF6 in CO2-saturated acetonitrile/water (4/1) at 100 mV s−1.
Table 2. Electrochemical and Spectroelectrochemical IR
Data of Complexes Ni-LN, Ni-LO, Ni-LS, and Ni-LSe
cathodic peak potential Epc vs
Fc/Fc+ (V)
MeCN MeCN/H2O (4/1) IR ν
̃
(cm−1) MeCN/H2O (4/1)
Ni-LN
Ni-LO
−1.73
−1.72
1984
1840
2158
−1.70
−1.72
1977
1828
2160
1987
1836
Ni-LS
−1.59
−1.52
−1.60
−1.54
Upon purging CO2 through acetonitrile/water (4/1)
solutions of complexes Ni-LN, Ni-LO, Ni-LS, and Ni-LSe, the
shape of the CV changed drastically in comparison to the
measurements under an argon atmosphere (Figure 5 bottom).
For Ni-LN, Ni-LO, Ni-LS, and Ni-LSe new diffusion-limited
signals can be observed. The reduction peak of the catalytic
conversion obviously is in the same potential range as the
cathodic peak current of the corresponding complex (Table 2).
After this initial reduction, catalytic response is observed at
Ni-LSe
2160
2035 (w)
1964 (w)
−2.14 V vs Fc/Fc+ 150 μA for Ni-LN, at −1.86 V vs Fc/Fc+
112 μA for Ni-LO, at −1.84 V vs Fc/Fc+ 122 μA for Ni-LS,
and at −1.70 V vs Fc/Fc+ 115 μA for Ni-LSe. Once more, the
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Figure 6. Spectroelectrochemical IR spectra in the CO region of 20 mM solutions of complexes Ni-LN, Ni-LO, Ni-LS, and Ni-LSe in acetonitrile/
water (4/1) saturated with CO2 and recorded during chronoamperometry at −1.75 V vs Fc/Fc+.
influence of the chalcogen is visible by the potential shift of the
catalytic signal to more positive values with increasing electron
density of the chalcogen atom. To elucidate the nature of the
catalytic species, rinse tests were performed at the correspond-
ing catalytic potentials for all nickel complexes (Figure S16). In
case of Ni-LN the rinse test was negative, consequently
revealing a homogeneous species to be catalytically active.
However, in case of all chalcogen-containing complexes Ni-LO,
Ni-LS, and Ni-LSe the rinse test was positive, hence suggesting
an electrodeposited catalytically active surface species rather
than a homogeneous catalysis. Although the change in shape
and the increase in current are good indicators for the
occurrence of any CO2RR, the hydrogen evolution reaction is a
typical parasitic reaction and final proof for a successful CO2
conversion must be determined via spectroelectrochemical
infrared spectroscopy as well as product analysis.
Spectroelectrochemical IR Studies. In an effort to verify
the potential of the compounds to act as precatalysts and to
identify nickel carbonyl intermediates generated during the
CO2RR, SEC-IR experiments were performed with complexes
Ni-LN, Ni-LO, Ni-LS, and Ni-LSe. For this purpose, an
electrochemical ATR cell attached to an IR spectrometer was
utilized and all measurements were subsequently performed in
CO2-saturated acetonitrile/water mixtures (4/1) containing
20 mM of the corresponding nickel complex and 0.1 M
[nBu4N]PF6 as electrolyte. The potential was held at −1.75 V
vs Fc/Fc+ before the IR scan was initiated.
Upon application of a constant potential, the IR spectrum of
Ni-LN shows two distinct new IR bands in the CO region at
1984 and 1840 cm−1 that can be assigned to low-valent Ni−
CO species, hence supporting the potential of Ni-LN to
catalyze the CO2 reduction (Figure 6).8,32 While the signal at
higher wavenumbers most likely represents a NiI−CO species
with a terminal CO bond, the signal at lower wavenumbers is
typically observed for bridging carbonyls.52 Likewise, nickel
complexes Ni-LO (1977, 1828 cm−1) and Ni-LS (1987,
1836 cm-1) show comparable CO bands, although at slightly
shifted wavenumbers, also indicating an ongoing CO2RR
(Figure 6 and Table 2). Additionally, complexes Ni-LO and Ni-
LS reveal a third CO frequency at higher wavenumbers around
2160 cm−1 that can be assigned to gaseous carbon monoxide
(2150 cm−1).53 In contrast, the complex Ni-LSe reveals only
weak CO bands around 2160 and 2035−1964 cm−1, indicating
low CO2RR activity. All Ni−CO species observed herein are
most likely inhibited or poisoned states, as the diffusion time
required for the formed intermediate to get from the electrode
to the ATR crystal is quite long and the lifetime of the highly
reactive intermediates is much shorter.
Nature of the Electrodeposited Species. In contrast to
Ni-LN, the chalcogen-containing complexes Ni-LO, Ni-LS, and
Ni-LSe do not act as homogeneous electrocatalysts. They rather
act as precursor molecules that produce an electrochemically
deposited catalytically active species, as was shown by our rinse
tests. In order to analyze the composition of the electrochemi-
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Figure 7. STEM images and EDX mappings of the electrodeposited species, using complexes Ni-LO (A), Ni-LS (B), and Ni-LSe (C) as precursors.
Color code: C, gray; N, blue; O, red; S, yellow; Se, brown; Ni, magenta.
cally generated heterogeneous materials and consequently to
further support the influence of the different chalcogens on the
electrochemical performance, TEM/EDX analysis was per-
formed.
the selectivity of the CO2RR, additional electrolysis experi-
ments were performed (Figure S21). A 2 mM solution of the
corresponding nickel complexes Ni-LN, Ni-LO, Ni-LS, and Ni-
LSe in CO2-saturated acetonitrile/water (4/1) with 0.1 M
[nBuN4]PF6 as electrolyte was used. During CPC at a defined
potential, gaseous products were quantified after 2, 4, and 6 h
via GC-MS analyses. Additionally, the liquid phase was
analyzed after 6 h. Note, since electrodeposited surface species
are also responsible for the catalytic activity of the nickel
complexes, no information about the concentration or number
of active sites is available. Consequently, a calculation of
turnover numbers or turnover frequencies is not possible. In
order to compare the effectivity of different (precursor)
complexes, the ppm values of the products formed are given in
Table 3.
Thus, controlled-potential coulometry (CPC) at −1.50 V vs
Fc/Fc+ for 12 h was conducted with solutions of Ni-LO, Ni-LS,
and Ni-LSe (2 mM) in acetonitrile/water (4/1). In the case of
Ni-LO a light yellow-green solid was observed on the glassy-
carbon electrode. In the case of Ni-LS, the observed solid was
light yellow, and in the case of Ni-LSe the solid was brown. The
subsequent analysis of the electrodeposited materials via
scanning transmission electron microscopy (STEM) with
integrated energy-dispersive X-ray spectroscopy (EDX)
revealed the generation of an amorphous, heterogeneous
multielement film (Figure 7 and Figure S17) and was not
solely the deposition of metallic nickel. Since carbon is the
background of the samples, no reasonable statement about its
proportion can be done. However, as seen from the elemental
mappings, a significant amount of carbon is also included in
the deposited materials (Figure 7).
Table 3. Quantification of the Electrocatalytically Generated
CO2RR Products and Hydrogen by GC-MS
a
a
FEH
time
(h)
CO
(ppm)
FECO
(%)
2
E vs Fc/Fc+ (V)
−2.14
(%)
The solid generated from Ni-LO revealed next to nickel
significant amounts of carbon, oxygen, and nitrogen with an
elemental ratio of Ni1O8.6N1.4. For Ni-LS additionally sulfur
was observed and the solid showed an elemental ratio of
Ni1S1.4O10.2N10.2. In the case of Ni-LSe, selenium was observed
and the solid showed an elemental ratio of Ni1Se1O6.6N1.1.
Although Ni-LS and Ni-LSe do not contain oxygen in their
macrocyclic ligands, the EDX analysis of the corresponding
deposited materials showed its presence. Moreover, the
amount of oxygen in the deposited material of Ni-LO is
quite high. These high O contents can be most-likely reasoned
by ex situ oxidation of the low-valent highly reactive catalytic
intermediates with molecular oxygen prior to STEM. Never-
theless, for the other two samples, the Ni:S/Se ratio will be
unaffected by such ex situ processes. In the case of Ni-LS and
Ni-LSe, the ratio of nickel to chalcogen of the deposited
material is close to 1:1, as in the precursor complexes.
Consequently, it can be concluded that the chalcogens are
likewise incorporated into the deposited films. This finding is
further supported by a more detailed STEM/EDX analysis. In
situ electrochemical STEM analysis of a 5 mM Ni-LS solution
in acetonitrile/water (4/1) showed the rapid growth of an
electrodeposited material on a glassy-carbon electrode upon
electrolysis. Additional EDX analysis of this in situ deposited
material showed likewise a nickel−sulfur ratio of almost 1:1
(Figures S18−S20). Changes observed in the electrocatalysis
are thus also a result of the different substitution patterns of
the original precursor system.
Ni-LN
Ni-LO
Ni-LS
Ni-LSe
2
4
6
2
4
6
2
4
6
2
4
6
964
1941
2200
715
1606
1569
539
802
1468
33
84
87
66
46
56
21
70
50
24
5
15
12
16
52
43
30
28
49
41
94
96
75
−1.86
−1.84
−1.70
51
45
2
1
a
Only the charge of catalytic processes was used for the calculation of
the Faradaic efficiencies. The quantification after 2 and 4 h is
restricted to gaseous compounds, while the quantification after 6 h
also includes liquid products. In all experiments a concentration for all
Ni complexes of 2 mM was used.
The CO2RR selectivity was then calculated and is reflected
by the Faradaic efficiencies (FE) obtained for the individual
products utilizing the nickel complexes Ni-LN, Ni-LO, Ni-LS,
and Ni-LSe (Table 3 and Figure S22). Within the first 4 h at a
potential of −2.14 V vs Fc/Fc+ and an overpotential of 1.20 V
for the generation of CO, Ni-LN revealed a FE of ∼86% for
CO and ∼14% for the generation of H2. After an additional
2 h, the selectivity changed and H2 was observed at a slightly
elevated FE of 16% at the expense of decrease in CO selectivity
(66% FE). Furthermore, the liquid phase revealed ethanol and
acetaldehyde (absolute amount 35 μL) each with a FE of 8%.
Electrocatalytic CO2RR Product Quantification. In
order to determine the formed products and the activity and
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The last two products, however, are not a result from CO2
reduction but rather originate from reduction of acetonitrile.
In comparison to Ni-LN, the selectivity for the generation of
CO deteriorates for the O-containing complex Ni-LO. Within
the first 4 h at a potential of −1.86 V vs Fc/Fc+ and an
overpotential of 0.92 V for the generation of CO, Ni-LO shows
an average FE of 51% for the generation of CO, while the
evolution of H2 is observed with a FE of 48%. After additional
2 h of CPC, the ratio between CO and H2 shifts from nearly
50:50 to a 40:60 ratio with enhanced generation of H2.
However, the amounts of CO and H2 sum only to 51% FE,
since acetaldehyde was found as a further product with a FE of
42% and an absolute amount of 412 μL in the liquid phase.
Again, the latter product is not a result of the CO2RR but
rather stems from the electrochemical hydrogenation of
acetonitrile.
unprecedented O-, S-, and Se-containing nickel complexes
Ni-LO, Ni-LS, and Ni-LSe along with their performances in the
electrocatalytic reduction of CO2. Herein, incorporation of the
different chalcogenide atoms induces a significantly altered
electrochemical behavior of the nickel complexes in compar-
ison to Ni-LN. A major effect of the chalcogen incorporation in
the isocyclam derivatives is evidenced by the deposition of the
Ni chalcogenide derived heterogeneous active species on the
electrode surface. Depending on the specific precursor
complex, stoichiometrically different deposits involving carbon
and the respective chalcogenide are formed on the electrode
surface, as could be proven by STEM and EDX measurements.
With increasing electron density of the chalcogen, the
generation of the catalytically active species becomes more
feasible and appears at more positive potentials. Moreover,
upon addition of CO2 the electrodeposited material of the
precursor complexes Ni-LO and Ni-LS revealed a facilitated
CO2RR at 0.3 V milder potentials in comparison to Ni-LN,
however, at the price of loss in selectivity for the CO2
reduction. However, in comparison to Ni-LN, a new reaction
pathway is opened and hydride transfer to acetonitrile leading
to the generation of acetaldehyde is observed. Notably,
incorporation of selenium leads to a total loss in activity
regarding CO2 reduction and the electrodeposited complex Ni-
LSe reveals a high activity for the hydrogen evolution reaction.
For the higher homologue Ni-LS, electrolysis at −1.84 V vs
Fc/Fc+ and an overpotential of 0.94 V for the generation of
CO resulted in a FE of 70% for CO and a FE of 28% for H2
after the first 2 h. However, as time went on, the distribution of
products changed drastically. After 4 h, the CO:H2 ratio shifted
to ca. 50:50 and after further 2 h of electrolysis a ratio of CO to
H2 of 37:63 with a total FE of 65% for both products was
observed. Likewise, Ni-LS revealed acetaldehyde with a FE of
28% and an absolute amount of 240 μL in the liquid phase
after 6 h of CPC.
Going further down in the chalcogen group results in
complete loss of the CO2RR activity, as was already expected
from the SEC-IR experiment. The selenium-containing
complex Ni-LSe herein shows a FE of 95% for the generation
of H2 and only 4% FE for the production of CO after 4 h at
−1.70 V vs Fc/Fc+ and an overpotential of 0.76 V for the
generation of CO. Additionally, after 6 h of CPC Ni-LSe
revealed a FE of 1% for CO, 75% for H2, 7% for ethanol, and
10% for acetaldehyde (absolute amount 40 μL).
In order to clarify the origin of the observed acetaldehyde,
electrolysis experiments with Ni-LO as catalyst were performed
in a CO2-saturated mixture of deuterated acetonitrile with
water (4/1). Analysis of the liquid phase via GC-MS revealed
the presence of deuterated acetaldehyde, hence confirming
acetonitrile as the origin rather than CO2 (Figure S23).
Although the selectivity for the CO production diminishes
with heavier chalcogens present in the catalytically active
material, the absolute catalytic activity, in terms of CO
concentration detected, is on the same order of magnitude for
Ni-LN, Ni-LO, and Ni-LS (Table 3). Furthermore, the overall
catalytic activity of the deposited materials generated from Ni-
LO and Ni-LS is drastically increased in comparison to Ni-LN
and, along with CO, major amounts of H2 and acetaldehyde
are generated during catalysis. Consequently, along with the
parasitic hydrogen evolution, the chalcogen-containing electro-
deposited film of the precursor complexes Ni-LO and Ni-LS
enable another competing reaction pathway, the hydro-
genation of nitriles. Moreover, the potential required to drive
the CO2RR at comparable currents is lowered drastically by
about 0.3 V for the electrodeposited materials of the precursor
complexes Ni-LO and Ni-LS.
EXPERIMENTAL SECTION
■
General Considerations. All reactions were performed under a
dry Ar or N2 atmosphere using standard Schlenk techniques or by
working in a glovebox. Starting materials and chemicals were obtained
from commercial suppliers and used without further purification. All
solvents were dried and degassed according to standard methods.54
Thin-layer chromatography was performed using Merck TLC
aluminum sheets, silica gel 60 F254. For column chromatography,
silica gel with a pore size of 60 Å from Acros Organics was used. The
used solvent mixtures are given by volume fractions. Bis(3-
tosyloxypropyl)tosylamide (1),43 dipropanolamine (2),42 diethylene-
triamine tritosylate (3-N),44 N,N′-(dithiodi-2,1-ethanediyl)bis-p-
toluenesulfonamide (4),45 N,N′-[selenobis(ethane-2,1-diyl)]bis-p-tol-
uenesulfonamide (3-Se),47 N-(2-bromoethyl)-p-toluenesulfonamide
(6),46 and 4,8,12-tritosyl-1-oxa-4,8,12-triazacyclotetradecane (7-O)41
were synthesized according to literature-known procedures with
minor alterations. 1H and 13C{1H} spectra were recorded with a
Bruker DPX-200 NMR, a Bruker DPX-250 NMR, or a Bruker AVIII-
300 NMR spectrometer, 77Se NMR spectra were recorded with a
Bruker DPX-250 NMR, and all are reported in parts per million
(ppm). Mass spectra were obtained with a Bruker Daltonics Esquire
6000 instrument. UV/vis/NIR spectra were recorded with a JASCO
V-670 instrument at 25 °C and are reported in nm. IR spectra were
recorded with a Bruker Tensor 27 FT-IR instrument attached to a
Pike Miracle ATR unit and are reported in cm−1. EPR spectra on
frozen solutions were recorded at T = 13 K on a Bruker EMX-plus X-
band spectrometer in perpendicular mode. The microwave frequency
amounted to 9.35 GHz with a field center of 3000 G and a sweep
width of 5500 G. CHN analyses were measured with an Elementar
vario MICRO cube. Scanning transmission electron microscopy
(STEM) was performed using a JEOL microscope (JEM-2800) with a
Schottky-type emission source working at 200 kV. EDX mapping was
performed with the equipped double SDD detectors, with a solid
angle of 0.98 steradian with a detection area of 100 mm2.
Caution! Perchlorate salts of metal complexes are potentially
explosive. They should be handled with care and prepared only in
small quantities.
Caution! Mustard gases are highly toxic blister agents, which cause
severe injury to the skin, eyes, or any other tissue they contact. They
should be handled with great care and prepared only in small
CONCLUSION
■
In summary, we herein describe a novel synthesis route for the
generation of isocyclam LN and the chalcogenide-containing
macrocycles LS, LSe and 7-S, 7-Se, 7-Te. In addition, we
showcase the synthesis and characterization of hitherto
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quantities. Moreover, all glassware should be treated with calcium
hypochlorite afterward.
NaOH to the cooled reaction mixture. A solution of 2-chloroethyl-
amine hydrochloride (5.8 g, 50.2 mmol) dissolved in 26.3 mL of 2 M
deoxygenated NaOH was added dropwise to the mixture, and it was
refluxed for an additional 12 h. Subsequently, the reaction mixture
was cooled to room temperature and the pH was adjusted to 2 using
6 M HCl. The organic layer was separated from the aqueous layer,
and the aqueous layer was washed with chloroform (2 × 40 mL). The
aqueous layer was then adjusted to a pH of 10 using 6 M NaOH and
again extracted with chloroform (3 × 40 mL). The organic layer was
dried over Na2SO4, filtered, and concentrated to give 5 (9.7 g, 71%)
Synthesis of Dipropanolamine (2). 3-Amino-1-propanol
(14.5 mL, 0.19 mol) and 3-chloro-1-propanol (8 mL, 0.1 mol)
were dissolved in 50 mL of water, and the reaction mixture was heated
at reflux for 24 h. To the cooled solution was added KOH (5.4 g,
0.1 mol), and the mixture was filtered and extracted with acetone.
Subsequently, the organic layer was concentrated to leave a yellow oil.
Dipropanolamine 2 was obtained by subsequent distillation (150−
180 °C, 1 mbar) as a colorless oil (6.7 g, 53%). 1H NMR (200 MHz,
CDCl3): δ 3.76 (s, 3 H, NH/OH), 3.69 (t, 4 H, CH2OH), 2.77 (t, 4
H, CH2NH), 1.67 (quintet, 4 H, CH2CH2CH2) ppm. 13C{1H} NMR
(50 MHz, CDCl3): δ 62.3 (CH2OH), 48.3 (CH2NH), 31.4
(CH2CH2CH2) ppm.
1
as a colorless solid. H NMR (200 MHz, CDCl3): δ 7.72 (d, 2 H,
H
aromatic), 7.28 (d, 2 H, Haromatic), 3.20 (bs, 2 H, NH), 3.09 (t, 2 H,
CH2NHTs), 2.82 (t, 2 H, CH2NH2), 2.59 (t, 2 H, CH2S), 2.54 (t, 2
H, CH2S), 2.39 (s, 3 H, CH3) ppm. 13C{1H} NMR (50 MHz,
CDCl3): δ 143.4, 137.2, 129.8, 127.1 (Caromatic), 42.8 (CH2NHTs),
41.1 (CH2NH2), 35.8 (CH2S), 32.1 (CH2S), 21.6 (CH3) ppm.
Synthesis of N,N′-[Thiobis(ethane-2,1-diyl)]bis-p-toluene-
sulfonamide (3-S). Sodium hydroxide (1.27 g, 31.8 mmol) was
dissolved in 100 mL of water, and 5 (6.9 g, 25.1 mmol) was then
added to the solution. Subsequently, p-toluenesulfonyl chloride
(4.79 g, 25.1 mmol) dissolved in 65 mL of DCM was added
dropwise to the vigorously stirred mixture. After complete addition,
the reaction mixture was stirred for an additional 12 h. The organic
layer was then separated from the aqueous layer, and the organic layer
was washed with 3 M HCl (2 × 45 mL), water (2 × 45 mL), and
brine (1 × 45 mL), dried over Na2SO4, filtered, and concentrated to
give 3-S (9.2 g, 86%) as a colorless solid. 1H NMR (200 MHz,
CDCl3): δ 7.74 (d, 4 H, Haromatic), 7.30 (d, 4 H, Haromatic), 5.19 (bt, 2
H, NH), 3.06 (bq, 4 H, CH2NHTs), 2.54 (t, 4 H, CH2S), 2.42 (s, 6
H, CH3) ppm. 13C{1H} NMR (50 MHz, CDCl3): δ 143.8, 136.9,
130.0, 127.2 (Caromatic), 42.3 (CH2NHTs), 31.9 (CH2S), 21.7 (CH3)
ppm. ESI-MS: calcd for [C18H24N2O4S3 + Na]+, 451.1; found 451.0.
Synthesis of N-(2-Bromoethyl)-p-toluenesulfonamide (6). 2-
Bromoethylamine hydrobromide (5.0 g, 24.4 mol) and p-
toluenesulfonyl chloride (4.0 g, 21.2 mmol) were suspended in
80 mL of DCM and cooled to 0 °C. Subsequently, triethylamine
(7.0 mL, 50.8 mmol) was added dropwise to the stirred mixture
within 10 min. The reaction mixture was stirred at 0 °C for an
additional 40 min. Afterward, the mixture was diluted with DCM,
until it was completely dissolved and washed with 2 M HCl (2 ×
150 mL) and brine (2 × 150 mL). The organic layer was dried over
MgSO4, filtered, and concentrated to give 6 (5.5 g, 93%) as a colorless
solid. 1H NMR (200 MHz, CDCl3): δ 7.76 (d, 2 H, Haromatic), 7.32 (d,
2 H, Haromatic), 5.14 (bt, 1 H, NH), 3.38−3.33 (m, 4 H,
BrCH2CH2NHTs), 2.43 (s, 3 H, CH3) ppm. 13C{1H} NMR (50
MHz, CDCl3): δ 144.0, 137.0, 130.0, 127.1 (Caromatic), 44.7
(CH2NHTs), 31.7 (CH2Br), 21.7 (CH3) ppm.
Synthesis of N,N′-[Selenobis(ethane-2,1-diyl)]bis-p-tolue-
nesulfonamide (3-Se). Selenium (0.87 g, 11 mmol) was suspended
in 45 mL of EtOH, and KBH4 (1.25 g, 23 mmol) was added to the
solution. Subsequently, the mixture was heated to reflux for 2 h, upon
which a colorless solution was formed. Afterward, 6 (5.5 g, 20 mmol)
dissolved in 55 mL of EtOH was added dropwise to the reaction
mixture. After the addition was completed, the reaction mixture was
refluxed for an additional 12 h. The cooled solution was then opened
to air and filtered and the solvent removed in vacuo. Subsequently,
the residue was dissolved in a minimum amount of hot EtOAc and
purified via a short column (Ø 5 cm, 15 cm) with Et2O as eluent. The
organic layer was concentrated to give 3-Se (4.7 g, 99%) as a colorless
solid. 1H NMR (250 MHz, CDCl3): δ 7.74 (d, 4 H, Haromatic), 7.30 (d,
4 H, Haromatic), 5.07 (t, 2 H, NH), 3.14 (q, 4 H, CH2NHTs), 2.59 (t, 4
H, CH2Se), 2.43 (s, 6 H, CH3) ppm. 13C{1H} NMR (63 MHz,
CDCl3): δ 143.8, 137.0, 130.0, 127.2 (Caromatic), 43.1 (CH2NHTs),
24.1 (CH2Se), 21.7 (CH3) ppm. 77Se NMR (48 MHz, CDCl3): δ
111.9 ppm. ESI-MS: calcd for [C18H24N2O4S2Se + Na]+, 499.0;
found, 498.7.
Synthesis of bis(3-tosyloxypropyl)tosylamide (1). Dipropa-
nolamine (2; 6.2 g, 46.6 mmol) was dissolved in 80 mL of DCM, and
the solution was cooled to 0 °C. Subsequently, triethylamine (21 mL,
151.5 mmol) was added to the mixture, followed by addition of tosyl
chloride (27.1 g, 142.2 mmol) over a period of 2 h. The mixture was
stirred at 0 °C for an additional 3 h and at room temperature
overnight. The white precipitate (triethylamine hydrochloride)
formed in course of the reaction was filtered off and washed with
DCM. Subsequently, the pale yellow filtrate was washed with water (4
× 40 mL), 1 M HCl (4 × 40 mL), water (4 × 40 mL), and saturated
NaHCO3 solution (1 × 40 mL). The organic layer was dried over
MgSO4, filtered, and evaporated to dryness. Purification via column
chromatography with PE/EtOAc (1.5/1 → 1/1 → 0/1) afforded
1
compound 1 (23 g, 83%) as a white solid. H NMR (200 MHz,
CDCl3): δ 7.77 (d, 4 H, CHaromatic), 7.61 (d, 2 H, CHaromatic), 7.35 (d,
4 H, CHaromatic), 7.29 (d, 2 H, CHaromatic), 4.02 (t, 4 H, CH2OTs),
3.08 (t, 4 H, CH2NTs), 2.45 (s, 6 H, CH3), 2.43 (s, 3 H, CH3), 1.87
(quintet, 4 H, CH2CH2CH2) ppm. 13C{1H} NMR (50 MHz,
CDCl3): δ 145.1, 143.9, 135.6, 132.9, 130.1, 130.0, 128.0, 127.3
(Caromatic), 67.8 (CH2OTs), 46.0 (CH2NTs), 28.9 (CH2CH2CH2),
21.8, 21.7 (CH3) ppm.
Synthesis of Diethylenetriamine Tritosylate (3-N). Dieth-
ylenetriamine (5.2 g, 50 mmol) and sodium hydroxide (6.0 g,
150 mmol) were dissolved in 50 mL of water. Subsequently, p-
toluenesulfonyl chloride (28.6 g, 150 mmol), dissolved in 150 mL of
Et2O was added dropwise to the mixture and stirred for 4.5 days. In
the course of the reaction, a colorless precipitate was formed.
Subsequently, the precipitate was filtered off, washed with water,
MeOH, and Et2O (each 100 mL), and dried under reduced pressure
1
to give 3-N (22.4 g, 79%) as a colorless solid. H NMR (250 MHz,
CDCl3): δ 7.66 (d, 6 H, Haromatic), 7.55 (d, 2 H, Haromatic), 7.38 (m, 6
H, Haromatic + NH), 3.03 (bt, 4 H, CH2NTs), 2.82 (bq, 4 H,
CH2NHTs), 2.39 (s, 9 H, CH3) ppm. 13C{1H} NMR (63 MHz,
CDCl3): δ 143.4, 142.7, 137.4, 135.3, 129.9, 129.7, 126.8, 126.5
(Caromatic), 43.4 (CH2NTs), 41.6 (CH2NHTs), 21.0 (CH3) ppm.
Synthesis of N,N′-(Dithiodi-2,1-ethanediyl)bis-p-toluenesul-
fonamide (4). Sodium hydroxide (9.96 g, 249 mmol) was dissolved
in 250 mL of water, and cystamine dihydrochloride
(13.1 g, 58.4 mmol) was added. Subsequently, p-toluenesulfonyl
chloride (22.2 g, 116.7 mmol) dissolved in 300 mL of DCM was
added dropwise to the vigorously stirred mixture. After the addition
was complete, the reaction mixture was stirred for an additional 12 h.
The organic layer was then separated from the aqueous layer, and the
organic layer was washed with 3 M HCl (2 × 45 mL), water
(2 × 45 mL), and brine (1 × 45 mL) and dried over Na2SO4, filtered,
1
and concentrated to give 4 (25.2 g, 94%) as a colorless solid. H
NMR (200 MHz, CDCl3): δ 7.74 (d, 4 H, Haromatic), 7.30 (d, 4 H,
H
aromatic), 5.32 (t, 2 H, NH), 3.21 (q, 4 H, CH2NHTs), 2.69 (t, 4 H,
CH2S), 2.41 (s, 6 H, CH3) ppm. 13C{1H} NMR (50 MHz, CDCl3): δ
143.8, 136.8, 129.9, 127.2 (Caromatic), 41.8 (CH2NHTs), 38.0 (CH2S),
21.7 (CH3) ppm.
Synthesis of N-[2-[(3-Aminoethyl)thio]ethyl]-p-toluenesul-
fonamide (5). Compound 4 (11.6 g, 25.1 mol) and triphenylphos-
phine (7.9 g, 30.1 mmol) were dissolved in 94 mL of deoxygenated
THF and 19 mL of deoxygenated water. The mixture was refluxed for
3 days, followed by the addition of 26.3 mL of 2 M deoxygenated
Synthesis of N,N′-[Tellurobis(ethane-2,1-diyl)]bis-p-tolue-
nesulfonamide (3-Te). Tellurium (1.28 g, 10 mmol) was suspended
in 45 mL of EtOH and 6 mL of water, followed by addition of KBH4
(1.13 g, 21 mmol) to the stirred solution. Subsequently, the mixture
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was heated to reflux for 2 h and a purple solution was formed.
Additional KBH4 (0.5 g, 9 mmol) was added, and the reaction
mixture subsequently turned colorless. Afterward, 6 (5.4 g, 19 mmol)
dissolved in 55 mL of EtOH was added dropwise to the reaction
mixture. The reaction mixture was then refluxed for an additional
12 h, opened to air, filtered, and concentrated. The residue was
dissolved in a minimum of hot EtOAc and purified with a short
column (Ø 5 cm, 15 cm) with EtOAc as eluent. The organic layer was
CH2NTs), 3.27 (t, 4 H, CH2NTs), 3.05 (t, 4 H, CH2NTs), 2.70 (t, 4
H, CH2Te), 2.43 (s, 3 H, CH3), 2.42 (s, 6 H, CH3), 1.88 (quintet, 4
H, CH2CH2CH2) ppm. 13C{1H} NMR (63 MHz, CDCl3): δ 143.9,
143.7, 136.3, 135.0, 130.0, 127.3, 127.1 (Caromatic), 52.5, 49.1, 47.4
(CH2NTs), 30.4 (CH2CH2CH2), 21.7, 21.6 (CH3), 2.5 (CH2Te)
ppm. ESI-MS: calcd for [C31H41N3O6S3Te + Na]+, 800.1; found,
800.2. Anal. Calcd for [C31H41N3O6S3Te]: C, 48.02; H, 5.33; N, 5.42.
Found: C, 47.7; H, 5.0; N, 5.3.
Synthesis of 1,4,8,12-Tetraazacyclotetradecane (LN). Com-
pound 7-N (2.0 g, 2.45 mmol) was dissolved in 5 mL of concentrated
H2SO4 and stirred at 100 °C for 5 days. After it was cooled to room
temperature, the reaction mixture was added dropwise into cold
ethanol and a colorless precipitate was formed. The solid was filtered
off and washed with cold diethyl ether. Afterward, the solid was
dissolved in 3.5 mL of water and 2.65 mL of concentrated HCl was
added. The mixture was adjusted to pH 14 using 50% NaOH, and the
aqueous layer was extracted with DCM (4 × 30 mL). The combined
organic layers were dried over Na2SO4, filtered, and concentrated to
1
concentrated to give 3-Te (4.1 g, 82%) as an orange solid. H NMR
(250 MHz, CDCl3): δ 7.74 (d, 4 H, Haromatic), 7.30 (d, 4 H, Haromatic),
5.29 (t, 2 H, NH), 3.21 (q, 4 H, CH2NHTs), 2.65 (t, 4 H, CH2Te),
2.41 (s, 6 H, CH3) ppm. 13C{1H} NMR (63 MHz, CDCl3): δ 143.7,
137.1, 129.9, 127.2 (Caromatic), 45.1 (CH2NHTs), 21.7 (CH3), 3.5
(CH2Te) ppm.
General Procedure for the Ring Closures and Formation of
7-N, -S, -Se, and -Te. The tosylamides 3-N, -S, -Se, and -Te (1
equiv) and Cs2CO3 (2.5 equiv) were mixed in DMF (15 mL mmol−1)
, and the resulting white suspension was stirred for 1 h. Subsequently,
a solution of compound 1 (1 equiv) dissolved in DMF (5 mL
mmol−1) was added dropwise via a syringe pump (3 mL h−1). The
reaction mixture was stirred for at least 3 days. Afterward, the mixture
was opened to air and ice (17.5 mL mmol−1) was added slowly to the
solution. When the ice was almost melted, the reaction mixture was
left in the refrigerator overnight. The formed precipitate was filtered
off, washed thoroughly with ice-cold water, taken up in DCM, dried
over Na2SO4 ,and concentrated under reduced pressure. The
tosylated macrocycles 7-N, -S, -Se, and -Te were obtained, after
purification via column chromatography.
1,4,7,11-Tetratosyl-1,4,7,11-tetraazacyclotetradecane (7-N).
After purification via column chromatography (DCM/MeOH (75/
1)) 7-N was obtained as a colorless solid in 76% yield. 1H NMR (200
MHz, CDCl3): δ 7.70 (d, 2 H, Haromatic), 7.65 (d, 4 H, Haromatic), 7.61
(d, 2 H, Haromatic), 7.33 (d, 2 H, Haromatic), 7.30 (d, 4 H, Haromatic), 7.27
(d, 2 H, Haromatic), 3.37−3.05 (m, 16 H, CH2NTs), 2.44 (s, 3 H,
CH3), 2.42 (s, 6 H, CH3), 2.41 (s, 3 H, CH3), 1.87 (quintet, 4 H,
CH2CH2CH2) ppm. 13C{1H} NMR (50 MHz, CDCl3): δ 143.9,
143.8, 135.8, 135.1, 135.0, 130.0, 129.9, 127.4, 127.2 (Caromatic), 49.9,
48.3, 48.2, 48.1 (CH2NTs), 29.7 (CH2CH2CH2), 21.6 (CH3) ppm.
ESI-MS: calcd for [C38H48N4O8S4 + Na]+, 839.2; found, 839.7. Anal.
Calcd for [C38H48N4O8S4]: C, 55.86; H, 5.92; N, 6.86. Found: C,
55.6; H, 6.7; N, 6.6.
1
give LN (426 mg, 87%) as a colorless solid. H NMR (200 MHz,
CDCl3): δ 3.24 (bs, 4 H, NH), 2.76−2.65 (m, 16 H, CH2NH), 1.74
(quintet, 4 H, CH2CH2CH2) ppm. 13C{1H} NMR (50 MHz,
CDCl3): δ 49.4, 48.4, 48.1, 46.7 (CH2NH), 28.0 (CH2CH2CH2)
ppm. ESI-MS: calcd for [C10H25N4]+, 201.2; found, 201.1. Anal.
Calcd for [C10H24N4]: C, 59.96; H, 12.08; N, 27.97. Found: C, 60.3;
H, 12.1; N, 28.1.
Synthesis of 1-Oxa-4,8,12-triazacyclotetradecane (LO). Com-
pound 7-O (2.41 g, 3.63 mmol) and phenol (3.85 g, 40 mmol) were
refluxed in 55 mL of 33% HBr in glacial acetic acid for 48 h. An
additional 30 mL of 33% HBr in acetic acid was added to the reaction,
and the mixture was refluxed for another 48 h. Subsequently, the
reaction mixture was cooled to room temperature and concentrated.
The residue was taken up in 40 mL of DCM/water (1/1) and the
aqueous layer washed with DCM (3 × 30 mL). Afterward, using 50%
NaOH the pH of the aqueous layer was adjusted to 14 and the layer
was extracted with DCM (4 × 35 mL). The combined organic layers
were dried over Na2SO4, filtered, and concentrated to give LO
1
(357 mg, 49%) as a colorless solid. H NMR (200 MHz, CDCl3):
δ 3.58 (t, 4 H, CH2O), 2.78−2.36 (m, 12 H, CH2NH), 2.36 (bs, 3 H,
NH), 1.71 (quintet, 4 H, CH2CH2CH2) ppm. 13C{1H} NMR (50
MHz, CDCl3): δ 69.0 (CH2O), 50.6, 49.6, 49.4 (CH2NH), 29.2
(CH2CH2CH2) ppm. ESI-MS: calcd for [C10H24N3O]+, 202.2; found,
202.1. Anal. Calcd for [C10H23N3O]: C, 59.66; H, 11.52; N, 20.87.
Found: C, 59.9; H, 11.8; N, 20.7.
4,8,12-Tritosyl-1-thia-4,8,12-triazacyclotetradecane (7-S). After
purification via column chromatography (PE/EtOAc (1.25/1)) 7-S
1
Synthesis of 1-Thia-4,8,12-triazacyclotetradecane (LS). Com-
pound 7-S (2.0 g, 2.94 mmol) and phenol (3.1 g, 33 mmol) were
heated under reflux in 45 mL of 33% HBr in glacial acetic acid for
48 h. An additional 25 mL of 33% HBr in acetic acid was added to the
reaction, and the mixture was refluxed for 96 h. Subsequently, the
reaction mixture was cooled to room temperature and concentrated.
The residue was taken up in 30 mL of DCM/water (1/1) and the
aqueous layer washed with DCM (3 × 20 mL). Afterward, the
aqueous layer was adjusted to pH 14 using 50% NaOH and the layer
was extracted with DCM (4 × 30 mL). The combined organic layers
were dried over Na2SO4, filtered, and concentrated to give LS
(492 mg, 77%) as a light brown solid. 1H NMR (200 MHz,
CDCl3): δ 2.71−2.62 (m, 16 H, CH2NH/CH2S), 2.31 (bs, 3 H, NH),
1.65 (quintet, 4 H, CH2CH2CH2) ppm. 13C{1H} NMR (50 MHz,
CDCl3): δ 48.9, 48.3, 47.4 (CH2NH), 31.7 (CH2S), 28.3
(CH2CH2CH2) ppm. ESI-MS: calcd for [C10H24N3S]+, 218.1;
found, 217.9. Anal. Calcd for [C10H23N3S]: C, 55.25; H, 10.67; N,
19.33. Found: C, 55.1; H, 10.8; N, 19.6.
was obtained as a colorless solid in 70% yield. H NMR (200 MHz,
CDCl3): δ 7.67 (d, 4 H, Haromatic), 7.64 (d, 2 H, Haromatic), 7.31 (d, 6
H, Haromatic), 3.24−3.11 (m, 12 H, CH2NTs), 2.73 (t, 4 H, CH2S),
2.42 (s, 9 H, CH3), 1.94 (quintet, 4 H, CH2CH2CH2) ppm. 13C{1H}
NMR (63 MHz, CDCl3): δ 143.8, 135.6, 130.1, 127.3, 127.2
(Caromatic), 49.7, 48.5 (CH2NTs), 31.5 (CH2S), 30.1 (CH2CH2CH2),
21.6 (CH3) ppm. ESI-MS: calcd for [C31H41N3O6S4 + Na]+, 702.2;
found, 701.1. Anal. Calcd for [C31H41N3O6S4]: C, 54.76; H, 6.08; N,
6.18. Found: C, 55.0; H, 5.7; N, 6.2.
4,8,12-Tritosyl-1-selena-4,8,12-triazacyclotetradecane (7-Se).
After purification via column chromatography (PE/EtOAc (1.25/
1)) 7-Se was obtained as a colorless solid in 85% yield. 1H NMR (250
MHz, CDCl3): δ 7.67 (d, 4 H, Haromatic), 7.63 (d, 2 H, Haromatic), 7.31
(d, 2 H, Haromatic), 7.30 (d, 4 H, Haromatic), 3.30−3.22 (m, 8 H,
CH2NTs), 3.09 (t, 4 H, CH2NTs), 2.69 (t, 4 H, CH2Se), 2.43 (s, 3 H,
CH3), 2.42 (s, 6 H, CH3), 1.91 (quintet, 4 H, CH2CH2CH2) ppm.
13C{1H} NMR (63 MHz, CDCl3): δ 143.9, 143.8, 136.0, 135.2, 130.0,
127.3, 127.2 (Caromatic), 50.4, 48.9, 48.0 (CH2NTs), 30.5
(CH2CH2CH2), 22.6 (CH2Se), 21.6 (CH3) ppm. 77Se NMR (48
MHz, CDCl3): δ 144.2 ppm. ESI-MS: calcd for [C31H41N3O6S3Se +
Na]+, 750.1; found, 750.0. Anal. Calcd for [C31H41N3O6S3Se]: C,
51.23; H, 5.69; N, 5.78. Found: C, 50.9; H, 5.7; N, 5.6.
Synthesis of 1-Selena-4,8,12-triazacyclotetradecane (LSe).
Naphthalene (1.85 g, 14.4 mmol) was dissolved in 10 mL of DME,
and sodium (270 mg, 11.7 mmol) was added to the solution. The
mixture was stirred for 2 h at room temperature, while a dark green
suspension of sodium naphthalenide was formed. A solution of 7-Se
(250 mg, 0.34 mmol) in 15 mL of DME was cooled to −45 °C, and
subsequently the sodium naphthalenide solution was added via
syringe. After the addition was complete, the reaction was quenched
with 3 mL of saturated NaHCO3 at −45 °C. The mixture was warmed
4,8,12-Tritosyl-1-tellura-4,8,12-triazacyclotetradecane (7-Te).
After purification via column chromatography (DCM/MeOH (100/
1
1)) 7-Te was obtained as a colorless solid in 64% yield. H NMR
(250 MHz, CDCl3): δ 7.67 (d, 4 H, Haromatic), 7.64 (d, 2 H, Haromatic),
7.32 (d, 2 H, Haromatic), 7.30 (d, 4 H, Haromatic), 3.38 (t, 4 H,
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to room temperature and stirred for an additional 12 h. Subsequently,
the mixture was concentrated and the residue was taken up in 20 mL
DCM:water, then the aqueous layer was adjusted to pH 2 using 6 M
HCl. Following this, the aqueous layer was washed with DCM (4 ×
10 mL). Subsequently, the aqueous layer was adjusted to a pH of 10
using 6 M NaOH and washed with DCM (4 × 10 mL). The organic
layer was dried over Na2SO4, filtered, and concentrated to give LSe
(70 mg, 78%) as a light brown solid. 1H NMR (250 MHz, CDCl3): δ
2.81−2.68 (m, 16 H, CH2NH/CH2Se) 2.51 (bs, 3 H, NH), 1.72
(quintet, 4 H, CH2CH2CH2) ppm. 13C{1H} NMR (63 MHz,
CDCl3): δ 49.0, 48.4, 48.0 (CH2NH), 28.3 (CH2CH2CH2), 25.0
(CH2Se) ppm. 77Se NMR (48 MHz, CDCl3): δ 44.9 ppm. ESI-MS:
calcd for [C10H24N3Se]+, 266.1; found, 266.0. Anal. Calcd for
[C10H23N3Se]: C, 45.45; H, 8.77; N, 15.90. Found: C, 45.3; H, 8.5;
N, 16.1.
conditions otherwise identical with those reported above. The
electrochemical cell for long-term electrolysis contained 10 mL of
electrolyte and a headspace of 43 mL. The cathodic peak potentials
Epc were identified at the local minimum of the cathodic current.
Spectroelectrochemistry (SEC-IR). Infrared spectroelectro-
chemistry (SEC-IR) measurements were carried out with an SP-02
cell (Spectroelectrochemistry Partners) attached to a Bruker Tensor
27 FT-IR spectrometer with a Pike Miracle ATR unit. A PalmSens3
or a PalmSens4 instrument was used as a potentiostat with a standard
three-electrode setup (WE, glassy-carbon electrode; RE, Ag wire; CE,
Pt wire). All measurements were carried out in CO2-saturated
acetonitrile/water (4/1) mixtures containing 20 mM of the NiII
species and 0.1 M [nBu4N]PF6 as the electrolyte. Prior to each
experiment, the electrochemical cell was degassed with Ar for 10 min
and the atmosphere was maintained throughout the measurement.
Furthermore, the working electrode was prepared by successive
polishing with 1.0 and 0.3 μm sandpaper and subsequent sonication in
acetonitrile for 10 min. The WE was placed 200 μm above the ATR
crystal, and the potential was held for 800 s before the IR scan was
initiated. The IR measurement (128 scans) was completed in roughly
260 s.
Quantification of CO2RR Products. Quantification of the
headspace gas composition and liquid phase composition of the
electrochemical cell was performed using a Shimadzu GCMS-QP2020
system equipped with two capillary columns and a MS detector. Gas-
phase separation was performed via hand injection using a Carboxen
1010 PLOT fused silica capillary GC column (30 m length ×
0.32 mm i.d., average thickness 15 μm) and liquid-phase separation
was performed via headspace analysis using a SH-Rtx-200 ms fused
silica capillary GC column (30 m length × 0.25 mm i.d., average
thickness 1 μm). Helium was used as carrier gas. The following
gaseous and liquid products/components were assayed via the GCMS
system: H2, O2, N2, CO, CH4, C2H4, C2H6, methanol, ethanol,
propanol, formate/formic acid, acetate/acetic acid, propionate/
propionic acid, acetaldehyde, and propionaldehyde.
General Procedure for the Syntheses of Complexes Ni-LN,
Ni-LO, Ni-LS, and Ni-LSe. Macrocycles LN, LO, LS, and LSe (1 equiv)
were suspended in acetonitrile (20 mL mmol−1), and [Ni(ClO4)2]·
6H2O (1 equiv) dissolved in acetonitrile (12 mL mmol−1) was added
to the macrocycle solution. Upon addition of the nickel salt, a
characteristic color change was observed and the reaction mixture was
stirred for 24 h. Subsequently, the mixture was concentrated and the
nickel complexes Ni-LN, Ni-LO, Ni-LS, and Ni-LSe were purified via
filter column chromatography, with acetonitrile as eluent.
[Ni(LN)(NCCH3)2](ClO4)2 (Ni-LN). After purification, Ni-LN was
obtained as a red solid in 92% yield. IR: ν 3275, 2939, 2876, 2309,
̃
2278, 1474, 1460, 1428, 1082, 1028 cm−1. UV/vis/NIR (acetoni-
trile): λ 335, 499, 714 nm. ESI-MS: calcd for [C10H24ClN4NiO4]+,
357.1; found, 356.9; calcd for [C10H23N4Ni]+, 257.1; found, 257.2.
Anal. Calcd for [C10H24Cl2N4NiO8] + 4H2O: C, 22.66; H, 6.09; N,
10.57. Found: C, 22.6; H, 5.7; N, 10.4.
[Ni(LO)(NCCH3)2](ClO4)2 (Ni-LO). After purification Ni-LO was
obtained as a blue solid in 80% yield. IR: ν
̃
3269, 2939, 2877,
2310, 2280, 1468, 1423, 1070, 1034 cm−1. UV/vis/NIR (acetoni-
trile):
λ 349, 538, 626, 799 nm. ESI-MS: calcd for
[C1 0 H2 3 ClN3 NiO5 ]+ , 358.1; found, 358.2; calcd for
[C10H22N3NiO]+, 258.1; found, 258.5. Anal. Calcd for
[C10H23Cl2N3NiO9] + CH3CN + 3H2O: C, 26.02; H, 5.82; N,
10.11. Found: C, 26.1; H, 5.6; N, 9.9.
X-ray Data Collection and Structure Solution Refinement.
Single crystals suitable for X-ray structure analysis were handled in
Paratone N oil, mounted on a fiber loop, and placed in a cold N2
stream throughout the measurement. X-ray diffraction data were
collected using either an Oxford Xcalibur-2 diffractometer (Mo Kα, λ
= 0.71073 Å) with a sapphire-II CCD camera or using a Rigaku
XTAlab Synergy diffractometer (Cu Kα, λ = 1.54184 Å) equipped
with a HyPix detector. The data obtained were analyzed with the
program packages CrysAlisPro and Platon.55 The structures were
solved by intrinsic phasing using SHELXT56 and refined against F2
(SHELXL57). Hydrogen atoms were added in calculated positions
[Ni(LS)(NCCH3)2](ClO4)2 (Ni-LS). After purification Ni-LS was
obtained as a purple solid in 90% yield. IR: ν 3258, 2939, 2876,
̃
2311, 2282, 1464, 1421, 1065 cm−1. UV/vis/NIR (acetonitrile): λ
356, 563, 826 (sh), 916 nm. ESI-MS: calcd for [C10H23ClN3NiO4S]+,
374.1; found, 374.1; calcd for [C10H22N3NiS]+, 274.1; found, 274.4.
Anal. Calcd for [C10H23Cl2N3NiO8S] + CH3CN + 2H2O: C, 26.11;
H, 5.48; N, 10.15. Found: C, 25.8; H, 5.7; N, 10.6.
58
and refined using a riding model. The program package OLEX2
[Ni(LSe)(NCCH3)2](ClO4)2 (Ni-LSe). After purification Ni-LSe was
served as a graphical user interface. Crystallographic data and
refinement parameters are presented in Table S3 in the Supporting
Information.
obtained as a purple-green solid in 50% yield. IR: ν 3256, 2939, 2876,
̃
2309, 2282, 1464, 1420, 1074 cm−1. UV/vis/NIR (acetonitrile): λ
357, 567, 830 (sh), 926 nm. ESI-MS: calcd for
[C10H23ClN3NiO4Se]+, 422.0; found, 422.1. Anal. Calcd for
[C10H23Cl2N3NiO8Se] + 2CH3CN + 0.2 H2O: C, 27.68; H, 4.88;
N, 11.53. Found: C, 27.9; H, 4.50; N, 11.6.
ASSOCIATED CONTENT
■
sı
* Supporting Information
Electrochemistry. The electrochemical studies were performed
using a PalmSens3 or a PalmSens4 potentiostat in a standard three-
electrode setup. A glassy-carbon electrode was used as the working
electrode (WE), a Ag wire in a Luggin capillary as the
pseudoreference electrode (PRE), and a Pt wire as the counter
electrode (CE). The working electrode was prepared by successive
polishing with 1.0 and 0.3 μm sandpaper and subsequent sonication in
acetonitrile for 10 min. Tetrabutylammonium hexafluorophosphate
([nBu4N]PF6, 0.1 M) was used as the electrolyte in all electrochemical
measurements either in anhydrous acetonitrile or an acetonitrile/
water mixture (4/1). Prior to each experiment, the electrochemical
cell was degassed with Ar for 10 min and an Ar or CO2 atmosphere
was maintained throughout the measurement. All cyclic voltammo-
grams were recorded at a scan rate of 100 mV s−1, and after every
experiment all pseudoreferenced potentials were referenced against
the ferrocene/ferrocenium couple (Fc/Fc+). Controlled-potential
coulometry (CPC) was performed at defined potentials under
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.organomet.0c00129.
NMR data and spectra, ESI-MS, IR, and UV/vis/NIR
spectra of the nickel complexes, details of X-ray studies,
additional electrochemical data of the nickel complexes,
and additional data of STEM/EDX analysis (PDF)
Accession Codes
CCDC 1961570−1961574 contain the supplementary crys-
tallographic data for this paper. These data can be obtained
free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by
emailing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
L
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(9) Benson, E. E.; Kubiak, C. P.; Sathrum, A. J.; Smieja, J. M.
Electrocatalytic and Homogeneous Approaches to Conversion of CO2
to Liquid Fuels. Chem. Soc. Rev. 2009, 38 (1), 89−99.
(10) Khezri, B.; Fisher, A. C.; Pumera, M. CO2 Reduction: The
Quest for Electrocatalytic Materials. J. Mater. Chem. A 2017, 5 (18),
8230−8246.
AUTHOR INFORMATION
■
Corresponding Authors
̈
Kallol Ray − Department of Chemistry, Humboldt-Universitat
zu Berlin, 12489 Berlin, Germany; orcid.org/0000-0003-
2074-8844; Email: Kallol.Ray@chemie.hu-berlin.de
(11) Zheng, T.; Jiang, K.; Wang, H. Recent Advances in
Electrochemical CO2-to-CO Conversion on Heterogeneous Catalysts.
Adv. Mater. 2018, 30 (48), 1802066.
Ulf-Peter Apfel − Department of Chemistry and Biochemistry,
̈
Ruhr-Universitat Bochum, 44801 Bochum, Germany;
Fraunhofer UMSICHT, 46047 Oberhausen, Germany;
(12) Khan, A.; Ullah, H.; Nasir, J. A.; Shuda, S.; Chen, W.; Khan, M.
A.; Zia-ur-Rehman. Metal- and Carbon-Based Materials as Hetero-
geneous Electrocatalysts for CO2 Reduction. J. Nanosci. Nanotechnol.
2018, 18 (5), 3031−3048.
(13) Piontek, S.; junge Puring, K.; Siegmund, D.; Smialkowski, M.;
Sinev, I.; Tetzlaff, D.; Roldan Cuenya, B.; Apfel, U.-P. Bio-Inspired
Design: Bulk Iron−Nickel Sulfide Allows for Efficient Solvent-
Dependent CO2 Reduction. Chem. Sci. 2019, 10 (4), 1075−1081.
orcid.org/0000-0002-1577-2420; Email: ulf.apfel@rub.de
Authors
Philipp Gerschel − Department of Chemistry and Biochemistry,
̈
Ruhr-Universitat Bochum, 44801 Bochum, Germany
Beatrice Battistella − Department of Chemistry, Humboldt-
̈
Universitat zu Berlin, 12489 Berlin, Germany
Daniel Siegmund − Fraunhofer UMSICHT, 46047
̈
(14) Kotrel, S.; Brauninger, S. Industrial Electrocatalysis. In
Oberhausen, Germany; orcid.org/0000-0003-2476-8965
̈
Handbook of Heterogeneous Catalysis; Ertl, G., Knozinger, H.,
Schuth, F., Weitkamp, J., Eds.; Wiley-VCH: Weinheim, Germany,
̈
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.organomet.0c00129
2008; p hetcat0103.
(15) Feng, D.-M.; Zhu, Y.-P.; Chen, P.; Ma, T.-Y. Recent Advances
in Transition-Metal-Mediated Electrocatalytic CO2 Reduction: From
Homogeneous to Heterogeneous Systems. Catalysts 2017, 7 (12),
373.
Notes
The authors declare no competing financial interest.
(16) Lee, K. J.; Elgrishi, N.; Kandemir, B.; Dempsey, J. L.
Electrochemical and Spectroscopic Methods for Evaluating Molecular
Electrocatalysts. Nat. Rev. Chem. 2017, 1 (5), 0039.
ACKNOWLEDGMENTS
■
This work was supported by the Fonds of the Chemical
Industry (Liebig Grant to U.-P.A. and Kekule fellowship to
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P.G.) and the Deutsche Forschungsgemeinschaft (Emmy
Noether Grant to U.-P.A., AP242/2-1 and AP242/5-1). K.R.
acknowledges financial support from the DFG (Cluster of
Excellence “Unifying Concepts in Catalysis”; ECX 314-2, and
the Heisenberg-Program). This work was supported by the
Fraunhofer Internal Programs under Grant No. Attract 097-
602175. Furthermore, we thank Julia Jokel for GC-MS support
and David Tetzlaff and Yen-Ting Chen for STEM/EDX
analysis.
̈
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