Electrocatalytic reduction of aldehydes and ketones on nickel(II) tetraazamacrocyclic complex-modified graphite felt electrode

Article abstract of DOI:10.1016/S0022-328X(02)01854-5

Electrocatalytic reduction of aldehydes and ketones was studied using a nickel(II) tetraazamacrocyclic complex-modified graphite felt (GF) electrode. The nickel(II) tetraazamacrocyclic complex-modified GF electrode was prepared by attaching nickel(II) (6-(2′-hydroxyethyl)-1,4,8,11-tetraazacyclotetradecane)perchlorate chemically to the carboxyl groups of a thin poly(acrylic acid) (PAA) layer coated on the GF. The modified electrode gave a reversible electron transfer for the nickel(II)-nickel(I) redox couple in cyclic voltammetry at -0.95 V versus Ag/AgCl. A preparative electrocatalytic reduction of aldehydes and ketones to the corresponding alcohol and pinacol compounds was successfully achieved on the modified electrode with an adequate current efficiency (46.9-75.9%), conversion (48.1-84.6%) and turnover number of the nickel catalyst (1053-2267).

Full text of DOI:10.1016/S0022-328X(02)01854-5

Journal of Organometallic Chemistry 662 (2002) 9ꢀ13
/
www.elsevier.com/locate/jorganchem
Electrocatalytic reduction of aldehydes and ketones on nickel(II)
tetraazamacrocyclic complex-modified graphite felt electrode
Yoshitomo Kashiwagi *, Chikara Kikuchi, Futoshi Kurashima, Jun-ichi Anzai
Graduate School of Pharmaceutical Sciences, Tohoku University, Aramaki, Aoba-ku, Sendai 980-8578, Japan
Received 17 June 2002; received in revised form 20 August 2002; accepted 20 August 2002
Abstract
Electrocatalytic reduction of aldehydes and ketones was studied using a nickel(II) tetraazamacrocyclic complex-modified graphite
felt (GF) electrode. The nickel(II) tetraazamacrocyclic complex-modified GF electrode was prepared by attaching nickel(II) (6-(2?-
hydroxyethyl)-1,4,8,11-tetraazacyclotetradecane)perchlorate chemically to the carboxyl groups of a thin poly(acrylic acid) (PAA)
layer coated on the GF. The modified electrode gave a reversible electron transfer for the nickel(II)ꢀ
voltammetry at ꢁ0.95 V versus Ag/AgCl. A preparative electrocatalytic reduction of aldehydes and ketones to the corresponding
alcohol and pinacol compounds was successfully achieved on the modified electrode with an adequate current efficiency (46.9ꢀ
75.9%), conversion (48.1ꢀ84.6%) and turnover number of the nickel catalyst (1053ꢀ2267).
# 2002 Elsevier Science B.V. All rights reserved.
/
nickel(I) redox couple in cyclic
/
/
/
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Keywords: Ni(II) tetraazamacrocyclic complexes; Mediator; Modified electrode; Electrocatalytic reduction
1. Introduction
lytic reduction of aldehydes and ketones to alcohols. We
used here nickel complex as a single mediator. Nickel
complexes such as nickel(II) cyclam (1,4,8,11-tetraaza-
cyclotetradecane) catalyze many electroorganic reac-
The reduction of aldehydes and ketones to alcohols is
one of the important subjects in organic synthesis. This
kind of reactions are usually performed by using
hydrides [1] dissolving metals [1], catalytic hydrogena-
tion [1,2] and baker yeast [3]. However, use of even a
catalytic amount of the hazardous metallic reagents or
expensive enzymes is a matter of economic and environ-
mental concern [4]. Organic synthesis using mediator-
modified electrodes which attached a catalyst to the
surface is a simple and clean process with easy isolation
of product from electrolyte solution [5]. Recently, we
have reported the electrocatalytic reduction of ketones
to alcohols with multiple mediator-modified electrode
system such as a poly(acrylic acid) (PAA)-coated
graphite felt (GF) electrode immobilizing all the com-
ponents of viologen, NADꢂ, diaphorase, and alcohol
dehydrogenase [6]. In the present paper, we describe a
single mediator-modified electrode for the electrocata-
tions such as carbonÃ
/
carbon bond formation [7ꢀ
/
9],
dehalogenation of organohalides [10ꢀ
/12], reduction of
CO2 [13,14], and oxidation of H2O2 [15]. However, the
use of nickel complexes in the electrocatalytic reduction
of aldehydes and ketones has not been reported.
2. Results and discussion
2.1. Catalysis of nickel(II) tetraazamacrocyclic
complex-modified GF electrode for aldehydes and ketones
The preparation method of the nickel(II) (6-(2?-
hydroxyethyl)-1,4,8,11-tetraazacyclotetradecane)per-
chlorate (1)-modified GF electrode is shown in Scheme
1. The reduction potential of nickel(II) to nickel(I) was
found at ꢁ
amount of electroactive 1 on the electrode surface was
ca. 2.4ꢃ
10ꢁ6 mol cmꢁ3. The details of characteristic
/0.95 V versus Ag/AgCl (Fig. 1) and the
/
* Corresponding author. Tel.: ꢂ
6840
E-mail address: kashi@mail.pharm.tohoku.ac.jp (Y. Kashiwagi).
/
81-22-217-6842; fax: ꢂ81-22-217-
/
and voltammetric behavior of the 1-modified GF
electrode was described in the previous report [12].
0022-328X/02/$ - see front matter # 2002 Elsevier Science B.V. All rights reserved.
PII: S 0 0 2 2 - 3 2 8 X ( 0 2 ) 0 1 8 5 4 - 5

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Y. Kashiwagi et al. / Journal of Organometallic Chemistry 662 (2002) 9ꢀ13
/
Scheme 1. Scheme for preparation of the 1-modified GF electrode.
2.2. Macroelectrolysis of aldehydes and ketones using the
nickel(II) tetraazamacrocyclic complex-modified GF
electrode
Based on the cyclic voltammetry results, the prepara-
tive electrolysis reaction of acetophenone was performed
at ꢁ1.0 V versus Ag/AgCl in CH3CN solution. During
/
electrolysis, the substrate and products were occasion-
ally analyzed by high performance liquid chromatogra-
phy (HLPC). A time course of the electrocatalytic
reduction of acetophenone on a 1-modified GF elec-
trode is shown Fig. 2. A half millimole of acetophenone
reacted about 85% in 15 h and was reduced to the
corresponding alcohol and pinacol compounds. The
current efficiency in the electrolysis was 75.9% during
the course of electrolysis, and 1-phenylethanol and 2,3-
diphenyl-2,3-butanediol were observed 39.2 (0.196) and
28.8% (0.144 mmol), respectively. The turnover number
based on the amount of 1 on the electrode surface (given
Fig. 1. Cyclic voltammograms of 1-modified GF electrode (5ꢃ
mm) at 100 mV sꢁ1 in 0.1 M TEAPꢂ
CH3CN with: (a) 0 M
acetophenone, (b) 0.1 M acetophenone, and (c) 0.2 M acetophenone.
(d) 0.1 M acetophenone on bare GF electrode (5ꢃ5ꢃ5 mm).
/
5ꢃ
/
5
/
/
/
When acetophenone was added to electrolytic solution
bathing the 1-modified GF electrode, an increase of the
nickel(II) reduction current was observed (Fig. 1). The
nickel(I) oxidation current disappeared, and the new
reduction peak current was proportional to the concen-
tration of acetophenone. The ratio of this new reduction
peak’s height to the reversible nickel(II) reduction peak
height decreased as scan rate increased. Acetophenone
was reduced directly on a bare GF electrode in CH3CN
by the ratio of mol of productsꢃ2 per mol of 1) was
/
calculated to be 2267 after 15 h of electrolysis. After
electrolysis, the reduction peak current of nickel(II) to
nickel(I) on the modified electrode was not decreased,
solution at about ꢁ1.85 V versus Ag/AgCl. All these
/
results are characteristic [16,17] of electrochemical
catalysis of the reduction of acetophenone to 1-pheny-
lethanol via the active (L)Ni(I) reductant. The 0.9 V
positive shift in the peak potential represents a large
decrease in the activation energy due to catalysis of the
reduction of acetophenone. Similar results were seen
with the other aldehydes and ketones used.
Fig. 2. Macroelectrolysis of acetophenone using a 1-modified GF
electrode in 0.1
M
TEAPꢂCH3CN. I, Acetophenone; k, 1-
/
phenylethanol; and m, 2,3-diphenyl-2,3-butanediol.

Y. Kashiwagi et al. / Journal of Organometallic Chemistry 662 (2002) 9ꢀ
/13
11
and the catalytic activity for acetophenone was also the
same as for a fresh one. This means that the 1-modified
GF electrode can be used repeatedly for macroelectro-
lysis. In fact, the 1-modified GF electrode has been used
repeatedly more than ten times.
The results from the reduction reactions of a variety
of carbonyl compounds are shown in Table 1. Prepara-
tive electrocatalytic reduction of the aldehydes and
ketones was achieved successfully on a 1-modified GF
electrode with adequate current efficiency (46.9ꢀ
conversion (48.1ꢀ84.6%) and turnover number of Ni
catalyst (1053ꢀ2267). Benzaldehyde and 4-methylben-
/
80.1%),
/
Scheme 2. Proposed mechanism for the electrocatalytic reduction of
carbonyl compounds on the 1-modified GF electrode.
/
zaldehyde as an aldehyde were obtained primary
alchohol and pinacol compounds. On the other hand,
ketones such as acetophenone, 4-methoxyacetophenone
and 4-methylbenzophenone gave secondary alcohol and
pinacol compounds.
On the basis of the observed products, the mechanism
of electrocatalytic reduction of aldehydes and ketones
with a 1-modified GF electrode is proposed in Scheme 2.
The metal complex undergoes one-electron reduction at
electrode (Eq. (1)). The reduced form ((L)Ni(I)) interacts
with the carbonyl compounds to yield an organometallic
intermediate (Eq. (2)). The organometallic intermediate
is subject to homolytic cleavage, and is led to (L)Ni(II)
and anionic radical intermediate (Eq. (3)). This anionic
radical intermediate can be consumed in such reactions
to yield alcohols as reduction (Eq. (4)) and dimerization
(Eq. (5)). In the formation of reduction product, the
anionic radical intermediate again was reduced to
dianionic intermediate by (L)Ni(I), and undergoes
protonation to form alcohol. On the other hand, the
anionic radical intermediate can dimerize and undergoes
protonation to form the pinacol compound.
3. Experimental
3.1. Chemicals and solutions
Dry PAA (average molar mass (MW) 1 400 000) was
prepared by freeze-drying a 25% w/v aqueous solution
Table 1
Electrocatalytic reduction of aldehydes and ketones using a 1-modified GF electrode

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Y. Kashiwagi et al. / Journal of Organometallic Chemistry 662 (2002) 9ꢀ13
/
obtained from Wako Pure Chemical Industries, Ltd. O-
Benzotriazol-1-yl-N,N,N?,N?-tetramethyluronium hexa-
fluorophosphate (HBTU) and 1-hydroxybenzotriazole
hydrate (HOBT) were from Nakarai Tesque. Di-n-butyl
sulfate (DBS) and 1,8-diazabicyclo[5.4.0]-7-undecene
(DBU) were from Tokyo Chemical Industry. N,N-
Dimethylformamide (DMF) and acetonitrile (CH3CN)
were purified by a method described in the literature
[18]. Tetraethylammonium perchlorate (TEAP), ammo-
nium perchlorate (NH4ClO4) and tetralin were from
Tokyo Chemical Industry. All other reagents used were
of commercially available reagent grade.
additional 6 h at r.t.. The 1-modified GF electrode was
then treated in acetone solution containing 48 mM of
DBS and 48 mM of DBU at 80 8C for 30 min.
3.4. Cyclic voltammetry
The 1-modified GF electrode (5ꢃ
/
5ꢃ5 mm) and a
/
platinum wire were employed as the working electrode
and the counter electrode, respectively. The cathode
potentials were referred to Ag/AgCl (saturated AgCl
and (CH3)3C2H5NCl in CH3CN). Cyclic potential
sweeps were generated by a Hokuto Denko Model
HABF-501 potentiostat/galvanostat. Cyclic voltammo-
3.2. Synthesis of 1
grams were recorded on a Graphtec model WX1200 X ꢀ
/
6-(2?-Hydroxyethyl)-1,4,8,11-tetraazacyclotetrade-
cane was prepared by the procedure of Tabushi et al.
[19]. Diethyl malonate (82.4 g, 0.51 mol) was added to
21 w/v% sodium ethoxide ethanol solution (100 ml). The
mixture was then added to 2-bromoethanol (62.3 g, 0.5
mol), and was refluxed for 4 h. The reaction mixture was
cooled and concentrated in vacuo. The residue was
chromatographed on a silica gel column with 4:1
Y recorder. All electrochemical measurements were
carried out at r.t. (ca. 20 8C) under nitrogen atmo-
sphere.
3.5. Macroelectrolysis
Preparative potential-controlled electrolysis was per-
formed in CH3CN, using an ‘H’ type divided cell
separated by a cationic exchange membrane (Nafion
117). The catholyte contained 0.5 mmol of substrate, 2
mmol of ammonium perchlorate (NH4ClO4) as a proton
source, 0.5 mmol of tetralin as gas chromatographic
standard and 0.5 mmol of tetraethylammonium per-
chlorate (TEAP) as a supporting electrolyte in a total
volume of 5 ml. The anolyte was 5 ml of CH3CN
containing 0.5 mmol of TEAP. Controlled potential
CH3ClꢀCH3OH to give diethyl-2-ethylhydroxymalo-
/
nate, as colorless oil (32.1 g, 31% yield). Then, con-
densation of this malonate (12.4 g, 60 mmol) with N,N?-
bis(1-aminoethyl)-1,3-propanediamine (9.6 g, 60 mmol)
in ethanol (80 ml) under reflux for 3 days gave 6-(2?-
hydroxyethyl)-5,7-dioxo-1,4,8,11-tetraazacyclotetrade-
cane (2.4 g, 15% yield), which was isolated through silica
gel chromatography eluted with 3:2 CH3ClꢀCH3OH.
/
This cyclic diamide (1.4 g, 5 mmol) thus obtained was
smoothly reduced with a large excess of diborane (0.6 g,
20 mmol) in refluxing tetrahydrofuran (50 ml) for 24 h.
electrolysis was carried out at ꢁ
under nitrogen atmosphere. The 1-modified GF elec-
trode, the size of 5ꢃ5ꢃ5 mm, was used as working
/1.0 V versus Ag/AgCl
After treatment with 6 N HCl and KOHꢀ
/
CH3OH, 6-
/
/
(2?-hydroxyethyl)-1,4,8,11-tetraazacyclotetradecane was
obtained in 78% yield (1.0 g) by recrystallization from
benzene. This tetraazacyclotetradecane (0.4 g, 1.5 mmol)
was dissolved in 10 ml of ethanol. A solution of Ni(II)
perchlorate hexahydrate (1.0 g, 2.7 mmol) in 10 ml of
H2O was added dropwise with stirring. It was stirred for
3 h at room temperature (r.t.). The product crystallized
from the reaction mixture and was filtered, washed with
ethanol, and dried, giving 0.55 g (66% yield) of 1.
cathode electrode. During electrolysis, the substrate and
product were arbitrarily analyzed by HLPC (Daisel
CHIRALCELL-OD column, 46 mm f ꢃ
/
25 cm, col-
2:33,
umn temperature 40 8C, 2-propanol:n-hexaneꢄ
/
flow rate; 0.7 ml minꢁ1, detection; UV absorption at
254 nm).
4. Conclusion
3.3. Preparation of 1-modified GF electrode
The modified electrode made by linking 1 to PAA-
The preparation method of the 1-modified GF
electrode is shown in Scheme 1. A GF electrode (Nippon
coated GF gave reversible Ni(II)ꢀNi(I) voltammetry.
/
The 1-modified GF electrode was active for the reduc-
tion of several carbonyl compounds. A preparative
electrocatalytic reduction of aldehydes and ketones to
alcohols was successfully achieved on the modified
electrode in adequate current efficiency, conversion
and turnover number of Ni catalyst. The 1-modified
GF electrode exhibited no inactivation and thus can be
used repeatedly for electrolysis.
Kynol Inc., 50ꢃ
immersing it in a 0.1 w/v% methanol solution of PAA.
10ꢁ6 mol
/
20ꢃ
/
5 mm) was coated with PAA by
The amount of coated PAA was 12.4ꢃ
/
monomer units of acrylic acid (MW 72) per 1 cm3 GF
electrode. This coated electrode was then treated with
16.6 mM of 1 in DMF in the presence of 20.8 mM of
HBTU and 20.6 mM of HOBT for 12 h at 4 8C and

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for Earth Environment, and by a Grant-in-Aid for
Scientific Research on Encouragement Research (No.
13750792) from the Ministry of Education, Culture,
Sports, Science and Technology of Japan.
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