Photocatalytic function of a polymer-supported B12 complex with a ruthenium trisbipyridine photosensitizer

Article abstract of DOI:10.1039/c1cc11970b

The hybrid polymer was synthesized by a radical polymerization of a B ₁₂ derivative and a Ru complex having styrene moieties in each peripheral position, and the hybrid polymer showed photocatalytic activity for molecular transformation with vi

Full text of DOI:10.1039/c1cc11970b

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Chemical Communications
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Volume 47 | Number 23 | 21 June 2011 | Pages 6481–6732
ISSN 1359-7345
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Yoshio Hisaeda et al.
Photocatalytic function of a polymer-supported B12 complex with a ruthenium trisbipyridine photosensitizer

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Photocatalytic function of a polymer-supported B12 complex with
a ruthenium trisbipyridine photosensitizerw
Hisashi Shimakoshi,a Masashi Nishi,a Akihiro Tanaka,ab Katsumi Chikamab and
Yoshio Hisaeda*a
Received 7th April 2011, Accepted 19th April 2011
DOI: 10.1039/c1cc11970b
The hybrid polymer was synthesized by a radical polymerization
of a B12 derivative and a Ru complex having styrene moieties
in each peripheral position, and the hybrid polymer showed
photocatalytic activity for molecular transformation with visible
light irradiation.
The Co(I) species of the cobalamin derivative (B12) is widely
known as a supernucleophile that forms an alkylated complex
by reaction with an alkyl halide.1,2 Because the alkylated
complex is a useful reagent for forming radical species as the
cobalt–carbon bond is readily cleaved homolytically by
thermolysis, photolysis and electrolysis, research interest is
focused on the application of this compound to various
molecular transformations.3,4 Recently, we reported the
dechlorination of 1,1-bis(4-chlorophenyl)-2,2,2-trichloroethane
(DDT) catalyzed by the B12 derivative, heptamethyl cobyrinate
perchlorate with a [Ru(II)(bpy)3]Cl2 photosensitizer by irradiation
with visible light.5 Although the B12 complex showed high
catalytic efficiency in the reaction, an excess of the photosensitizer
was required for the reaction. To alleviate this problem, we
synthesized a new B12 catalyst as shown in Fig. 1. In this
compound, the photosensitizing unit and the B12 catalytic unit
are incorporated in the same polymer, which may allow efficient
electron transfer from the Ru(bpy)3 moiety to the cobalt center in
the B12 complex. In this study, the synthesis and catalysis of this
hybrid polymer composed of the B12 complex and a Ru(bpy)3
photosensitizer are reported.
Fig. 1 Synthesis of polymer-supported B12 catalyst and photosensitizer.
polydispersity of the polymer were determined by gel permea-
tion chromatography (GPC) (Mw = 39 000, Mn = 27 000,
Mw/Mn = 1.4) (Fig. S1, ESIw). The morphology and size
of the polymer were analyzed by transmission electron
microscopy (TEM) and dynamic light scattering (DLS). From
the TEM image, a warped shape of the polymer was observed
as shown in Fig. S2 (ESIw) and the size of the polymer was
estimated to be about 7 nm. The size was also supported by
DLS (Fig. S3, ESIw).
The UV-vis spectrum of the hybrid polymer showed the
typical shape for the B12 and Ru(bpy)3 complexes with
absorption maxima at 587 nm and 456 nm, respectively
(Fig. S4, ESIw). The contents of the B12 and the Ru(bpy)3
complex in the polymer were determined by UV-vis,z and the
mole ratio of each unit was estimated to be B12 :Ru:Styrene =
1 : 1 : 98. These values were quite similar to the initial ratio of
the starting monomers.y From these data (molecular weight,
size of polymer, and contents of B12 and Ru complexes in the
polymer), we can estimate the number of B12 and Ru
complexes in one polymer unit as both three molecules. In
this polymer, the neighboring ruthenium photosensitizer is
This copolymer was synthesized from the corresponding
monomers (B12 and Ru(bpy)3 derivative (1a, 2), and styrene
monomers) by radical polymerization using azobisisobuty-
ronitrile (AIBN) (ESIw). Each of the monomers has a styrene
moiety with an amide linkage to the B12 and ruthenium
tris-bipyridine complexes. After polymerization, unreacted
monomers were easily removed by the Soxhlet extraction
method using methanol as the solvent. Molecular weight and
a Department of Chemistry and Biochemistry, Graduate School of
Engineering, Kyushu University, Motooka, Fukuoka 819-0395,
Japan. E-mail: yhisatcm@mail.cstm.kyushu-u.ac.jp;
Fax: +81-92-802-2827
b Synthesis Research Dept., Chemical Research Lab., Nissan Chemical
Industries, Ltd., Funabashi 274-8507, Japan
w Electronic supplementary information (ESI) available: Details of
experimental procedures. See: DOI: 10.1039/c1cc11970b
c
6548 Chem. Commun., 2011, 47, 6548–6550
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Table 1 Photocatalytic reaction of B12 complexa
Yieldb (%)
Conversionb (%)
TONc
EB
ST
Entry Catalyst
1
2
3
4
5
B
12–Ru polymer
98
0
82
0
59
0
8
0
4
0
0
270
0
189
—
0
B12–Ru polymerd
B12, [Ru(bpy)3]Cl2 66
[Ru(bpy)3]Cl2
B12
e
0
0
f
0
a
Conditions: [B12] = [Ru complex] = 1 ꢀ 10ꢁ4 M in polymer,
Fig. 2 UV-vis spectral change in the polymer (B12 unit, 5 ꢀ 10ꢁ4 M)
in the presence of triethanolamine (1 M) by visible light irradiation
(l Z 420 nm) under N2 in DMF. (a) Before irradiation (blue line),
(b) after 5 minute irradiation (red line).
[substrate] = 3 ꢀ 10ꢁ2 M, [triethanolamine] = 1 M. Solvent, DMF,
l Z 420 nm under N2 at room temperature. Reaction time, 24 h.
Conversion was estimated by the recovery of the substrate. Yields were
b
c
based on initial concentration of the substrate. Turnover numbers (TON)
d
were based on [B12]. In the dark. e [Ru complex] = 1 ꢀ 10ꢁ4 M.
f Dicyano heptamethyl cobyrinate was used; [B12] = 1 ꢀ 10ꢁ4 M.
expected to be an electron donor for B12 activation to form
reactive Co(I) species.
Formation of the Co(I) species of the B12 complex in the
polymer was monitored by UV-vis spectral change under
irradiation with visible light (l Z 420 nm). The UV-vis
spectrum of the starting polymer having a Co(III) form of
Table 2 Photocatalytic reaction of the B12 complex under diluted
conditionsa
Yieldb (%)
Conversionb (%)
TONc
B12 was changed to a new spectrum after visible light irradiation
EB
ST
Entry Catalyst
with absorption maxima at 390 nm in DMF containing a
sacrificial reductant (triethanolamine) as shown in Fig. 2, which
is typical for the Co(I) state of B12.6
1
2
B12–Ru polymer
B12, [Ru(bpy)3]Cl2
27
3
22
1
1
1
690
60
a
Conditions: [B12] = [Ru complex] = 1 ꢀ 10ꢁ5 M in polymer,
The photoreduction of B12 in the polymer was also investigated
by ESR spectroscopy. When the Co(III) state of the B12–Ru
hybrid polymer was dissolved in DMF, the diamagnetic starting
polymer showed no ESR signal (see Fig. S5(a), ESIw). After
irradiation by visible light in the presence of triethanolamine, the
typical Co(II) low-spin signal appeared (g1 = 2.39, g2 = 2.29,
[substrate] = 3 ꢀ 10ꢁ2 M, [triethanolamine] = 1 M. Solvent, DMF,
l
24 h. Conversion was estimated by the recovery of the substrate.
Z 420 nm under N2 at room temperature. Reaction time,
b
c
Yields were based on initial concentration of the substrate. Turnover
numbers (TON) were based on [B12].
Co
Co
g3 = 2.00, A2 = 48 G, A3 = 136 G).7 Upon subsequent
irradiation by visible light, this signal disappeared to form the
diamagnetic Co(I) species. According to these ESR spectral
changes, stepwise one-electron reduction of cobalt center occurred
by electron transfer from the ruthenium photosensitizer in the
polymer to form the Co(I) state of B12. Thus, we carried out a
catalytic reaction using this hybrid polymer.
efficiency of the polymer system was dramatically enhanced
compared to that of the non-polymer system.
Indeed, a UV-vis spectral change for the formation of the
Co(I) species by visible light irradiation was still observed
under highly diluted conditions (o1 ꢀ 10ꢁ5 M) in the polymer
system (Fig. S10a, ESIw), while such a spectral change was not
observed in the non-polymer system (Fig. S10b, ESIw). At a
high concentration (1 ꢀ 10ꢁ4 M), the distance between the
ruthenium photosensitizer and the B12 catalyst is very small
(but not so close at least over 10 A since no concentration
quenching was observed in ruthenium unit, data not shown) in
both the polymer and non-polymer systems. In contrast, at a
low concentration (1 ꢀ 10ꢁ5 M), the Ru photosensitizer and
B12 catalyst were separated in the non-polymer system,
whereas both units are close in the polymer system as shown
in Fig. 3. Therefore, efficient electron transfer from the Ru
complex to the B12 catalyst should occur in the hybrid
polymer. In other words, fixation of the Ru and B12 catalyst
in the same polymer unit retains photo-catalytic efficiency
under a diluted condition and allows effective utilization of a
small amount of the catalyst.
The catalytic reaction was carried out using phenethyl
bromide as a substrate.z The results are shown in Table 1.
After 24 h with visible light irradiation, most of the phenethyl
bromide was converted to ethylbenzene and styrene in 82 and
8% yields, respectively. The reaction did not proceed under
dark conditions (entry 2 in Table 1).
In contrast, the conversion of phenethyl bromide was
decreased to 66% when we used a 1 : 1 mixture of the
Ru complex ([Ru(bpy)3]Cl2) and B12 catalyst (dicyano
heptamethyl cobyrinate) (entry 3 in Table 1). Intramolecular
electron transfer from the Ru complex to B12 should be
accelerated in the polymer. Of course, the reaction did not
proceed when we used the Ru complex or B12 catalyst alone
(entries 4 and 5 in Table 1). To emphasize this polymer-
supported effect, a catalytic reaction was carried out at a
more dilute concentration as shown in Table 2. Under this
condition, the effect of the polymer was clearly observed
compared to the previous condition (Table 1), and the reaction
Furthermore, it is noteworthy that because [Ru(bpy)3]Cl2 is
used as an excellent photo-redox catalyst due to its reducing
ability (Ru(bpy)3+, ꢁ1.33 V vs. SCE in CH3CN)8 and was
developed in the field of green organic chemistry,9 substrates
c
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Chem. Commun., 2011, 47, 6548–6550 6549

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and a Grant-in-Aid for Scientific Research (A) (No. 21245016)
from the Japan Society for the Promotion of Science (JSPS).
Notes and references
z The contents of B12 and Ru(bpy)3 complexes in the polymer
were determined by UV-vis spectroscopy using the standard data
for dicyano heptamethyl cobyrinate, l(e) = 587 (1.14 ꢀ 104), 456
(2.47 ꢀ 103) and Ru(bpy)3 monomer, l(e) = 456 (1.60 ꢀ 104) in
CH2Cl2.
y It was noted that the copolymerization of the Co(II) state of the B12
monomer (1b) (ESIw), Ru monomer and styrene (starting with a
1 : 1 : 98 mole ratio) failed to incorporate the B12 unit in the polymer
(obtained 0.16 : 0.21 : 99.63 mole ratio). Due to the high radical affinity
of the Co(II) complex, the radical polymerization by AIBN was
presumably inhibited.
z General procedure: a 2 mL DMF solution of polymer (B12 and Ru
complexes contents, both 1.0 ꢀ 10ꢁ4 M), phenethyl bromide (3 ꢀ 10ꢁ2 M),
triethanolamine (1 M) and diphenyl (5 mg) as internal standard was
degassed by nitrogen bubbling for 30 minutes; then the solution was stirred
at room temperature under irradiation by a 200 W tungsten lamp with a
420 nm cut-off filter (Sigma Koki, 42 L) and a heat cut-off filter (Sigma
Koki, 30 H). After 24 h, the solution was passed through a silica gel
column (CH2Cl2 eluent) to remove the polymer and triethanol-
amine, and the products were analyzed by GC-MS.
Fig. 3 Schematic representation of concentration dependency for
electron transfer behaviours of catalysts.
are limited to electron-deficient compounds to undergo
efficient one-electron reduction, whereas our catalyst utilizing
the supernucleophilicity of the Co(I) species overcomes this
problem (compare entries 1 and 4 in Table 1).10
As for the mechanism, intrapolymer and interpolymer
electron transfer are possible in the hybrid polymer. Based
on this question, we carried out the following experiment. We
synthesized a B12 polymer and a Ru complex polymer (ESIw),
and a catalytic reaction was carried out using a 1 : 1 mixture of
these homo-polymers (reaction conditions were the same as in
Table 2). Catalytic efficiency was decreased to 9% conversion
of the substrate when we used a mixture of these homo-
polymers. Therefore, the predominant electron transfer from
the Ru complex to B12 should occur in the same polymer unit.11
Finally, we investigated the photosensitizing mechanism of
the Ru complex in the polymer. The Ru complex is activated
by visible light irradiation, and in general, reductive and
oxidative quenching mechanisms exist in the ruthenium tris-
bipyridine complex.8 To elucidate the quenching mechanism,
the steady-state emission from the triplet state of the Ru
complex in the polymer and its lifetime were measured. After
exciting the metal to ligand charge transfer (MLCT) transition
band of the Ru complex at 456 nm, a strong emission at
638 nm was observed with a fluorescence quantum yield,
Ff = 5.3% in DMF. Because these values were similar to
those of [Ru(bpy)3]Cl2 (Ff = 6.8% in DMF),12 the triplet state
of the Ru complex in the polymer is not quenched by B12.
Therefore, electron transfer to B12 mainly occurs by a
reductive quenching mechanism of the Ru complex. After
visible light irradiation, the triplet state of the Ru complex is
quenched by a sacrificial reductant to form one-electron
reduced species, and that reduces the B12 to form the Co(I)
species. The thus-formed Co(I) species attacked phenethyl
bromide to form ethylbenzene and styrene.6
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proceeded with the hybrid polymer composed of the B12
catalyst and a Ru photosensitizer under irradiation with
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This work was partially supported by a Grant-in-Aid for
Scientific Research on Priority Areas (452) and Innovative
Areas (2204) and the Global COE Program ‘‘Science for
Future Molecular Systems’’ from the Ministry of Education,
Culture, Sports, Science and Technology (MEXT) of Japan
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c
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