Hydrophobic vitamin B12. Part 18. Preparation of a sol-gel modified electrode trapped with a vitamin B12 derivative and its photoelectrochemical reactivity

Article abstract of DOI:10.1039/b212863b

A vitamin B₁₂ derivative, heptapropyl cobyrinate perchlorate, was readily trapped onto an indium tin oxide (ITO) electrode by a sol-gel reaction. The complex was physically retained in a silica gel film which is formed on an ITO electrode. The thickness of the film could be controlled by the withdrawing speed of the dip coating process. Formation of a sol-gel film was confirmed by SEM measurements, and the total amount of the complex in the film was determined by UV-VIS absorption spectra. The complex exhibits the Co^II/Co^I redox couple at -0.42 V vs. Ag-AgCl. The amount of the electroactive complex in the sol-gel film deduced from electrochemical measurements is 3.0 X 10^-11 and 6.2 x 10^-11 mol cm^-2 for thicknesses of 170 and 330 nm, respectively. This electreoactive complex shows a high reactivity towards organic halides, and the controlled-potential electrolysis of benzyl bromide using the sol-gel modified electrode at -1.20 V vs. Ag-AgCl in aqueous solution containing 0.1 M KCl afforded dehalogenated products, bibenzyl and toluene, with a total turnover number of >1000 for 1 h.

Full text of DOI:10.1039/b212863b

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Hydrophobic vitamin B . Part 18.† Preparation of a sol–gel
12
modified electrode trapped with a vitamin B derivative and its
1
2
photoelectrochemical reactivity
a
a
a
b
b
Hisashi Shimakoshi, Aki Nakazato, Mami Tokunaga, Kiyofumi Katagiri, Katsuhiko Ariga,
b
a
Jun-ichi Kikuchi and Yoshio Hisaeda*
Department of Chemistry and Biochemistry, Graduate School of Engineering,
a
Kyushu University, Hakozaki, Higashi-ku, Fukuoka 812-8581, Japan.
E-mail: yhisatcm@mbox.nc.kyushu-u.ac.jp; Fax: 81-92-632-4718; Tel: 81-92-642-3592
Graduate School of Materials Science, Nara Institute of Science and Technology,
b
8
916-5 Takayama, Ikoma, Nara 630-0101, Japan
Received 2nd January 2003, Accepted 11th April 2003
First published as an Advance Article on the web 29th April 2003
A vitamin B12 derivative, heptapropyl cobyrinate perchlorate, was readily trapped onto an indium tin oxide (ITO)
electrode by a sol–gel reaction. The complex was physically retained in a silica gel film which is formed on an ITO
electrode. The thickness of the film could be controlled by the withdrawing speed of the dip coating process.
Formation of a sol–gel film was confirmed by SEM measurements, and the total amount of the complex in the film
II
I
was determined by UV–VIS absorption spectra. The complex exhibits the Co /Co redox couple at Ϫ0.42 V vs.
Ag–AgCl. The amount of the electroactive complex in the sol–gel film deduced from electrochemical measurements
Ϫ11
Ϫ11
Ϫ2
is 3.0 × 10 and 6.2 × 10 mol cm for thicknesses of 170 and 330 nm, respectively. This electroactive complex
shows a high reactivity towards organic halides, and the controlled-potential electrolysis of benzyl bromide using
the sol–gel modified electrode at Ϫ1.20 V vs. Ag–AgCl in aqueous solution containing 0.1 M KCl afforded
dehalogenated products, bibenzyl and toluene, with a total turnover number of >1000 for 1 h.
Introduction
ethoxysilane (TEOS, Shin-etsu Chemicals) was used after
distillation under reduced pressure. Absolute ethanol was
used after distillation. Heptamethyl cobyrinate perchlorate,
Vitamin B -dependent enzymes, involving a cobalt species as
12
1
the catalytic center, mediate various reactions. In order to
simulate the catalytic functions of vitamin B12 as exerted in the
hydrophobic active sites of enzymes concerned, we have been
dealing with hydrophobic vitamin B12 derivatives which have
ester groups in place of the peripheral amide moieties of the
[
Cob(II)7C ester]ClO , and heptapropyl cobyrinate perchlor-
1 4
ate, [Cob(II)7C ester]ClO , were synthesized by the previously
3
4
2,27
reported method. The ITO-coated glass plates (5 × 50 mm)
and the slide glass substrate were cleaned by sonication in 30%
H O aqueous solution for 30 min followed by sonication in
2
2
2
naturally occurring vitamin B , and succeeded in performing
various electroorganic reactions such as 1,2-migration of
12
ethanol for 1 h at 30 ЊC, and then dried with nitrogen gas.
3
–10
11–13
functional groups,
asymmetric reactions,
ring-expansion
14
Preparation of the sol–gel modified electrode
reactions, and synthesis of large-membered macrocyclic lac-
15
tones using hydrophobic vitamin B12 derivatives as a catalyst
in organic solvents. In a recent development of electroorganic
chemistry, it is considered that the immobilization of catalysts
onto an electrode has many advantageous features such as util-
ization of small amounts of catalyst species, ready separation
of products, and application to specific electro-organic syn-
The silica gel films were prepared by hydrolysis and conden-
sation of TEOS. A typical silica solution was prepared by mix-
ing TEOS (0.62 g, 3.0 mmol), 0.1 M aq. HCl (0.22 g, 12 mmol),
ethanol (1.11 g, 24 mmol) and [Cob(II)7C ester]ClO (40.5 mg,
3
4
Ϫ2
3.0 × 10 mmol) for 24 h at room temperature. The resulting
solution was deposited on a freshly cleaned ITO electrode or a
slide glass substrate (Matsunami Glass Industries) by dip coat-
16
theses. With this viewpoint, we and other groups reported
some methods for immobilization of vitamin B derivatives
Ϫ1
12
ing at withdrawing speeds of 50 and 100 mm min . The silica
1
7–22
onto electrodes,
and performed various electroorganic reac-
gel film formed on the substrate was dried (aged) for 12 h at
80 ЊC.
tions. Unfortunately, all of these methods require a specific
modification of the electrode surface so that a tedious pro-
cedure is needed for the preparation of the modified electrode.
In contrast to these methods, the sol–gel method is a convenient
technique for incorporation of various compounds into sol–gel
General analyses and measurements
The UV–VIS absorption spectra were measured on a Hitachi
U-3300 spectrophotometer at room temperature, and a silica
glass slide with undoped gel film was used as a reference. The
EPR spectra were obtained on a JEOL JES-FE1G X-band
spectrometer equipped with an Advantest TR-5213 microwave
counter and an Echo Electronics EFM-200 NMR field meter.
The sol solution containing [Cob(II)7C ester]ClO was poured
23–26
films.
There is no need for a pretreatment of the electrode,
and it is easy to trap various compounds simply by co-mixing.
In this paper, we report the incorporation of a vitamin B12
derivative onto an indium tin oxide (ITO) electrode using the
sol–gel method as shown in Fig. 1 and examine the catalytic
activity for the dehalogenation of an organic halide.
3
4
onto a glass plate, and the solvent was evaporated at 80 ЊC to
form a red film. This film was dried under reduced pressure in
the EPR cell, and the EPR spectrum was obtained at 77 K. The
GLC analyses were carried out on a Shimadzu GC-9A appar-
atus equipped with a Shimadzu C-R6A Chromatopac. The
GC-MS was obtained using a Shimadzu QP5050A. Scanning
electron micrographs (SEMs) were recorded on a Hitachi
S-5000 (HV = 25 kV) installed at the Centre of Advanced
Instrumental Analysis, Kyushu University.
Experimental
Materials
All solvents and chemicals used in the syntheses were of reagent
grade and were used without further purification. Tetra-
1
5
†
Hydrophobic vitamin B . Part 17.
12
2
308
D a l t o n T r a n s . , 2 0 0 3 , 2 3 0 8 – 2 3 1 2
T h i s j o u r n a l i s © T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 3

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Fig. 1 Schematic representation for the preparation of a sol–gel modified electrode containing the trapped vitamin B12 derivative
Cob(II)7C ester]ClO .
[
3
4
Cyclic voltammetry
Cyclic voltammograms (CVs) were obtained using a BAS CV
0W electrochemical analyzer. A three-electrode cell equipped
5
with the sol–gel modified electrode and a platinum wire as the
working and counter electrodes were used, respectively. An Ag–
AgCl (3.0 M NaCl) electrode served as the reference. Aqueous
Ϫ1
solutions containing KCl (1.0 × 10 M) were deaerated prior
to each measurement, and the inside of the cell was maintained
under an argon atmosphere throughout each measurement.
All measurements were carried out at room temperature. The
Ϫ1
scan rate was varied over the range from 10 to 300 mV s . The
II
III
III
E1/2 value of the Fe /Fe redox couple for K [Fe (CN) ] in
3
6
Fig. 2 Cross-sectional SEM photograph for the sol–gel modified ITO
electrode containing the vitamin B derivative [Cob(II)7C ester]ClO at
aqueous solution was 0.21 V vs. Ag–AgCl with this setup.
1
2
3
4
Ϫ1
5
0 mm min withdrawing speed for the dip coating.
Electrolysis of benzyl bromide
The thicknesses of gel films for withdawing speeds of 50 and
The controlled-potential electrolysis at Ϫ1.20 V vs. Ag–AgCl
was carried out in a one-compartment cell equipped with a
sol–gel modified electrode as a cathode and a magnesium rod
Ϫ1
1
00 mm min were determined by SEM measurements to
be 170 and 330 nm, respectively. We could prepare a sol–gel
modified electrode containing [Cob(II)7C ester]ClO in place of
1
4
(
diameter: 3 mm) as a sacrificial anode at room temperature
[
Cob(II)7C ester]ClO , but the [Cob(II)7C ester]ClO dissolved
3 4 1 4
under an argon atmosphere during irradiation with a 500-W
tungsten lamp at a distance of 30 cm. The applied potential
between the sol–gel modified and reference electrodes during
the electrolysis was maintained constant with a Hokuto Denko
HA-501 potentiostat/galvanostat, and the reaction was moni-
tored on a Hokuto Denko HF-201 coulomb/ampere-hour
meter. Initial concentrations: benzyl bromide, 1.0 × 10 M;
aqueous solution containing 0.1 M KCl. The products were
analyzed by GC and GC-MS with a DB-1 capillary column
out slightly from the gel in aqueous solution. Therefore, we
used the sol–gel modified electrode with [Cob(II)7C ester]ClO4
3
in the electrochemical reactions.
The amounts of vitamin B12 derivative in the gel films were
determined by UV–VIS absorption spectra. Absorption spectra
of a gel film were obtained by using a slide glass as substrate
as shown in Fig. 3(a), since the ITO substrate has a weak
absorption in the UV-VIS region. The absorption spectrum
showed the intense α- and γ-bands of vitamin B12 at 470 and
Ϫ2
(
J&W Scientific; length 30 m; ID 0.25 mm, film 0.25 µm).
3
13 nm, respectively, and the spectrum is similar to that for
[
Cob(II)7C ester]ClO in CH Cl as shown in Fig. 3(b). These
3
4
2
2
electronic spectra indicate that the structure of the vitamin B12
derivative is not changed, at least to the limit of the sensitivity
of the measurements. The total amounts of the B complex
Results and discussion
12
Preparation and characterization of the sol–gel modified
electrode
Ϫ9
Ϫ9
Ϫ2
were 1.0 × 10 and 3.7 × 10 mol cm for thicknesses of 170
and 330 nm, respectively. These values are comparable to that
for the polymer-coated B modified electrode we previously
The sol–gel modified electrode on ITO was prepared from
12
18
TEOS and [Cob(II)7C ester]ClO in ethanol containing aque-
reported.
The EPR spectra of [Cob(II)7C ester]ClO were measured in
3
4
ous HCl as described in the Experimental section (see Fig. 1).
After the dip coating of the silica gel solution containing the
vitamin B12 derivative, a homogeneous red gel film was formed
on the ITO electrode. Formation of the gel film was also
directly observed by SEM measurements as shown in Fig. 2. In
a dip coating process, it is well known that the film thickness
3
4
the gel film, as a powder, and in a frozen solution of chloro-
form–benzene (2 : 1 v/v) at 77 K as shown in Fig. 4. The
spectrum (Fig. 4(a)) in the gel film is very similar to that for
powdered [Cob(II)7C ester]ClO as shown in Fig. 4(b). This
3
4
result indicates that [Cob(II)7C ester]ClO is trapped into the
3
4
28
is regulated by the withdrawing speed of the substrate.
gel film with high density. These spectra and the correspond-
D a l t o n T r a n s . , 2 0 0 3 , 2 3 0 8 – 2 3 1 2
2309

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Fig. 3 Electronic spectra of (a) a sol–gel modified glass plate
containing the vitamin B12 derivative [Cob(II)7C ester]ClO and
3
4
Ϫ4
Fig. 4 EPR spectra of (a) [Cob(II)7C ester]ClO in the gel film,
(
b) 5.0 × 10 M [Cob(II)7C ester]ClO in CH Cl .
3
4
3
4
2
2
Ϫ3
(
b) powdered [Cob(II)7C ester]ClO and (c) 6.3 × 10 M [Cob(II)-
3
4
7
C ester]ClO in CHCl –C H (2 : 1 v/v) at 77 K.
3 4 3 6 6
ing spin Hamiltonian parameters are comparable to those of
2,29
vitamin B12 derivatives.
These EPR and electronic spectra
clearly indicate that the cobalt() state was maintained even
after the sol–gel process.
It is noted that the B12 complex containing hydrophobic peri-
pheral substituents within the gel film was quite stable and was
not dissolved out when the electrode was immersed in water.
Thus, the sol–gel method readily trapped a large amount of the
B12 complex to give a film with stable electrochemical activity as
described below.
Electrochemistry of sol–gel modified electrodes
Before the electrochemical measurements, the sol–gel modified
electrode was immersed in an aqueous solution containing
0
.1 M KCl in order to incorporate the electrolyte into the gel,
since the silica gel film has poor conductivity. The B complex
12
II
I
in the film exhibited the Co /Co redox couple at Ϫ0.42 V vs.
Ag–AgCl as shown in Fig. 5. The corresponding potential of
heptamethyl cobyrinate perchlorate [Cob(II)7C ester]ClO in
1
4
aqueous solution is Ϫ0.54 V vs. Ag–AgCl. The value of the
voltammetric peak current (i ) depends on the square root of
p
Fig. 5 Cyclic voltammograms of the sol–gel modified electrode
containing the vitamin B12 derivative [Cob(II)7C ester]ClO in aqueous
solution containing 0.1 M KCl. Scan rate is 10 mV s : (a) film
thickness: 330 nm, (b) film thickness: 170 nm.
the scan rate (v). Therefore, it is likely that the electron transfer
of the B12 complex in the gel film is a diffusion-driven process.
The anodic and cathodic peaks are expected to be at the same
voltage for a rigidly surface-immobilized species. The cyclic
voltammogram exhibited the behaviour seen for diffusion con-
trolled reactions because the B12 complex can move in the gel.
The amount of electroactive catalysts in the films was estimated
3
4
Ϫ1
(thickness: 330 nm). The total amounts of B12 on ITO deter-
Ϫ9
Ϫ9
mined by UV-VIS spectroscopy were 1.0 × 10 and 3.7 × 10
II
Ϫ2
by integrating the Co reduction peaks and by applying Fara-
mol cm for thicknesses of 170 and 330 nm, respectively. This
experimental result shows that only a small percentage of the
B12 complex on ITO is active for the electrolysis.
day’s law, yielding a surface concentration Γ of B of 3.0 ×
0
12
Ϫ11
Ϫ11
Ϫ2
Ϫ2
1
0
mol cm (thickness: 170 nm) and 6.2 × 10 mol cm
2
310
D a l t o n T r a n s . , 2 0 0 3 , 2 3 0 8 – 2 3 1 2

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a
Table 1 Product analyses for the controlled-potential electrolyses of benzyl bromide using the modified electrode
d
Electrolysis conditions
Product ratio
Toluene
Total
turnover
number
b
c
Ϫ1
e
Dopant in sol–gel electrode
hν
Charge /F mol
Time/h
Conversion (%)
Bibenzyl
[
[
Cob(II)7C ester]ClO
Irradiation
In the dark
Irradiation
0.1
0.015
0.019
2
2
2
8.5
0
0
6
0
0
94
0
0
2260
–
–
3
4
Cob(II)7C ester]ClO
3
4
None
a
Controlled-potential electrolyses were carried out in aqueous solution at Ϫ1.20 V vs. Ag–AgCl under an argon atmosphere. Initial concentration:
Ϫ2
b
c
benzyl bromide, 1.0 × 10 M; KCl, 0.1 M. Under irradiation using a 500-W tungsten lamp at a distance of 30 cm. Electrical charge passed per mol
of benzyl bromide. Products were analyzed by GC and GC-MS. Total turnover number was estimated from the total amount of [Cob(II)7C3-
ester]ClO in the sol–gel modified electrode, 3.7 × 10 mol cm
d
e
Ϫ9
Ϫ2
.
4
Electrolysis of benzyl bromide
Monovalent cobalt in reduced B12 derivatives is a strong nucleo-
phile and reacts with various organic halides to give organo-
30
cobalt complexes with cobalt–carbon bonds. A compound
with a cobalt–carbon bond is homolytically cleaved by photo-
lysis, electrolysis or thermolysis to form the corresponding radi-
31
cal species. In order to examine the catalytic activity of the B
12
Ϫ2
Ϫ11
modified electrode (thickness, 330 nm; 6.2 × 10 mol cm ),
the electrolysis of benzyl bromide was carried out in aqueous
solution. The electrolysis of benzyl bromide was performed at
Ϫ1.20 V vs. Ag–AgCl under the irradiation of visible light, and
the products were analyzed by GC-MS. Product analyses under
various conditions are shown in Table 1. After an electrical
Ϫ1
charge of 0.1 F mol was passed based on the initial concen-
tration of benzyl bromide, bibenzyl and toluene were detected
as products (eqn. (1)).
Fig. 6 Proposed mechanism for the electrolysis of benzyl bromide
mediated by the vitamin B12 derivative [Cob(II)7C ester]ClO in a
3
4
sol–gel film.
(
1)
and deposited on an ITO electrode. The B12 complex held in gel
on the electrode possesses electrochemical activity and exhibits
high reactivity for dehalogenation of an organic halide. It has
been reported that certain bacteria can use tetrachloroethane as
an electron acceptor by reducing it to cis-dichloroethane,
and this enzyme, reductive dehalogenase, contains a vitamin B12
The major product was bibenzyl, and total turnover number
was 2260 for 2 h estimated from the total amount of vitamin B12
derivative in the sol–gel electrode. Products were not obtained
in the dark as shown in Table 1. This reaction did not proceed
when we used a B12 undoped sol–gel ITO electrode or a bare
ITO electrode under the same conditions. Therefore, the
trapped complex catalyzes the electrolysis of organic halides
under irradiation with visible light.
34
as cofactor. Therefore, the application of the B12 complex to
catalytic dehalogenation reactions of various organic halides is
35
quite interesting from the viewpoint of green electrochemistry.
Acknowledgements
We thank Prof. N. Kimizuka and Mr. K. Kuroiwa, Kyushu
University, for help in measuring the SEM. We also thank
Prof. A. Matsuda, Osaka Prefecture University, for supply of
the ITO-coated glass plates. The present work was partly
supported by a Grant-in-Aid for Scientific Research from the
Ministry of Education, Culture, Sports, Science, and Tech-
nology of Japan. The authors are also grateful for financial
supports from the Showa Shell Sekiyu Foundation and
Tokuyama Science Foundation.
The electrolysis of benzyl bromide was also investigated
upon the addition of a spin-trapping reagent, 5,5Ј-dimethyl-
pyrroline N-oxide (DMPO). Upon the addition of DMPO,
the formation of bibenzyl and toluene was completely
inhibited. This result indicates that radical species are generated
as electrolysis intermediates under the present conditions
and a possible mechanism is shown in Fig. 6. The reaction is
initiated by the electrochemical reduction of Co() to Co()
species. The Co() species, which is a strong nucleophile, reacts
with benzyl bromide to yield the benzyl–cobalt complex. The
cobalt–carbon bond of the complex then homolytically cleaves
to form a benzyl radical upon irradiation with visible light. The
benzyl radicals are efficiently coupled to form bibenzyl on the
modified electrode before they diffuse out. It was reported that
the electrolysis of benzyl bromide mediated by vitamin B12 at
Ϫ1.20 V vs. SCE gave bibenzyl as a major product in DMF or a
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D a l t o n T r a n s . , 2 0 0 3 , 2 3 0 8 – 2 3 1 2