Calix[8]arene-based Ni(II) complexes for electrocatalytic CO2 reduction

Article abstract of DOI:10.1016/j.ica.2020.119607

The electrochemical behavior and catalytic activity for the electroreduction of CO₂ of complexes of Ni^(II) containing phenanthroline-based ligands, with or without a calixarene scaffold, were tested. The complexes were characterized by spectroscopic techniques, and their electrocatalytic properties determined by cyclic voltammetry. With water as proton source the complex [(1,5-(2,9-dimethyl-1,10-phenanthro)-p-tert-butylcalix[8]arene)NiCl₂] (1) presented a significant increase in current at E = ?2.36 V (relative to Ag/AgCl reference electrode) when reduced under an atmosphere of CO₂, indicating that an electrocatalytic process occurs. Thus, calix[8]arenes that feature a phenanthroyl moiety as bidentate N-ligands, and an intramolecular proton source in the phenolic –OH groups, afford Ni^(II) electrocatalysts for the reduction of CO₂.

Full text of DOI:10.1016/j.ica.2020.119607

InorganicaChimicaActa507(2020)119607
Contents lists available at ScienceDirect
Inorganica Chimica Acta
journal homepage: www.elsevier.com/locate/ica
Research paper
Calix[8]arene-based Ni(II) complexes for electrocatalytic CO2 reduction
Carlos A. Reyes-Mata, Ivan Castillo⁎
Instituto de Química, Universidad Nacional Autónoma de México, Circuito Exterior, CU, Ciudad de México 04510, Mexico
A R T I C L E I N F O
A B S T R A C T
Keywords:
Calixarene
Nickel
Electrochemistry
Carbon dioxide
The electrochemical behavior and catalytic activity for the electroreduction of CO2 of complexes of Ni(II) con-
taining phenanthroline-based ligands, with or without a calixarene scaffold, were tested. The complexes were
characterized by spectroscopic techniques, and their electrocatalytic properties determined by cyclic voltam-
metry. With water as proton source the complex [(1,5-(2,9-dimethyl-1,10-phenanthro)-p-tert-butylcalix[8]arene)
NiCl2] (1) presented a significant increase in current at E = −2.36 V (relative to Ag/AgCl reference electrode)
when reduced under an atmosphere of CO2, indicating that an electrocatalytic process occurs. Thus, calix[8]
arenes that feature a phenanthroyl moiety as bidentate N-ligands, and an intramolecular proton source in the
phenolic –OH groups, afford Ni(II) electrocatalysts for the reduction of CO2.
1. Introduction
have thus exploited a calix[8]arene-based copper catalyst in selective
and efficient CeS cross-coupling reactions [16]. Calix[8]arene was thus
The great activity of nickel in catalytic systems has been studied
extensively [1], such that at the beginning of the 20th century, Paul
Sabatier referred to it as a “spirited horse”; this was partly due to the
difficulty in controlling its reactivity, which affects the selectivity of a
desired chemical transformation [2]. Careful design of ligand scaffolds
has allowed the taming of nickel reactivity, with examples in poly-
merizations (e.g. ethylene) [3], Suzuki-Miyaura couplings [4], CeS
bond formation [5], activation of unsaturated molecules [6], and CO2
reduction. The latter represents a global challenge due to the deleter-
ious effect of CO2 as a greenhouse gas. The transition metals are good
candidates to carry out this transformation due to the ease of access to
various oxidation states, allowing the electron/proton transfer; one of
the earth-abundant candidates is nickel [7]. This is particularly relevant
since CO2 reduction has been inspired by the reactivity of CO-dehy-
drogenase, which has a nickel center at its active site [8]. Nevertheless,
reductive carboxylation with nitrogen-containing ligands has been
widely studied using different organic substrates as alkynes [9], organic
(pseudo)halides [10], and esters [11] to produce the corresponding
carboxylic acids with CO2. On the other hand, through electroreduc-
tion, specifically with nickel/nitrogen-based catalysts, carbon mon-
oxide [12], formate [13], and oxalate [14] can be produced. Despite the
number of nickel-based catalysts with nitrogen-containing motifs, su-
pramolecular catalysts with this metal are scarce.
functionalized with a phenanthroline motif to afford the ligand C8Phen
(Fig. 1), with its Cu(I) complexes as efficient catalysts for CeS couplings
that result in a variety of diaryl sulfides. The selectivity of the substrates
was affected by the steric restriction provided by the calixarene cavity,
leading to enhanced reactivity of bromoarenes relative to their io-
doarene counterparts. Confined environments may thus result in dif-
ferent reactivity and selectivity from that observed at a metal center
exposed to bulk solvent, allowing reaction pathways that favor mole-
cular collisions within the cavity, while also enforcing size restrictions.
Based on these results, we are interested in expanding the scope of these
compounds towards CO2 reduction. Herein, we report the results of the
catalytic electroreduction studies of nickel complexes with C8Phen,
and neocuproine (Neo) ligands for CO2 reduction.
2. Experimental
2.1. Reagents and techniques
All reagents and solvents were obtained from commercial sources.
Air-sensitive compounds were handled under an inert atmosphere using
standard Schlenk techniques or a nitrogen-filled glovebox. The synth-
esis of p-tert-butylcalix[8]arene was carried out according to the lit-
erature procedure [16]. IR spectra were acquired with a Perkin Elmer
203-B FT-IR spectrophotometer in the range of 4000–400 cm−1 as KBr
pellets. 1H NMR spectra were recorded with a JEOL Eclipse 300 spec-
trometer, and tetramethylsilane as internal standard. Positive-ion Fast
In this context, calix[8]arenes represent an ideal platform for the
binding of metal ions within their large cavities, while functionalization
afford the opportunity to introduce different donor groups [15]. We
⁎ Corresponding author.
E-mail address: joseivan@unam.mx (I. Castillo).
https://doi.org/10.1016/j.ica.2020.119607
Received 21 November 2019; Received in revised form 17 March 2020; Accepted 18 March 2020
Availableonline19March2020
0020-1693/©2020ElsevierB.V.Allrightsreserved.

C.A. Reyes-Mata and I. Castillo
InorganicaChimicaActa507(2020)119607
cooling to room temperature, THF was added and the mixture was
heated to 50 °C overnight. After cooling to room temperature, NeoBr2
(350 mg, 0.02 mmol) was added to the mixture and stirred for 36 h. The
solvent was then evaporated under reduced pressure to afford a yellow
solid that was dissolved in a CHCl3/toluene 8:1 mixture, and after slow
evaporation a crystalline solid was obtained. This solid was washed
with 50 mL of HCl (0.1 M) and 30 mL of CH2Cl2 in order to remove
cesium as CsCl. Finally, the product was washed with a saturated so-
lution of NaHCO3 for neutralization, affording C8Phen in 86% yield
(800 mg); m.p. 245 °C (dec). IR (KBr) νmax/cm−1: 3230 (OH), 1594
(CN). 1H NMR (C2D2Cl4, 300 MHz): δ 9.52 (s, 6H, OH), 8.39 (d,
J = 8.26HZ, 2 H, Arphen), 7.89 (m, 2 H, Arphen), 7.24 (s, 2 H, Arphen),
7.10 (m, 16H, Arcalix), 5.12 (s, 4 H, CH2phen), 4.3 (m, J = 13 Hz, 8 H,
CH2calix), 3.54 (m, J = 13 Hz, 8 H, CH2calix), 1.30 (m, 54H, t-Bu), 1.04
(m, 18H, t-Bu). FAB-MS m/z = 1502 [C8PhenH]+
.
2.2.4. Synthesis of complexes
Fig. 1. Schematic representation of C8Phen and Neo.
2.2.4.1. Synthesis of C8PhenNiCl2 (1). The complex C8PhenNiCl2 was
obtained from the reaction of C8Phen (50 mg, 0.033 mmol) and
NiCl2·6H2O (7.80 mg, 0.033 mmol) in THF and stirring for 2 h at room
temperature. Volatiles were evaporated and the solid was washed with
diethylether to afford C8PhenNiCl2 as a pale yellow solid in 90% yield
Atom Bombardment mass spectra (FAB+) were measured on a JEOL
JMS-SX-102A spectrometer, and direct analyses in real time mass
spectra (DART) were acquired with a JEOL JMS-T100LC spectrometer.
Combustion analyses were recorded with a Thermo Scientific Flash
2000. Melting points were determined with an Electrothermal Mel-
Temp, without corrections.
Catalytic tests were carried out with standard Schlenk techniques,
monitored by thin layer chromatography using silica gel GF-254 Merck,
and revealed with short wave UV–vis light (254 nm). The products were
purified by chromatography column using basic alumina 90 Macherey-
Nagel as stationary phase and hexane as eluant.
(48
mg);
m.p.
270
°C
(dec).
Anal.
Cald.
for
C
102H120Cl2N2NiO8•2CH2Cl2·H2O; C: 68.65, H: 6.98, N: 1.54; Found
C: 68.35, H: 6.77, N: 1.26. IR (KBr) νmax/cm−1: 3271 (OH), 1597 (CN).
1H NMR (CDCl3, 300 MHz): δ 9.7 (s, 6H, OH), 8.3 (s, 2 H, Arphen), 8.15
(s, 2 H, Arphen), 7.8 (s, 2 H, Arphen), 7.3 (m, 16H, Arcalix), 5.2 (s, 4 H,
CH2phen), 4.45 (s, 8 H, CH2calix), 3.5 (s, 8 H, CH2calix), 1.3 (m, 72H, t-
Bu). FAB-MS m/z = 1559 [C8PhenNi]+
.
2.2.4.2. Synthesis of C8PhenNi(OAc)2 (2). C8PhenNi(OAc)2 was
obtained from the reaction of C8Phen (50 mg, 0.033 mmol) and Ni
(OAc)2*4H2O (8.2 mg, 0.033 mmol) in THF and stirring for 2 h at room
temperature. Volatiles were evaporated and the solid was washed with
diethylether to afford C8PhenNi(OAc)2 as a yellow solid in 95% yield
2.2. Synthetic procedures
2.2.1. Synthesis of p-tert-butylcalix[8]arene
The p-tert-butylcalix[8]arene was synthesized according to the lit-
erature procedure [17], from p-tert-butylphenol (4.00 g, 0.026 mol),
paraformaldehyde (3.60 g, 0.120 mol), and sodium hydroxide (355 mg,
0.008 mol) in xylene, by heating to reflux for 24 h under inert atmo-
sphere. After cooling to room temperature, the solid formed was filtered
and washed with cold xylene. The product was obtained as a white
crystalline solid in 80% yield (3.40 g); m.p. > 250 °C (dec). 1H NMR
(CDCl3, 300 MHz): δ 9.62 (s, 8H; OH), 7.17 (m, 16H; Ar), 4.36 (m, 8H;
eCH2e), 3.50 (m, 8H; eCH2e), 1.25 (s, 72H; t-Bu).
(53
mg);
m.p.
290
°C
(dec).
Anal.
Cald.
for
C
106H126N2NiO12•CH2Cl2•H2O; C: 72.13, H: 7.35, N: 1.57; Found C:
72.26, H: 7.19, N: 1.47. IR (KBr) νmax/cm−1: 3249 (OH), 1598 (CN),
1573 and 1479 (CH3COO). 1H NMR (CDCl3, 300 MHz): δ 9.63 (s, 6H,
OH), 8.24 (s, 2 H, Arphen), 7.88 (s, 2 H, Arphen), 7.18 (s, 16H, Arcalix),
4.36 (m, 8 H, CH2calix), 3.34 (m, 8 H, CH2calix), 1.26 (m, 72H, t-Bu).
FAB-MS m/z = 1559 [C8FenNi]+
.
2.2.4.3. Synthesis of NeoNiCl2 (3). The complex NeoNiCl2 was
synthesized from the reaction of neocuproine (50 mg, 0.24 mmol)
and NiCl2·6H2O (57 mg, 0.24 mmol) in MeOH, and stirring for 2 h at
room temperature. After evaporation of volatiles, the solid was washed
with diethylether to afford NeoNiCl2 as a dark green solid in 95% yield
(77 mg); m.p. > 350 °C (dec). Anal. Cald. for C14H12Cl2N2Ni·3H2O; C:
2.2.2. Synthesis of 2,9-dimethyl-1,10-phenanthroline (NeoBr2)
2,9-dimethyl-1,10-phenanthroline was synthesized from neocu-
proine (1.50 g, 0.007 mol) and three equivalents of recrystallized N-
bromosuccinimide (4.00 g, 0.02 mol) in CH3CN by refluxing for 18 h
under inert atmosphere. After cooling to room temperature and eva-
poration under reduced pressure, the mixture was redissolved in die-
thylether and vacuum filtered. The solid was neutralized with a satu-
rated solution of NaHCO3. After, the solid was dissolved in THF and
cooled to 0 °C, two equivalents of both HPO(OEt)2 and i-Pr2NEt were
added, the mixture was warmed to room temperature, and stirred for
24 h for debromination. Then the solvent was evaporated under reduce
pressure, and the product was purified by column chromatography
using CH2Cl2 as eluant to obtain NeoBr2 in 30% yield (800 mg); m.p.
110–115 °C (dec). 1H NMR (CDCl3, 300 MHz): δ 8.26 (d, J = 8.34 Hz,
2H, Ar), 7.92 (d, J = 8.33 Hz, 2H, Ar), 7.81 (s, 2H, Ar), 4.97 (s, 4H,
42.91, H: 4.63, N: 7.15; Found C: 42.34, H: 4.02, N: 6.96. IR (KBr) νmax
/
cm−1: 3282 (H2O), 1613–1504 (CeH ar). 1H NMR (MeOD, 300 MHz): δ
(ppm) = 68 (s, 2H, Arphen), 20 (s, 2H, Arphen), 18 (s, 2H, Arphen), −2 (s,
2H, CH3). DART-MS m/z = 359 [NeoNiCl2Na]+, 301 [NeoNiCl]+
.
2.2.4.4. Synthesis of NeoNi(OAc)2 (4). The complex NeoNi(OAc)2 was
synthesized from the reaction of neocuproine (50 mg, 0.24 mmol) and
Ni(OAc)2·4H2O (60 mg, 0.24 mmol) in MeOH, and stirring for 2 h at
room temperature. Volatiles were evaporated under reduced pressure
and the solid was washed with diethylether to obtain the complex
CH2Br). DART-MS m/z = 367 [NeoBr2H]+
.
NeoNi(OAc)2 as
a light green solid in 98% yield (90 mg);
m.p. > 350 °C (dec). Anal. Cald. for C18H18N2NiO4·H2O; C: 53.64,
2.2.3. Synthesis of 1,5-(2,9-dimethyl-1,10-phenanthro)-p-tert-butylcalix
[8]arene (C8Phen)
The ligand C8Phen was obtained by adapting a literature method
[18]. p-tert-butylcalix[8]arene (1.00 g, 0.77 mmol) and CsF (1.17 g,
7.70 mmol) were dried for two hours at 120 °C under vacuum. After
H: 5.00, N: 6.95; Found C: 53.81, H: 4.71, N: 6.75. IR (KBr) νmax/cm−1
:
3447 (H2O), 1538 and 1449 (CH3COO). 1H NMR (MeOD, 300 MHz): δ
(ppm) = 53 (s, 8H, Arphen, CH3COO), 20 (s, 2H, Arphen), 17.5 (s, 2H,
Arphen), −9.5 (s, 6H, CH3). DART-MS m/z = 325 [NeoNiOAc]+, 209
[NeoH]+
.
2

C.A. Reyes-Mata and I. Castillo
InorganicaChimicaActa507(2020)119607
2.3. Cyclic voltammetry
toward the Ni(II) centers. The IR spectra of all compounds are presented
in the ESI Figs. S1–S5.
Electrochemical characterization of the complexes and CO2 elec-
troreduction studies were carried out using a CH instruments 1200b
potentiostat with a three electrodes cell equipped with a 3 mm diameter
glassy carbon working electrode, platinum wire as counter electrode,
and Ag+/Ag reference electrode with a tetrabutylammonium bromide
(Bu4NBr) solution. The supporting electrolyte used was tetra-
butylammonium hexafluorophosphate (Bu4NPF6) 0.1 M in 5 mL THF or
5 mL CH3CN:THF (95:5). The measured potentials at the working
Positive-ion Fast Atom Bombardment (FAB+) mass spectrometry
analysis of 1 and 2 reveals in both cases a peak at m/z = 1559 that is
assigned as [C8PhenNi]+, which corresponds to a Ni-reduced species
that may form in the ionization chamber [21]. Complexes 3 and 4 were
characterized by Direct Analysis in Real Time (DART) mass spectro-
metry. For 3, the spectrum shows a peak at m/z = 359, assigned to
[NeoNiCl2Na]+, a peak at m/z = 301 assigned as [NeoNiCl]+, and at
m/z = 209 corresponding to the protonated ligand [NeoH]+. Analysis
of 4 reveals a peak at m/z = 325, which was assigned as [Neo-
NiOAc]+, as well as the peak corresponding to [NeoH]+ (ESI Figs.
S6–S9).
electrode are reported with respect to the ferrocenium/ferrocene (Fc+
/
Fc) couple, by adding ferrocene to the solutions at the end of each
experiment. Dioxygen was removed by bubbling with dinitrogen.
1H NMR spectroscopy of C8Phen was recorded in different solvents,
with better definition of the signals in C2D2Cl4 as shown in Fig. 2. This
corresponds to the dicesium salt of doubly deprotonated C8Phen,
namely [C8Phen-2H]Cs2, since the cations appear to provide rigidity
that allows assignment of the signals, as previously reported by our
group in a variable-temperature study [18]. The protic H atoms are
expected to be exchangeable, giving rise to a broad signal at 9.55 and a
shoulder near the phenantrhoyl resonances at 8.45 ppm, consistent
with the two signals expected based on symmetry considerations. This
behavior was further probed by addition of 1 μL of D2O to a solution of
[C8Phen-2H]Cs2, resulting in disappearance of the broad signals (ESI,
Fig. S10).
3. Results and discussion
3.1. Synthesis and characterization
The first step was the synthesis of p-tert-butylcalix[8]arene (C8)
according to the methodology reported by Gutsche [17], which af-
forded C8 as a white powder in good yield. The second step was to
functionalize the neocuproine through a bromination reaction to obtain
NeoBr2; while the yield is moderate, characterization confirms its
identity for subsequent steps. Finally, the ligand 1,5-(2,9-dimethyl-
1,10-phenanthro)-p-tert-buthylcalix[8]arene (C8Phen) was obtained
from the initial reaction of C8 and CsF as template for 12 h in THF,
followed by addition of NeoBr2 (Scheme 1). This results in the selective
functionalization at the 1,5 positions of the calixarene. C8Phen was
purified after eliminating cesium by washing with 0.1 N aqueous HCl
by column chromatography with CH2Cl2 as eluent, obtaining the ligand
in 80% yield.
The 1H NMR spectra of the complexes were recorded initially in
CDCl3. The spectrum of 1 (Fig. S11) shows the signals of the methylene
groups of the calixarene framework as two doublets between 3.50 and
4.50 ppm as an AX system; the phenantroline-CH2-O protons at
5.2 ppm, and the signal of the OH groups around 9.70 ppm. The signals
of the aromatic protons appear between 8 and 8.50 ppm. Complex 2
show a similar spectrum, highlighting the signals of the AX system at
3.34–4.36 ppm and the signal for the OH group at 9.63 ppm (Fig. S12).
The complexes are not amenable for characterization by 13C NMR
spectroscopy, likely due to their flexible nature, and the presence of the
Ni(II) paramagnetic centers that result in broad and poorly resolved
signals. Nonetheless, the broadening and paramagnetic shift is not as
marked as in the neocuproine complexes, likely due to the distance of
the phenolic moieties to the paramagnetic Ni(II) ions.
The complexes of C8Phen with Ni(II) salts were prepared by mixing
the ligand with NiCl2 and Ni(OAc)2 in THF/MeOH and stirring for 2 h;
we expected to determine if the different counterions may have an ef-
fect on the reactivity. The solids obtained as light yellow powders
correspond to C8PhenNiCl2 (1) and C8PhenNi(OAc)2 (2) in good
yields, 85% and 90%, respectively. In order to compare the influence of
the calixarene motif, the corresponding complexes with neocuproine
NeoNiCl2 (3) and NeoNi(OAc)2 (4) were also synthesized. The reac-
tions were carried out in an analogous fashion, affording the complexes
as green solids in 95% yield for the complex 3 and 98% yield for
complex 4.
Characterization of C8Phen and the nickel complexes by IR spec-
troscopy was undertaken, in order to determine the presence of acetate
counterions in complexes 2 and 4; the symmetric and asymmetric bands
of those groups appear at 1479 and 1573 cm−1 for 2, and at 1449 and
1538 cm−1 for 4 [19,20]. The separation between the symmetric and
asymmetric stretching bands of the acetate groups provide information
about their coordination mode. For both 2 and 4, the difference of 94
and 89 cm−1 indicates a bidentate coordination mode of the acetates
In contrast, the 1H NMR spectra of the neocuproine complexes (3
and 4) showed resonances between 70 and −10 ppm, distinctive of
paramagnetic species. In the high field region between −2 and
−10 ppm, the signals observed were assigned to the protons of the
Neo-methyl groups. The assignment was carried out starting from the
integration of the signals for the aromatic protons of Neo between 17
and 21 ppm, corresponding to 2H each. This allowed us to determine
that the signal at high field corresponds to the six methyl group protons,
while the signals at low field were assigned to the remaining aromatic
protons (Figs. S13 and S14). In the case of 4, the intense signal at
53 ppm integrates to 8H, and was thus assigned to the aromatic protons
Scheme 1. Synthesis of C8Phen.
3

C.A. Reyes-Mata and I. Castillo
InorganicaChimicaActa507(2020)119607
Fig. 2. 1H NMR spectrum of [C8Phen-2H]Cs2 in C2D2Cl4 at 25 °C (chemical shifts and assignments indicated).
of Neo (likely those in para position relative to the N-donors) over-
lapping with the signal corresponding to the acetate Me-groups.
1 mM solutions of 1 in a CH3CN/THF (95:5) mixture were placed in a
three-electrode cell, saturated with CO2 by bubbling between each
measurement. Increasing amounts of water were added to the cell to
evaluate the influence of the proton source in the reaction. Fig. 4b
shows that with 10% (v/v) of water added, the current measured
reaches a minimum value. This happens under CO2 atmosphere only,
compared to when the proton source is present under N2 atmosphere
(Fig. S15), indicating that an electrocatalytic process occurs in the
presence of CO2. With regard to complex 2, its activity was significantly
lower than that of 1, and therefore we did not pursue its study further.
Apparently, the presence of acetate counterions hinders the reactivity
of the nickel centers in these systems.
In a preliminary assessment to establish the influence of the in-
tramolecular protons contained in the ligand C8Phen, the electro-
catalytic activity towards CO2 reduction was carried out using Ni(II)
complexes with the methylated ligand C8PhenMe (Fig. 5). The first
results show that the presence of the intramolecular protons is neces-
sary for CO2 reduction to occur, as it has been demonstrated previously
in related proton-transfer processes [23].
3.2. Cyclic voltammetry
3.2.1. Electrocatalytic reduction activity
Initially, the behavior of the ligands Neo and C8Phen was inter-
rogated by cyclic voltammetry (Fig. 3). It can be observed that Neo
presents two irreversible reduction waves at −2.53 V and −2.76 V vs
Fc+/Fc. The first reduction can be attributed to the formation of the
radical anion Neo%−, and the second reduction wave may be due to the
generation of Neo2−, as has been reported previously [22]. C8Phen
shows a similar behavior, with two reduction processes at −2.51 and
−2.77 V (Fig. 3). The reduction processes occur at very similar po-
tentials, but they are better defined in the case of C8Phen.
The cyclic voltammogram of 1 presents two reduction waves: the
first one at −2.01 V assigned to the Ni(II)/Ni(I) redox couple, and at
−2.36 V assigned to the Ni(I)/Ni(0) reduction (Fig. 4a). Since CO2 re-
duction is a Proton Coupled Electron Transfer (PCET) reaction, we
tested different proton sources to analyze the behavior of 1 under an
atmosphere of CO2, with water providing the best results. For this,
For comparison purposes, characterization of 3 and 4 that feature
the Neo ligand but lack the calixarene moiety was undertaken by cyclic
Fig. 3. Cyclic voltammograms of Neo (left), and C8Phen (right) under an atmosphere of N2. Conditions: 0.1 M Bu4NPF6 in CH3CN:THF (95:5) saturated with N2 at
100 mV s−1, glassy carbon working electrode, Ag/AgCl reference electrode. SE = Supporting Electrolyte.
4

C.A. Reyes-Mata and I. Castillo
InorganicaChimicaActa507(2020)119607
Fig. 4. Cyclic voltammograms of 1: a) under an atmosphere of N2; b) under an atmosphere of CO2 and different percentages of water. Conditions: 0.1 M Bu4NPF6 in
CH3CN/THF at 100 mV s−1
.
the complexes disappear, likely due to an electron transfer process
followed by a chemical step (ESI Fig. S16). This is consistent with ir-
reversible binding of CO2 to the reduced Ni(0) center, as described by
Kubiak and coworkers [24]. Based on the current reached, the elec-
trocatalytic activity of these complexes was not significantly better than
that observed for the calixarene-based complex, e.g. for 3 vs 1 (compare
ESI Fig. S17 with Fig. 4).
4. Conclusions
The electrocatalytic reduction of CO2 is assisted by an external
proton source, and the presence of an intramolecular source provided
by C8Phen further facilitates this transformation, as determined based
on comparisons with the Neo-based complexes. Characterization of the
calixarene-based complex 1 by cyclic voltammetry showed two irre-
versible waves attributed to the two reduction processes centered on Ni.
A better activity towards the reduction of CO2 was observed when 10%
(v/v) water is added, with the current reaching a minimum value re-
lative to that determined under N2 atmosphere. This indicates that an
electrocatalytic process occurs under CO2 atmosphere. Complexes 3
and 4 did not show significantly electrocatalytic activity, but their be-
havior in cyclic voltammetry allowed the electrochemical character-
ization of the nickel centers (Ni(II)/Ni(I) and (Ni(I)/Ni(0)) in the case of 3;
for 4 only an irreversible reduction wave was observed. Under an at-
mosphere of CO2, the S-shape waves of the Neo-based complexes dis-
appear, which can be attributed to an irreversible binding of the CO2 to
the nickel centers. The selectivity of the CO2 reduction products by 1 is
currently underway.
Fig. 5. Schematic representation of C8PhenMe.
voltammetry. Complex 3 showed three redox processes (Fig. 6a), the
two first ones assigned as nickel-centered at E1/2 = −1.20 (Ni(II)/Ni(I)
)
with a difference between the anodic and cathodic peak potentials
(ΔEp) of 70 mV and E1/2 = −1.53 V (Ni(I)/Ni(0)) with ΔEp = 80 mV,
and the one at more negative potential assigned to ligand-centered
reduction observed at E1/2 = −2.23 V (ΔEp = 70 mV). 4 presents an
irreversible reduction at −1.79 V, assigned to the Ni(II)/Ni(I) couple,
and a S-shape wave reduction of the ligand at less negative potential
relative to that of 3 (E1/2 = −2.15 V; ΔEp = 135 mV, Fig. 6b). When
the atmosphere of dinitrogen is replaced by CO2, the S-shape waves of
Fig. 6. Cyclic voltammograms of a) 3, and b) 4. Conditions: 0.1 M Bu4NPF6 in CH3CN/THF (95:5) saturated with N2 at 100 mV s−1
.
5

C.A. Reyes-Mata and I. Castillo
InorganicaChimicaActa507(2020)119607
CRediT authorship contribution statement
11627–11630;
(b) Y. Zhang, X. Xu, S. Zhu, Nat. Commun. 10 (2019) 1–9.
[7] J.W. Wang, W.J. Liu, D.C. Zhong, T.B. Lu, Coord. Chem. Rev. 378 (2019) 237–261.
[8] (a) T. Fogeron, P. Retailleau, M. Gomez-Mingot, Y. Li, M. Fontecave,
Organometallics 38 (2019) 1344–1350;
Carlos A. Reyes-Mata: Investigation, Writing - original draft. Ivan
Castillo: Supervision, Writing - review & editing.
(b) T. Fogeron, T.K. Todorova, J.P. Porcher, M. Gomez-Mingot, L.M. Chamoreau,
C. Mellot-Draznieks, Y. Li, M. Fontecave, ACS Catal. 8 (2018) 2030–2038.
[9] X. Wang, M. Nakajima, R. Martin, J. Am. Chem. Soc. 137 (2015) 8924–8927.
[10] M. Börjesson, T. Moragas, D. Gallego, R. Martin, ACS Catal. 6 (2016) 6739–6749.
[11] T. Moragas, J. Cornella, R. Martin, J. Am. Chem. Soc. 136 (2014) 17702–17705.
[12] B. Fisher, R. Eisenberg, J. Am. Chem. Soc. 102 (1980) 7361–7363.
[13] J.P. Collin, A. Jouaiti, J.P. Sauvage, Inorg. Chem. 27 (1988) 1986–1990.
[14] M. Rudolph, S. Dautz, E.G. Jager, J. Am. Chem. Soc. 122 (2000) 10821–10830.
[15] I. Abdellah, P. Kasongo, A. Labattut, R. Guillot, E. Schulz, C. Martini, V. Huc, Dalton
Trans. 47 (2018) 13843–13848.
Acknowledgements
The authors thank Luis Velasco for FAB-MS, Javier Pérez for DART-
MS, Virginia Gómez-Vidales for EPR measurements, María de la Paz
Orta for combustion analysis, Rocío Patiño for IR, Instituto de Química,
UNAM, and Conacyt (Beca 308307) for financial support.
[16] E. Guzmán-Percástegui, D.J. Hernández, I. Castillo, Chem. Commun. 52 (2016)
Appendix A. Supplementary data
3111–3114.
[17] C.E. Braun, C.D. Cook, J. Charles, Merritt, J.E. Rousseau, Org. Synth. 31 (1951) 77.
[18] D.J. Hernández, H. Vázquez-Lima, P. Guadarrama, D. Martínez-Otero, I. Castillo,
Tetrahedron Lett. 54 (2013) 4930–4933.
Supplementary data to this article can be found online at https://
doi.org/10.1016/j.ica.2020.119607.
[19] R. Baird, Solubility and structure studies of nickel (II) acetate, Masters Theses,
Missouri, 1964.
[20] K. Nakamoto, Infrared and Raman Spectra of Inorganic and Coordination
Compounds: Part B: Applications in Coordination, Organometallic, and
Bioinorganic Chemistry, Wiley, New Jersey, 2008.
[21] J.N. Kyranos, P. Vouros, Biol. Mass Spectrom. 19 (1990) 628–634.
[22] a) H. Yi, A. Jutand, A. Lei, Chem. Commun. 51 (2015) 545–548;
b) A. Klein, A. Kaiser, B. Sarkar, M. Wanner, J. Fiedler, Eur. J. Inorg. Chem. (2007)
965–976;
References
[1] (a) I.T. Trotuş, T. Zimmermann, F. Schüth, Chem. Rev. 114 (2014) 1761–1782;
(b) G. Wilke, Angew. Chem. Int. Ed. 27 (1988) 185–206.
[2] V.P. Ananikov, ACS Catal. 5 (2015) 1964–1971.
[3] (a) M.A. Zuideveld, P. Wehrmann, C. Röhr, S. Mecking, Angew. Chem. Int. Ed. 43
(2004) 869–873;
c) D.G. Yakhvarov, A. Petr, V. Kataev, B. Büchner, S. Gómez-Ruiz, E. Hey-Hawkins,
S.V. Kvashennikova, Y.S. Ganushevich, V.I. Morozov, O.G. Sinyashin, J. Organomet.
Chem. 750 (2014) 59–64.
(b) L.A. Brown, W.C. Anderson, N.E. Mitchell, K.R. Gmernicki, Polymers (Basel) 10
(2018) 1–9.
[4] C.A. Malapit, J.R. Bour, C.E. Brigham, M.S. Sanford, Nature 563 (2018) 100–104.
[5] S.C. Lee, H.H. Liao, A. Chatupheeraphat, M. Rueping, Chem. A Eur. J. 24 (2018)
3608–3612.
[23] H. Diebler, F. Secco, M. Venturini, J. Phys. Chem. 88 (1984) 4229–4232.
[24] A.L. Ostericher, T.M. Porter, M.H. Reineke, C.P. Kubiak, Dalton Trans. (2019)
15841–15848.
[6] (a) L. Cheng, M.M. Li, L.J. Xiao, J.H. Xie, Q.L. Zhou, J. Am. Chem. Soc. 140 (2018)
6