Redox behavior of ruthenium(bipyridine)(terpyridine)(carbonyl) complex-modified carbon electrode and reactivity toward electrochemical reduction of CO2

Article abstract of DOI:10.1246/cl.2010.1134

Electrochemical reduction of a diazonium complex [Ru-(bpy)(trpy-ph-N ₂⁺)(CO)](PF₆)₃([1](PF ₆)₃) (bpy = 2,2′-bipyridine, trpy-ph-N ₂⁺ = 4-(2, 2′:6′,2″-terpyridine- 4′-yl)benzenediazonium) using a GC electrode forms multilayer films of the complexes on the electrode surface, on which the reduced form of the complex showed strong affinity toward CO₂.

Full text of DOI:10.1246/cl.2010.1134

doi:10.1246/cl.2010.1134
1134
Published on the web September 28, 2010
Redox Behavior of Ruthenium(Bipyridine)(Terpyridine)(Carbonyl) Complex-modified
Carbon Electrode and Reactivity toward Electrochemical Reduction of CO2
Yuhei Tsukahara, Tohru Wada, and Koji Tanaka*
Coordination Chemistry Laboratories, Institute for Molecular Science, 5-1 Higashiyama, Myodaiji, Okazaki, Aichi 444-8787
(Received August 9, 2010; CL-100680; E-mail: ktanaka@ims.ac.jp)
Electrochemical reduction of a diazonium complex [Ru-
(bpy)(trpy-ph-N2+)(CO)](PF6)3 ([1](PF6)3) (bpy = 2,2¤-bipyri-
dine, trpy-ph-N2+ = 4-(2,2¤:6¤,2¤¤-terpyridine-4¤-yl)benzenedi-
azonium) using a GC electrode forms multilayer films of the
complexes on the electrode surface, on which the reduced form
of the complex showed strong affinity toward CO2.
Electrocatalytic reactions using functionalized electrodes
modified with redox-active1 molecular catalysts, or proteins2
have led to supplementary advantages in terms of enhancement
of catalytic efficiency because of minimizing the amounts of
Scheme 1. Synthesis of diazonium ruthenium complex
[1](PF6)3.
catalysts and facilitating electron transfer between catalysts and
electrodes.3 However, bonding strengths between electrode
surfaces and catalyst molecules are not always strong enough
to be utilized in electrocatalytic reductions. For example, self-
assembled thiolate monolayers (SAMs) on gold surfaces have
relatively narrow electrochemical potential windows and desorb
at potentials more negative than ¹1.0 V (vs. SCE) in aqueous
media.4 In fact, modified electrodes with molecular catalysts
have hardly been applied for electrochemical CO2 reductions
since electrolysis in those reactions are usually conduced at
potentials more negative than ¹1.0 V. Among various method-
ologies to obtain functionalized electrodes modified with redox-
active molecules, electrografting by irreversible reduction of
diazonium compounds is a feasible process to form a strong C-C
bonds between redox-active molecules and surfaces of carbon
electrodes.5
¹1
Figure 1. Repetitive CV of [1]3+ (10¹3 M) at 50 mV s in
We describe here modification of a ruthenium(bipyridine)-
(terpyridine)(carbonyl) framework on a carbon electrode with a
C-C bond to clarify the reactivity of the complex embedded
in the films toward electrochemical CO2 reduction, since [Ru-
(bpy)(trpy)(CO)](PF6)2 ([2](PF6)2) (trpy = 2,2¤:6¤,2¤¤-terpyri-
dine) has the ability to catalyze electrochemical reduction of
CO2 at ¹1.60 V (vs. Ag/Ag+) in DMF/H2O.6
A [Ru(bpy)(trpy-ph-NH2)Cl]Cl (trpy-ph-NH2 = 4¤-(4-ami-
nophenyl)-2,2¤:6¤,2¤¤-terpyridine) complex was obtained by
reaction of [Ru(bpy)(dmso)2Cl2] with trpy-ph-NH2. Treatment
of [Ru(bpy)(trpy-ph-NH2)Cl]Cl with CO (20 atm) followed by
NH4PF6 afforded yellow crystals of [Ru(bpy)(trpy-ph-NH2)-
(CO)](PF6)2 ([3](PF6)2) in 63% yield. The reaction of [3](PF6)2
with sodium nitrate gave a diazonium complex, [Ru(bpy)(trpy-
ph-N2+)(CO)](PF6)3 ([1](PF6)3), in 78% yield (Scheme 1). The
IR spectra of [1](PF6)3 showed characteristic ¯(N¸N) and
¯(C¸O) at 2282 and 2004 cm¹1, respectively.
CH3CN containing 0.1 M Bu4NPF6 using a GC electrode
(S = 0.071 cm2) in a potential region 0 and ¹1.4 V.
¹1.14 V) assigned to reduction of diazonium and two reversible
(trpy/trpy ) redox couples (E1/2 = ¹1.54 and ¹1.70).7 The
¹•
¹0.80 V cathodic wave of [1]3+, therefore, is associated with the
irreversible reduction of the aryldiazonium affording N2 and
aryl radical that is responsible for formation of a C-C bond with
the carbon electrode surface. Positive shift of the cathodic peak
potential of [1]3+ compared with [4]3+ is attributed to an
electron-withdrawing effect of CO in the former. Repetitive
potential sweeps between ¹1.40 and 0 V caused an increase the
peak currents of the E1/2 = ¹1.30 V redox couple of [1]3+ and
positive shift of the redox potential to E1/2 = ¹1.24 V due to
grafting of the complexes on the carbon electrode surface.
Modification on a GC electrode was carried out under the
electrolysis of [1](PF6)3 at ¹1.00 V in the presence of TBAPF6
(0.1 M) in CH3CN for 8 min. Then the carbon electrode was
rinsed with CH3CN to remove any physisorbed species. The CV
of the modified glassy carbon electrode 1/GC thus obtained
The cyclic voltammogram (CV) of [1](PF6)3 using a glassy
carbon (GC) electrode displays one irreversible cathodic peak
¹•
at ¹0.80 V (vs. Ag/Ag+) and one reversible (trpy/trpy )
redox couple at E1/2 = ¹1.30 V in the first potential scan in
CH3CN (Figure 1). An analogous [Ru(trpy)(trpy-ph-N2+)](PF6)3
exhibits the trpy/trpy redox couple at E1/2 = ¹1.25 V, that is
¹•
([4](PF6)3) exhibits one irreversible cathodic wave (Epc
=
close to the value of [2](PF6)2 (E1/2 = ¹1.33 V) in CH3CN
Chem. Lett. 2010, 39, 1134-1135
© 2010 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/

1135
of CO2 in CH3CN/H2O (4:1 v/v) containing 0.1 M of TBAPF6
for 0.5 h produced CO (0.9 ¯mol), HCOOH (0.3 ¯mol), and H2
(1 ¯mol) with current efficiencies 30, 10, and 30%, respectively,
but the cathodic currents dramatically decreased within 0.5 h.
Scanning electron micrographs (SEM) of the 1/GC showed
rough granular surface compared to those of unmodified GC
(Figure S39). After electrochemical reduction of CO2 using the
1/GC, the SEM images revealed disappearance of the rough
granular surface in some part of the modified region due to
partial detachment of the multilayer film from the GC surface.
On the other hand, [2](PF6)2 (1.0 mM) worked stably as a
catalyst in electrochemical CO2 reduction and produced CO and
HCOOH at ¹1.70 V in CH3CN/H2O. Detachment of the multi-
layer film from the 1/GC surface in the present study, therefore,
may be caused mainly by collapse of the surface structure of
the GC plate rather than degradation of the metal complex
and fragmentation of the multilayer films during the CO2
reduction. Taking into account achievement of a high current
density (6 mA cm¹1) of 1/GC in the electrochemical CO2
reduction under 20 atm of CO2, methodological developments
to depress detachment of redox active catalysts from electrode
surfaces may open a new area in a utilization of CO2 as a C1
resource.
¹1
Figure 2. (a) CV of 1/GC (S = 0.071 cm2) at 10 mV s in
CH3CN containing 0.1 M Bu4NPF6 in a potential region from
¹0.80 to ¹1.40 V (blue line) and from ¹0.80 to ¹1.75 V (red
line) under N2; (b) under N2 (black line), under 1 atm of CO2
(red line), and under 20 atm of CO2 (blue line) in a potential
region to ¹1.75 V.
(Figure 2a).6 The IR reflection absorption spectroscopy (IRRAS)
of the modified electrode 1/GC showed characteristic peaks
at 2009 and 862 cm assignable to ¯(C≡O) and ¯(PF6 ),
respectively, whereas any bands associated with ¯(N¸N)
completely disappeared (Figure S19). These results demonstrate
the retention of Ru(bpy)(trpy)(CO) framework on the surface of
the carbon electrode.
¹1
¹
The closed-packed monolayer coverage (Γ) of the complex
¹10
on a perfectly flat surface is estimated to be ca. 1.6 © 10
We thank Mr. Takuya Iizuka and Dr. Shin-ichi Kimura
(IMS) for IRRAS measurements and Mr. Satoru Nakao (IMS)
for SEM measurements. This work was supported by the Grant-
in-Aid for Scientific Research on Priority Areas (No. 20002005)
from Ministry of Education, Culture, Sports, Science and
Technology, Japan.
¹2
mol cm based on the crystal structure of the amino derivative
[3](PF6)2 determined by X-ray analysis (Figure S29). An actual
¹8
coverage (Γ) of the complex was calculated as 3.1 © 10
¹2
mol cm from the area of the cathodic (and anodic) wave.
The Γ value decreased only to 2 © 10¹9 mol cm even when a
¹2
highly oriented pyrolytic graphite (HOPG) with a basal plane
was used in place of a GC plate, indicating formation of a multi-
layer film consisting of the Ru(bpy)(trpy)(CO) unit, as reported
somewhere.8 The multilayer film would be formed by a
repetitive attack of Ru(trpy-ph•) generated in the irreversible
reduction of [Ru(bpy)(trpy-ph-N2+)(CO)]3+ to carbon on bpy or
trpy of the adsorbed Ru(bpy)(trpy-ph)(CO) complexes on the
carbon surface by considering that bpy would be located at the
most outside layer of the modified film. An electrografting
polyfilm, therefore, would consist almost entirely of an Ru(trpy-
aryl)(bpy-Ru) bridge.
References and Notes
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The anodic peak of the E1/2 = ¹1.25 V redox couple
disappeared after the potential sweep is expanded to ¹1.75 V
(Figure 2a). It is worthy of note that the CVof the homogeneous
solution containing the analogous [Ru(bpy)(trpy)(CO)]2+ [2]2+
¹•
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6
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couple at ¹1.33 V and one irreversible cathodic peak of the
¹•
(bpy/bpy ) wave at ¹1.69 V that is caused by cleavage of the
Ru-CO bond of [2]0.6 The disappearance of anodic wave (trpy/
¹•
trpy ) of the 1/GC after the potential sweep to ¹1.75 V,
therefore, is associated with the Ru-CO bond cleavage caused
by successive trpy and bpy localized reductions. The cathodic
currents of the 1/GC increased at potentials more negative than
7
8
9
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Supporting Information is available electronically on the CSJ-
Journal Web site, http://www.csj.jp/journals/chem-lett/index.
html.
¹•
the irreversible cathodic (bpy/bpy ) wave under 1 atm of CO2.
Furthermore, irreversible cathodic currents increased sixfold
when the CV was measured under 20 atm of CO2 (Figure 2b),
indicating that the rate-determining step of the present electro-
chemical CO2 reduction involves CO2 attack at the Ru center.
¹8
Controlled-potential electrolysis using the 1/GC (5 © 10
mol, surface area S = 2 cm2) at ¹1.70 (vs. Ag/Ag+) under 1 atm
Chem. Lett. 2010, 39, 1134-1135
© 2010 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/