Ligand-Assisted Electrochemical CO2 Reduction by Ru-Polypyridyl Complexes

Article abstract of DOI:10.1002/ejic.202000259

The introduction of a simple triazole moiety on a terpyridine ligand in the complex [Ru(tpy)(bpy)(Cl)]⁺ ([1]⁺; tpy = 2,2':6',2''-terpyridine; bpy = 2,2'-bipyridine) gives rise to a suitable electrocatalyst for the reduction of CO₂ to CO. The triazole group in [Ru(tpyaz)(bpy)(Cl)]⁺ {[2]⁺; tpyaz = 4'-[4-(tert-butylphenyl)-1H-1,2,3-triazol-4-yl]-2,2':6',2''-terpyridine} enables the complex [2]⁺ to bind with CO₂ even at the one-electron reduced state of the complex. Further 2^nd-electron addition after CO₂-coordination to the catalyst facilitates efficient CO₂ reduction, based on cyclic voltammetry analysis.

Full text of DOI:10.1002/ejic.202000259

European Journal of Inorganic Chemistry
Accepted Article
Title: Ligand-assisted electrochemical CO2 reduction by Ru-polypyridyl
complexes
Authors: Debashis Ghosh, Takashi Kajiwara, Susumu Kitagawa, and
Koji Tanaka
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To be cited as: Eur. J. Inorg. Chem. 10.1002/ejic.202000259
Link to VoR: https://doi.org/10.1002/ejic.202000259
0
1/2020

European Journal of Inorganic Chemistry
10.1002/ejic.202000259
FULL PAPER
Ligand-assisted electrochemical CO
2
reduction by Ru-polypyridyl
complexes
[a]
[b]
[b]
[b]
Debashis Ghosh,* Takashi Kajiwara, Susumu Kitagawa and Koji Tanaka*
Dedication ((optional))
4b]
Abstract: The introduction of a simple triazole moiety on a terpyridine
HCOOH reduction in MeCN/H
2
O mixed solvents.[ Herein, we
+
+
II
ligand in the complex [Ru(tpy)(bpy)(Cl)] ([1] ; tpy = 2,2':6',2''-
terpyridine; bpy 2,2'-bipyridine) gives rise to suitable
electrocatalyst for the reduction of CO to CO. The triazole group in
have synthesized two new Ru -polypyridyl complexes,
=
a
[Ru(tpyaz)(bpy)(Cl)][PF
6
], [2][PF
6
]
and [Ru(tpyaz)(tpy)][PF
6 2
] ,
2
[3][PF (Figure 1). We tested their CO
6
]
2
2
reduction abilities by
cyclic voltammetry under different reaction conditions and
compared the results to previously reported complexes
+
+
[
Ru(tpyaz)(bpy)(Cl)] ([2] ; tpyaz
=
4'-(4-(t-butylphenyl)-1H-1,2,3-
+
triazol-4-yl)-2,2':6',2''-terpyridine) enables the complex [2] to bind
with CO
2
even at the one-electron reduced state of the complex.
[Ru(tpy)(bpy)(Cl)][PF
Controlled-potential electrolysis at -1.55 V (vs. FeCp
6
], [1][PF
6
], and [Ru(tpy)
2
][PF
6
]
2
6 2
, [4][PF ] .
nd
+/0
Further 2 -electron addition after CO
2
-coordination to the catalyst
2
using
facilitates efficient CO
analysis.
2
reduction, based on cyclic voltammetry
ferrocene as internal standard) under CO atmosphere
a
2
+
+
+
produced CO as the main product using [1] and [2] , with [2]
showing a higher CO/HCOOH ratio. In contrast, [3]2 and [4]2+
+
2
generated H as the main reduction product under the same
conditions.
Introduction
Nature has developed effective catalysts for CO
light-independent reactions of photosynthesis. The efficient
reduction and fixation of CO greenhouse gas, would
2
reduction in the
2
, a
significantly decrease its associated environmental risks and offer
a sustainable route for the production of carbon-based feedstocks.
Therefore, it is highly desirable to develop new, simpler CO
2
reduction catalysts that use solar or electrical energy to produce
energy-dense products, e.g. formic acid (HCOOH), carbon
monoxide (CO), formaldehyde (HCHO) and methanol (CH
Currently, many encouraging results have been reported for
photochemical[1] and electrochemical[2,3] CO
reductions that
3
OH).
6 6 6 2 6 2 6
Figure 1: Chemical structures of [1][PF ], [2][PF ], [3][PF ] and [4][PF ] ; [PF ]
hexafluorophosphate as counter ion.
2
require two electrons to generate CO or HCOOH. Among various
metal complexes, Ru-polypyridyl complexes have received much
attention,[
2a,d,e,3,4b]
due to their efficiency and stability. Moreover,
Results and Discussion
Ru-polypyridyl complexes have significantly lower overpotentials
2
associated with CO reduction compared to those of other active
Ligand tpyaz was synthesized based on a previously reported
metal complexes.[
2a,3]
In this regard, we recently developed new
Ru -NAD (NAD = nicotinamide adenine dinucleotide) polypyridyl
complexes[4a] and explored their electrocatalytic activity for CO
method with slight modifications (Scheme S1, see SI for full
II
synthetic details).[5] Complexes [1][PF ][6] and [4][PF
[7]
6 6 2
]
were
2
to
synthesized according to literature procedures.
Synthesis and crystal structures of complexes [2][PF
3][PF
Complexes [2][PF
two-step synthesis starting from RuCl
respectively. As such, [RuCl (tpyaz)] was obtained by reacting
6
] and
[
[
a]
b]
Dr. Debashis Ghosh
Department of Applied Chemistry
Karunya Institute of Technology and Sciences (Deemed to be University)
Coimbatore-641114, Tamil Nadu, India
E-mail: deba.kamarpukur@gmail.com
Dr. Takashi Kajiwara, Prof. Susumu Kitagawa and Prof. Koji Tanaka
Institute for Integrated Cell-Material Sciences (KUIAS/iCeMS)
Kyoto University
[
6 2
]
6
] and [3][PF
6
]
2
are both readily available via a
·xH (Scheme 1),
3
2
O
3
RuCl
3
·xH
2
O first with tpyaz (75% yield), followed by bpy in the
+
presence of triethylamine to give [2] as a reddish-brown solid,
Yoshida, Sakyo-ku, Kyoto 606-8501, Japan
Email: koji.tanaka@icems.kyoto-u.ac.jp
which was isolated as the hexafluorophosphate salt in 82% yield
(
3
Scheme 1a). Alternatively, [RuCl (tpyaz)] was treated with tpy in
2
+
presence of triethylamine to give [3] as a deep red solid, which
Supporting information for this article is given via a link at the end of the
document: CVs of complexes [1]+ and [2]+ in presence of mixed
was isolated as hexafluorophosphate salt in 85% yield (Scheme
H
[
2
O/CH
3
CN solutions, characterization data for tpyaz-ligand, complexes
1
6 6 2
b). Fig. 2 shows the molecular structures of [2][PF ] and [3][PF ]
2]+ and [3]2+, 1H, 13C NMR spectra of corresponding ligand and
determined by X-ray crystallography (Table S1).[ Both [2][PF
8]
]
6
complexes. This Supporting Information file is available free of charge on
the Willey publication website at DOI.
and [3][PF were recrystallized from MeCN/Et O. The ruthenium
6
]
2
2
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European Journal of Inorganic Chemistry
10.1002/ejic.202000259
FULL PAPER
ion in complex [2][PF
by five nitrogen atoms (three of tpyaz and two of bpy ligand) and
one Cl ligand. The complex [3][PF also displays an octahedral
6
] shows an octahedral configuration formed
pyridine and triazole in [2][PF
6
6 2
] (Figure 2a) and [3][PF ] (Figure
o
+
o
2+
2b) (12.12 for [2] and 3.56 for [3] ) and between triazole and
o
+
o
2+
6
]
2
the next benzene ring (11.18 for [2] and 15.32 for [3] ) indicate
configuration formed by six nitrogen atoms (three of tpyaz and
three of tpy ligand). The dihedral angles between the central
that both complexes are almost coplanar
.
6 6 2
Scheme 1: Synthesis of complexes [2][PF ] and [3][PF ] .
The Ru-N bond lengths in both complexes were comparable to
ligand in [2][PF
compared to the pure tpyaz ligand.
6
] and [3][PF
6
]
2
were slightly downfield shifted
those in [4] . The 1H NMR spectrum of [2][PF
2
+ [9]
] in CD
CN
[5]
This is due to the
] and
6
3
displayed eleven signals integrated to 23 protons in the aromatic
complexation of the tpyaz ligand with Ru both in [2][PF
[3][PF
6
region, while that of [3][PF
protons also in the aromatic region. The H signals of the tpyaz-
6
]
2
revealed 11 signals integrated to 26
6 2
] .
1
a)
b)
Figure 2: Crystal structure of complexes (a) [Ru(tpyaz)(bpy)(Cl)]+ [2]+ and (b) [Ru(tpyaz)(tpy)]2+ [3]2+ with 50% thermal ellipsoids. The counter anions, the solvent
molecules and the hydrogen atoms are omitted for clarity.
Electrochemical behaviour of complexes [1] -[4]2+ under inert
+
couple at E1/2 = -1.57 V and an irreversible redox at Epc = -1.86 V.
and CO
2
atmosphere.
They are assignable to redox reactions localized on the tpy moiety
+
+
The cyclic voltammogram (CV) of [1] in MeCN under an argon
atmosphere (Figure 3, black) showed a reversible reduction
couple at E1/2 = -1.63 and an irreversible redox wave at Epc = -
of tpyaz and bpy, respectively, similar to [1] in CH
3
CN (cf. Figure
3, black). Introduction of CO
solution of [1] resulted in the appearance of catalytic current near
the second redox wave (threshold potentials of -1.76 V, cf. Figure
2
(solvent saturation for 20 min) to the
+
1
.87 V. These redox waves are attributed to redox reactions
localized on tpy and bpy, respectively, in accordance to a previous
report.[3d] Likewise, the CV of [2]+ in CH
CN under inert
atmosphere (Figure 4, black) showed a reversible reduction
+
3, red). Interestingly, complex [2] under similar conditions
3
showed significant two-step current enhancement near both first
and second redox waves (threshold potentials of -1.55 V and -
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European Journal of Inorganic Chemistry
10.1002/ejic.202000259
FULL PAPER
1
.78 V, respectively, cf. Figure 4, red). This result indicates that
addition of a second electron to [3]2+ caused increased cathodic
current at -1.70 V (cf. Figure 5, red).
the triazole moiety of tpyaz may have direct interaction with CO
causing increase of current near the first redox wave.
2
2
+
[
3] under Ar
-
40
20
0
-20
0
2+
+
[3] under CO2
[
[
1] under Ar
+
-
1] under CO2
2
0
0
20
40
60
80
4
60
-
2.0 -1.5 -1.0 -0.5
0.0
0.5
1.0
+
/0
V vs [FeCp2]
-2.0
-1.5
-1.0
-0.5
0.0
0.5
2+
2
Figure 5. Cyclic voltammograms of [3] under Ar (black) and CO (red) at a
+
/0
scan rate  = 100 mV s . Conditions: glassy carbon electrode, 1 mM [3]2+, 0.1
-1
V vs [FeCp2]
M [Bu N][PF ], in CH CN.
4
6
3
+
Figure 3. Cyclic voltammograms of [1] under Ar (black) and CO
2
(red) at a scan
-1
+
rate  = 100 mV s . Conditions: glassy carbon electrode, 1 mM [1] , 0.1 M
[
4 6 3
Bu N][PF ], in CH CN.
-20
0
-
40
+
[
2] under Ar
+
2+
-
20
0
[2] under CO
[4] under Ar
2
20
2
+
[
4] under CO2
2+
[
4] under CO2
4
6
8
0
0
0
2
4
6
8
0
0
0
0
-2.0
-1.5
-1.0
-0.5
0.0
/0
0.5
1.0
+
V vs [FeCp ]
1
00
20
2
1
2
+
Figure 6. Cyclic voltammograms of [4] under Ar (black) and CO
a scan rate  = 100 mV s . Conditions: glassy carbon electrode, 1 mM [4]2+, 0.1
M [Bu N][PF ], in CH CN.
4 6 3
2
(red, blue) at
-1
-2.0
-1.5
-1.0
-0.5
0.0
0.5
+
/0
V vs [FeCp2]
+
In contrast, the addition of a second electron to [4]2+ (at -1.77 V)
did not cause the same current effect (cf. Figure 6, red). These
findings clearly indicate that the triazole moiety in [3]2+ enables
Figure 4. Cyclic voltammograms of [2] under Ar (black) and CO
2
(red) at a scan
-1
+
rate  = 100 mV s . Conditions: glassy carbon electrode, 1 mM [2] , 0.1 M
[
4 6 3
Bu N][PF ], in CH CN.
direct interaction with CO
2
that leads to the increase in current
near the second redox wave.
Inspired by these observations, we recorded CVs of [3]2+ in
CH CN under both an argon and CO atmosphere and compared
the results with those of the reported complex [4] . The CV of
3
2
+
2+
Bulk electrolysis of complexes [1] -[4] under CO
To evaluate the efficiency of complex [2] in CO
its first redox wave (threshold potential of -1.55 V), controlled-
potential electrolysis was performed for both [1] and [2] at -1.55
2
2
+
+
2
+
2
reduction near
[
3
3] in CH CN under an argon atmosphere (Figure 5, black)
showed two reversible reduction couples at E1/2 = -1.50 and -1.77
V, which are attributed to ligand redox reactions localized on tpyaz
and tpy, respectively. The same pattern was observed for [4]2+ in
+
+
V (see Table 1). Only trace amounts of CO were detected for both
1]+ (Table 1, entry 1) and [2] (Table 1, entry 2). It shows that
+
[
CH
assignable to redox reactions localized on the two tpy ligands; cf.
Figure 6, black.[10] More importantly, under CO
atmosphere, the
3
CN (E1/2 = -1.52 V and -1.77 V). Here the signals are
2
addition of only one electron is not enough to drive CO reduction
+
+
for catalyst [1] or [2] . Recent reports, however, pointed out that
2
addition of a proton source can facilitate two-electron and two-
proton reduction of CO to CO and HCOOH in CH
2 3
CN.[11] We
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European Journal of Inorganic Chemistry
10.1002/ejic.202000259
FULL PAPER
consequently decided to add H
and [2] were recorded under a CO
2
O as a proton source. CVs of [1]+
atmosphere with increasing
O (cf. Figures S1 and S2, respectively). For
density (see Figure S6 and Table S2) and current efficiency (25.6
+
and 41.6% for [1] and [2] , respectively) was higher for [2]+
+
+
2
+
concentration of H
2
compared to [1] . With increasing water concentration (10% H
2
O
+
+
both [1] and [2] , the catalytic current increased significantly and
at the same time threshold potentials shifted to more positive
values (-1.65 V and -1.55 V, for [1]+ and [2] , respectively, cf.
Figure S1 and S2). This large enhancement of catalytic current is
in CH
3
CN), the tendency for CO
2
reduction over H
2
evolution
+
+
decreased for both [1] (Table 1, entry 5) and [2] (Table 1, entry
+
2 3
6). Therefore, 5% H O in CH CN was considered the best solvent
mixture for both catalysts. For comparison, controlled-potential
electrolysis was also performed for [3]2 and [4]2+ at -1.55 V in 5%
+
likely due to increased acidity in the CO
solution (eq 1).[
2
-saturated, aqueous
4b,11]
2+
2+
2 3
H O in CH CN. Neither complex [3] (Table 1, entry 7) nor [4]
(
Table 1, entry 8) proved to be suitable catalysts under these
+
reaction conditions. The discrepancy in catalytic behavior for [2]
and [3]2+ is probably caused by the different ligand structure. It is
+
Controlled-potential electrolysis was then performed for [1] and
+
[
1
2] at -1.55 V in CO
2
-saturated MeCN in presence of 5% and
plausible to assume that, after electron induced dissociation of the
+
0 % H
2
O (see Table 1). To our surprise, complex [2] showed a
-
+
2
Cl ligand in [2] , metal-centered CO reduction is significantly
higher CO/HCOOH selectivity in presence of 5% H
entry 4), whereas [1] produced large amounts of hydrogen and
2
O (Table 1,
slower in [3]2+ due to the presence of an occupying, tridentate tpy-
ligand compared to [2] with a vacant coordination site.
+
+
minor quantities of CO (Table 1, entry 3). Moreover, current
Table 1: Results of bulk electrolysis for complexes [1] - [4]2+[a]
+
Entry
Catalyst
% of H
2
O in
Coulombs
consumed in 6
h
Product [mol][b]
Selectivity (CO / HCOO-)
Selectivity (CO +
-
CH CN
3
HCOO / H
2
)
CO
H
2
HCOO-
1
2
3
4
5
6
7
8
[1][PF
[2][PF
[1][PF
[2][PF
[1][PF
[2][PF
6
6
6
6
6
6
]
]
]
]
]
]
0
0
2.1
3
0.25(2.2).
0.5(3.5)
15.9(17)
49(38)
N.D
N.D.
N.D.
N.D
-
-
-
-
2
5
18.3
25
23
29
6
26.7(28.6)
7.7(5.9)
39.6(33.3)
18.9(12.6)
5(16)
8(8.6)
0.9
7
5
4.8(3.6)
12.1(10)
10.4(6.9)
Trace.
N.D
10.2
1.5
5
10
10
5
18.3(15.4)
51.6(34.4)
Trace
0.77
3.3
-
[3][PF
[4][PF
6
]
2
-
6
]
2
5
4
N.D
4(19)
-
-
[
a] Controlled-potential electrolysis was carried out at -1.55 V (vs FeCp +/0) using 1 mM complex in corresponding solvent system in the presence of 0.1 M [n-
2
Bu N][PF ] as the supporting electrolyte; working electrode: glassy carbon; counter electrode: Pt. [b] current efficiency (%) given in parenthesis. Turnovers of the
4
6
catalysts with respect to CO under 5% of H
supporting information.
2
O in CH
3
CN: [1]+ = 0.71; [2]+ = 2.5 after 6 h hours. For calculations of current efficiency and turnovers, please see
-
to react at the Ru-center.[4b]
Based on the experimental data, it is reasonable to propose
release of Cl and allowing CO
2
+
the following reaction pathway for CO
2
activation by [2] (Scheme
Addition of small amounts of water lowers the overpotential for
+
[4b,11]
2
): Cyclic voltammograms of [2] (Figure 4) displayed significant
CO
2
reduction
2
(Figure S1 and S2) due to CO -assisted acidity
current enhancement near the first redox wave, while bulk
electrolysis displayed no occurrence of significant CO reduction
near the applied potential of -1.55 V. It shows that the addition of
increase in aqueous solutions (eq 1). Although the mechanism for
+
2
CO
2
reduction by [2] is not fully understood yet, it is clear from
+
the obtained experimental data that the tpyaz moiety in [2] plays
one electron to the tpyaz moiety enables weak interaction of the
an important role in the initial CO
interactions with CO . More detailed investigations regarding
these mechanistic steps are currently underway in our laboratory.
2
activation stage due to weak
+
triazo-group with CO
2
. The second electron injection into [2] then
2
results in the appearance of strong catalytic current. At this point,
the second electron is located at the bpy moiety causing the
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European Journal of Inorganic Chemistry
10.1002/ejic.202000259
FULL PAPER
+
2
Scheme 2. Proposed scheme for CO activation by [2] . Red-highlighted ligand fragments indicate the one-electron-reduced state.
room temperature. The controlled-potential electrolysis was performed in
an H-type electrolysis cell with two compartments: one for a glassy carbon
working electrode and the Ag/Ag+ reference electrode, and the other for
the Pt counter electrode, which was separated from the working electrode
by a Nafion membrane. Both compartments were made airtight by
attaching silicon septa with vacuum grease. Initially, to each compartment
was added 20 mL of a solution containing 0.1 M [n-Bu N][PF ] as the
supporting electrolyte. Afterwards, 1 mM of the corresponding complex
was added to the working electrode compartment, and the mixture was
stirred for 10 min. Both compartments were initially degassed by Ar
saturation (15 min), before the compartment of the working electrode was
saturated with 1 atm of CO2 for 20 min. The controlled-potential electrolysis
was performed using a HOKUTO DENKO HZ-5000 potentiostat. During
the electrolysis, gaseous aliquots (0.5 mL) were sampled from the
headspace using a gas-tight syringe, and injected into the GC. Liquid
samples (500 L) were taken from the reaction mixture, before they were
treated with a 0.1 M aqueous solution of NaOH (250 L). The solutions
were then evaporated to dryness on a rotary evaporator. The resulting
solid was treated with 500 L of water and filtered, before the quantities of
HCOOH were measured using HPLC analysis. The quantification of H2,
CO and HCOOH were carried out using a calibration curve (Figure S3,S4
and S5 respectively).
Conclusions
In summary, we have demonstrated that incorporating a triazole
moiety in the ligand framework can significantly enhance the
performance of CO reduction by using Ru-polypyridyl complexes.
2
Both CV and bulk electrolysis results evidently indicate that
4
6
+
complex [2] containing a triazole ring is more efficient and
+
selective towards CO formation than [1] which lacks the triazole.
Further exploration of this moiety and related functional groups for
more efficient Ru-catalysts is underway in our laboratory.
Experimental Section
Materials and methods
Complexes [1]+ [6] and [4]2+ [7] were synthesized according to literature
reports and characterized by NMR and ESI-MS spectroscopy. Detailed
synthetic procedures for complexes [2]+ and [3]2+ were described in the
Supporting Information and characterized by NMR, ESI-MS, elemental
data
and
X-ray
crystallography.
Tetra-n-butylammonium
hexafluorophosphate [n-Bu4N][PF6] was purchased from Tokyo Kasei
Organic Chemicals and recrystallized from ethanol. CO2 was purchased
from NIPPON EKITAN Corporation (research grade, 99.99% purity; < 3
ppm H2O) and used as received. All other chemicals (reagent grade) were
obtained from common commercial suppliers and used as received. Air-
sensitive materials were prepared and manipulated using Schlenk
techniques and/or an argon-filled glovebox (Glovebox Japan Inc., model
number, GBJF100).
Acknowledgments ((optional))
This work was funded by Ministry of Education, Culture, Sports,
Science, and Technology of Japan (MEXT) Project for Regional
Innovation Strategy Support Program “Next-generation Energy
System Creation Strategy for Kyoto” and “ The Advanced
Catalytic Transformation Program for Carbon Utilization (ACT-C,
Grant Number JPMJCR12YB) from Japan Science and
Technology Agency (JST)”. Dr. D. Ghosh is thankful to Dr. David
C. Fabry, Tokyo Institute of Technology, Japan, for helpful
discussions.
1H and 13C NMR spectra were measured in CD3CN on a BRUKER
AVANCE III spectrometer at 293 K. The chemical shifts are given in ppm
relative to the residual solvent signal of CHD2CN ( 1.94) or solvent signal
1
3
[12]
of CD3CN ( 1.32).
Electrospray-ionization mass spectrometry (ESI-
MS) was performed on a Thermo Fisher Scientific EXACTIVE instrument.
Gaseous products were analyzed using a SHIMADZU GC-2014 gas
chromatograph equipped with a thermal conductivity detector (TCD).
Keywords: Carbon dioxide reduction • Electrochemistry •
Homogeneous • Polypyridyl ligand • Ruthenium
-
Detection and quantification of the formate anion (HCOO ) were carried
out by SHIMADZU Prominence HPLC analysis.
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1]
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model 660A electrochemical analyzer. The electrode setup included a
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2
glassy carbon disc (0.071 cm ) working electrode that was polished with
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Entry for the Table of Contents (Please choose one layout)
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By incorporating a triazole moiety in
the ligand framework in newly
synthesized Ru-polypyridyl
2
Electrochemical CO reduction*
Debashis Ghosh,* Takashi Kajiwara,
Susumu Kitagawa and Koji Tanaka*
complexes, efficient electrocatalytic
reduction of CO
2
to CO was achieved
O (95/5) solvent.
Page No. – Page No.
in mixed CH CN/H
3
2
Ligand-assisted electrochemical CO
reduction by Ru-polypyridyl
complexes
2
*
one or two words that highlight the emphasis of the paper or the field of the study
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