Selective production of acetone in the electrochemical reduction of CO2 catalyzed by a Ru-naphthyridine complex

Article abstract of DOI:10.1002/(SICI)1521-3773(19990201)38:3<362::AID-ANIE362>3.0.CO;2-T

The controlled potential electrolysis of [Ru(bpy)(napy)₂(CO)₂](BF₄)₂ (1; bpy = 2,2'-bipyridine, napy = 1,8-naphthyridine) in the presence of LiBF₄ in CO₂-saturated DMSO at -1.65 V (vs. Ag/Ag⁺) produced CO and Li₂CO₃ [Eq. (a)], while similar electrolysis in the presence of (CH₃)₄NBF₄ resulted in formation of acetone together with (CH₃)₃N and {(CH₃)₄N}₂CO₃ [Eq. (b)]. This represents the first almost selective generation of acetone upon electrochemical reduction of CO₂. The selectivity is ascribed to depression of reductive cleavage of the Ru-CO bond of 1 due to an attack of the nonbonded nitrogen atom of napy at the carbonyl carbon atom.

Full text of DOI:10.1002/(SICI)1521-3773(19990201)38:3<362::AID-ANIE362>3.0.CO;2-T

COMMUNICATIONS
Selective Production of Acetone in the
Electrochemical Reduction of CO Catalyzed
2
by a Ru ± Naphthyridine Complex
Tetsunori Mizukawa, Kiyoshi Tsuge,
Hiroshi Nakajima, and Koji Tanaka*
Electrochemical reduction of CO with metal complexes is
2
a feasible technique for the utilization of CO as a C source,
2
1
though the reduction products usually have been limited to
CO and/or HCOOH. Metal-catalyzed reductive disproportio-
nation of CO [Eq. (1)] proceeds through oxide transfer from
2
a metal ± CO moiety to CO [Eq. (2)] followed by reductive
2
2
cleavage of the resultant metal ± CO bond.
Figure 1. Molecular structure of the dication of 2 with atom labeling.
2�
2
CO
2
‡ 2e� �ꢀ CO ‡ CO
3
(1)
(2)
n‡
�ꢀ [M�CO](n‡2)‡ ‡ CO
2�
� 1
[
M�CO
2
]
‡ CO
2
3
The cyclic voltammogram of 1 (0.1 mmolL ) in DMSO in
�
1
the presence of (CH ) NBF (0.1 molL ) under N shows an
3
4
4
2
Acylation of the metal ± CO bond would lead to a new
methodology for carbon ± carbon bond formation in the
irreversible cathodic wave at � 1.40 V (all potentionals are
‡
given versus that of Ag/Ag ) and three (quasi-)reversible
reduction of CO . Polypyridyl ruthenium carbonyl complexes
redox couples at E ˆ � 1.63, � 1.80, and � 2.02 V with peak
2
pc
2
‡
such as [Ru(bpy) (quinoline)(CO)]
and [Ru(bpy)-
separations (DE ) of 60, 80 and 94 mV, respectively (Fig-
2
p±p
(
terpyridine)(CO)]2 (bpy ˆ 2,2'-bipyridyl) work as catalysts
‡
ure 2). The bis-napy complex 2 displays two irreversible
[
1]
for the reduction of CO according to Equation (1), and one-
2
electron reduction of these complexes causes a bathochromic
�
1 [2]
shift of their n(CO) bands by about 30 ± 40 cm . The mono-
naphthyridine complex [Ru(bpy) (napy)(CO)](PF ) (1;
2
6 2
napy ˆ 1,8-naphthyridine) undergoes a pronounced batho-
�
1
chromic shift of the n(CO) band (D nÄ ˆ 418 cm ) upon one-
electron reduction due to nucleophilic attack of the non-
bonded nitrogen atom of the monodentate napy ligand on the
[
3]
carbonyl carbon atom [Eq. (3)].
Figure 2. Cyclic voltammograms of 1 and 2 in the presence of (CH
3
)
4
NBF
4
under N
versus Ag/Ag .
2
(ÐÐ) and CO
2
(- - - -) in DMSO. The potential E is given in V
‡
Such a metallacyclization would suppress the reductive
cleavage of the Ru�CO bond (CO evolution), and enable
cathodic waves at E ˆ � 0.76 and � 0.98 V as well as three
pc
reduction of the CO group derived from CO . Here we report
2
quasi-reversible couples at E ˆ � 1.44, � 1.58, and � 1.94 V
pc
the first selective production of acetone in the electrochemical
reduction of CO catalyzed by [Ru(bpy)(napy) (CO) ](PF )
with DE ˆ 80 mV under the same conditions. One irrever-
p±p
2
2
2
6
2
sible cathodic wave of 1 (E ˆ � 1.42 V) and two irreversible
pc
(
2) in the presence of (CH ) NBF .
3
4
4
waves of 2 (E ˆ � 0.76 and � 0.98 V) result from reduction of
pc
The two CO ligands of 2 are coordinated to Ru in cis
the napy ligands followed by formation of the Ru-C(O)-N-N
positions, and the two napy moieties are linked to Ru in a
monodentate manner, as depicted in the crystal structure
ring [Eq. (3)]. Introduction of CO to the solutions of 1 and 2
2
[
4]
in DMSO causes strong catalytic currents due to the reduction
of CO (Figure 2), since electrochemical reduction of CO
(Figure 1).
2
2
[
*] Prof. Dr. K. Tanaka, T. Mizukawa, Dr. K. Tsuge
Institute for Molecular Science
takes place at potentials more negative than � 2.4 V in the
absence of these complexes. It is noteworthy that the
Myodaiji, Okazaki, Aichi, 444-8585 (Japan)
Fax: (‡81)564-5245
reduction of CO by 2 occurs at potentials more negative
2
than � 1.4 V, while the threshold potential of the CO
2
E-mail: ktanaka@ims.ac.jp
reduction by 1 is close to � 2.0 V.
Dr. H. Nakajima
Japan Advanced Institute of Science and Technology
Tatsunokuchi, Ishikawa (Japan)
In agreement with Equation (1), only CO and Li CO were
2
3
produced in the controlled potential electrolysis of 2
3
62
ꢀ WILEY-VCH Verlag GmbH, D-69451 Weinheim, 1999
1433-7851/99/3803-0362 $ 17.50+.50/0
Angew. Chem. Int. Ed. 1999, 38, No. 3

COMMUNICATIONS
�
1
(
0.6 mmolL ) at � 1.65 V with a glassy carbon (GC) elec-
(napy)
2
Cl]BF
4
in ethanol was heated at 608C under CO (40 atm) for
2
[5]
15 h. Concentration of the solution and treatment with an aqueous solution
of NaBF gave [Ru(bpy)(napy) (CO) ](BF as a yellow powder in 70%
yield. Elemental analysis calcd for C28
11.24; found: C 44.83, H 2.39, N 11.24; IR: nÄ ˆ 2077, 2027 cm (CO).
trode (4 cm ) in CO -saturated DMSO (30 mL) containing
2
4
2
2
4 2
)
�
1
LiBF (0.1 molL ) as electrolyte. On the other hand, when
4
H
20
N
6
B
2
F
8
Ru: C 44.98, H 2.68, N
(
CH ) NBF was used as an electrolyte under otherwise
� 1
3
4
4
similar conditions, acetone was selectively generated with a
traces of CO. The amounts of these products increased with
the electricity, as shown in Figure 3. Besides acetone and CO,
Received: June 8, 1998 [Z11960IE]
German version: Angew. Chem. 1999, 111, 373 ± 374
Keywords: carbon dioxide ´ Ketones ´ N ligands ´ reduc-
tions ´ ruthenium
[1] a) A. G. M. Mostafa Hossain, T. Nagaoka, K. Ogura, Electrochim. Acta
1
1
997, 42(16), 2577; b) G. N. A. Nallas, K. J. Brewer, Inorg. Chim. Acta
996, 253, 7; c) A. G. M. Mostafa Hossain, T. Nagaoka, K. Ogura,
Electrochim. Acta 1996, 41(17), 2773; d) S. Matsuoka, T. Kohzuki, C.
Pac, S. Yanagida, Chem. Lett. 1990, 11, 2047; e) S. Ikeda, Y. Saito, M.
Yoshida, N. Noda, M. Maeda, K. Ito, J. Electroanal. Chem. Interfacial
Electrochem. 1989, 260, 335; f) N. A. Maiorova, O. A. Khazova, Yu. B.
Vasilꢁev, Electrokhimiya 1986, 22, 1196; g) Yu. B. Vasilꢁev, V. S.
Bagotskii, O. A. Khazova, N. A. Maiorova, J. Electroanal. Chem.
Interfacial Electrochem. 1985, 189, 295.
[2] a) H. Nakajima, Y. Kushi, H. Nagao, K. Tanaka, Organometallics 1995,
Figure 3. The amounts x of acetone (*) and CO (*) generated in the
14; b) K. Toyohara, K. Tanaka, unpublished results.
electrochemical reduction of CO
CH NBF
in DMSO at � 1.60 V. The dashed line represents the
theroretical amount of acetone expected based on Equation (4).
2
catalyzed by 2 in the presence of
[3] H. Nakajima, K. Tanaka, Chem. Lett. 1995, 10, 891.
(
3
)
4
4
[4] a) Crystal data for 1: monoclinic, P2 /n, a ˆ 10.696(2), b ˆ 14.989(3),
1
3
c ˆ 18.395(2) Š, b ˆ 99.58(1)8, Vˆ 2907.7(8) Š , Z ˆ 4,
1
calcd
ˆ
�
3
1
.707 gcm , 20.2 < 2q < 23.68, MoKa (l ˆ 0.71069 Š); of 7296 reflec-
tions observed, 6936 were independent. The structure was solved by
(
CH ) N and {(CH ) N} CO were the only other products
direct methods (SAPI91) and refined against jF j with 2230 reflections
3
3
3
4
2
3
‡
detected in the solution. Thus, (CH ) N works not only as an
(I > 3.0s(I)) and 234 parameters; R ˆ 0.071 and
R
w
ˆ 0.060.
3
4
b) Crystallographic data (excluding structure factors) for the structure
reported in this paper have been deposited with the Cambridge
Crystallographic Data Center as supplementary publication no.
CCDC-101753. Copies of the data can be obtained free of charge on
application to CCDC, 12 Union Road, Cambridge CB21EZ, UK (fax:
(‡44)1223-336-033; e-mail: deposit@ccdc.cam.ac.uk).
electrolyte, but also as a methylation reagent for the catalytic
generation of acetone in the electrochemical reduction of CO2
catalyzed by 2 [Eq. (4)].
�
‡
2�
3
2
CO
2
‡ 4e ‡ 2(CH
3
)
4
N
�ꢀ CH
3
C(O)CH
3
‡ CO
‡ 2(CH
3
)
3
N
(4)
�
[
[
5] The concentration of residual water in the DMSO was 21 mmolL 1.
6] The electrochemical reduction of CO
2
also took place in CH
is likely to deposit on the electrode. An
orange solution obtained upon concentration of the electrolyzed
solution of in CH CN became yellow after exposure to air.
3
CN,
Based on the stoichiometry of Equation (4), the current
efficiency of acetone was 70% at 1008C (turnover number
.5) and that of CO was less than 1%. This first selective
formation of acetone in the electrochemical reduction of CO2
Eq. (4)] is associated with the suppression of reductive
2
�
though in this case CO
3
8
2
3
Appearance of the two n(CO) bands typical of 2 in the IR spectra of
the resultant yellow solution strongly indicates that the reduced form of
[
2
is the stable catalyst in the formation of acetone.
cleavage of the Ru�CO bond [Eq. (1)] by the metallacycliza-
� 2
[
7] An initial current density of 5 mAcm dropped to about one-third at
[
6]
tion shown in Equation (3). Indeed, the current densities of
1
3 4 4
00 C due to consumption of (CH ) NBF . Renewed addition of
the GC electrodes in the reduction of CO catalyzed by 2 were
electrolyte to the solution resulted in almost complete recovery of the
initial current density.
8] Only a small amount of isopropyl alcohol was detected (current
2
�
2
[7]
5
and 0.25 mAcm in the presence of (CH ) NBF [Eq. (4)]
3 4 4
[
and LiBF [Eq. (1)], respectively. Moreover, the electrolysis
4
efficiency of about 10%) in the electrochemical reduction of CO
catalyzed by 1 at � 2.00 V in DMSO in the presence of (CH NBF
2
potential is of fundamental importance for the catalytic
generation of acetone, since acetone undergoes irreversible
reduction at potentials of � 2.0 Von GC electrodes in DMSO.
Therefore, from the viewpoints of the threshold potential for
3
)
4
4
.
the reduction of CO (Figure 2), 1 is not a suitable catalyst for
2
the reduction shown in Equation (4) even though CO
evolution is depressed by the metallacyclization reaction of
[
8]
Equation (3).
Experimental Section
To a solution of RuCl
(
(
3
´ 3H
1 mmol), and the solution was heated under reflux for 2 h. Then Et
0.5 mL) and napy (3 mmol) were added, and the solution was heated
2
O (1 mmol) in ethanol (50 mL) was added bpy
3
N
under reflux for 2 h. The reaction mixture was concentrated to about 5 mL.
Addition of NaBF (2 mmol) in H O resulted in precipitation of purple
Ru(bpy)(napy) Cl]BF in 80% yield. suspension of [Ru(bpy)-
4
2
[
2
4
A
Angew. Chem. Int. Ed. 1999, 38, No. 3
ꢀ WILEY-VCH Verlag GmbH, D-69451 Weinheim, 1999
1433-7851/99/3803-0363 $ 17.50+.50/0
363