B12-TiO2 Hybrid Catalyst for Light-Driven Hydrogen Production and Hydrogenation of C-C Multiple Bonds

Article abstract of DOI:10.1002/cplu.201402081

The B₁₂-TiO₂ hybrid catalyst mediates H₂O reduction to form hydrogen under UV irradiation (turnover number of one per hour). The catalyst also mediates reductions of alkenes such as styrene derivatives and alkylacrylates (maximum turnover number of 100 per hour) under mild conditions of room temperature, ordinary pressure, and water or alcohol as solvent.

Full text of DOI:10.1002/cplu.201402081

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DOI: 10.1002/cplu.201402081
B12-TiO2 Hybrid Catalyst for Light-Driven Hydrogen
Production and Hydrogenation of CÀC Multiple Bonds
Hisashi Shimakoshi* and Yoshio Hisaeda*[a]
The B12-TiO2 hybrid catalyst mediates H2O reduction to form
hydrogen under UV irradiation (turnover number of one per
hour). The catalyst also mediates reductions of alkenes such as
styrene derivatives and alkylacrylates (maximum turnover
number of 100 per hour) under mild conditions of room tem-
perature, ordinary pressure, and water or alcohol as solvent.
radiation. This prompted us to investigate further applications
of the B12-TiO2 catalyst utilizing the high reactivity of the CoI
species of the B12 complex. We now report the new catalysis of
B12-TiO2 for H2O reduction to form hydrogen, as shown in
Figure 1. Cobalt complexes have been studied as excellent cat-
Naturally occurring B12 (cobalamin)-dependent enzymes cata-
lyze various molecular transformations that are of particular in-
terest from the viewpoint of biological chemistry as well as
synthetic organic chemistry and catalytic chemistry.[1a–f] For ex-
ample, B12-dependent enzymes catalyze rearrangement reac-
tions as typified by the conversion of methylmalonyl-CoA to
succinyl-CoA, the reversible interconversion of l-glutamate and
threo-b-methyl-l-aspartate, and the methylation reaction as in
the synthesis of methionine. All these reactions are mediated
by the cobalt alkylated complex, which is generally formed by
the reaction of the CoI state of B12 with various electrophiles in
vitro.[2a,b] Therefore, the investigation of the reactivity of the
CoI species of B12 is important to elucidate the catalytic ability
of the enzyme. During the course of these studies, a variety of
molecular transformations were achieved by cobalamin deriva-
tives, including not only B12-mimic reactions,[2a] but also bioin-
spired reactions such as cyclopropane ring cleavage,[3] the re-
duction of a variety of substrates such as nitrite,[4] nitrate,[4,5a]
hydroxylamine,[5b] oxyhalogens,[5c] organic disulfides,[5d] unsatu-
rated esters,[5e,6] thiosulfate,[7] and sulfite,[7] reductive radical cy-
clization,[8a–c] the photoreduction of CO2,[9] and the reductive
dehalogenation of organic halides.[10–15]
Figure 1. Photoinduced hydrogen evolution and alkene reduction catalyzed
by the B12-TiO2 hybrid.
alysts for hydrogen production, and the cobalt hydride com-
plex is thought to be an intermediate in the reaction, which
could be formed by the reaction of CoI and a proton.[17a–f] As
metal hydride complexes are used widely for the reduction of
unsaturated compounds such as alkenes,[18] the application of
the B12-TiO2 catalyst for the hydrogenation of CÀC multiple
bonds was also investigated.
Hydrogen evolution by the B12-TiO2 was investigated in an
aqueous ethylenediamine-N,N,N’,N’-tetraacetic acid disodium
salt (EDTA·2Na, sacrificial reagent) (0.1m) solution, in which the
B12-TiO2 (10 mg, anatase, B12 content=3.43ꢀ10À7 mol) was sus-
pended under anaerobic conditions. The results for the hydro-
gen production are summarized in Table 1. During UV irradia-
tion (365 nm, 1.76 mWcmÀ2 at a distance of 12 cm), hydrogen
gas evolved gradually, as shown in Figure 2. Without the B12
complex (i.e., using bare TiO2) or UV irradiation, only a little hy-
drogen gas evolved (entries 4 and 5 in Table 1). If the B12 com-
plex was not immobilized on TiO2 but dissolved in the solution,
in which heptamethyl cobyrinate perchlorate (see Supporting
Information, Figure S1) was used as the B12 complex, the hy-
drogen evolution efficiency decreased (entry 3 in Table 1). The
immobilization of the B12 complex on the surface of TiO2
should enhance the reaction, probably owing to the short dis-
tance between the B12 complex and TiO2. The hydrogen evolu-
tion efficiency was also dependent on the TiO2 crystal type.
Anatase-type TiO2 was superior to rutile TiO2 for this reaction,
as shown in Figure 2 (entries 1 and 2 in Table 1), because the
Recently, we reported the unique catalysis of a B12-titanium
oxide (TiO2) hybrid catalyst, in which the B12 complex, cyanoa-
quacobyrinic acid (CoIII oxidation state), is immobilized on the
surface of TiO2 and the B12 complex is reductively activated to
form the CoI species by electron transfer from TiO2 under UV ir-
radiation.[16a–d] The hybrid catalyst mediated the dehalogena-
tion of various organic halides, and was applied to the radical-
mediated organic reaction[16a] via an alkylated complex as a cat-
alytic intermediate. The great advantage of the catalyst is the
facile and efficient formation of the CoI species simply by UV ir-
[a] Prof. Dr. H. Shimakoshi, Prof. Dr. Y. Hisaeda
Department of Chemistry and Biochemistry, Graduate Schoolof Engineering
Kyushu University, 744 Motooka, Nishi-ku, Fukuoka (Japan)
Fax: (+81)92-802-2830
E-mail: shimakoshi@mail.cstm.kyushu-u.ac.jp
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/cplu.201402081.
This article is part of the “Early Career Series”. To view the complete
series, visit: http://chempluschem.org/earlycareer.
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The B12-TiO2 catalyst also showed a high activity for the re-
duction of alkenes. Upon dissolution of an atropic acid (1) in
the solution, hydrogen evolution by the B12-TiO2 (anatase) was
completely inhibited and 2-phenyl propionic acid (2) was ob-
tained quantitatively within 1 h, as shown by Equation (1).
Table 1. Hydrogen production by the B12-TiO2 under UV irradiation.[a]
Entry
Catalyst
Crystal type of TiO2
H2 [mL]
TON[b]
[c]
1
2
3
4
5
6
B12-TiO2
B12-TiO2
anatase
rutile
anatase
anatase
anatase
anatase
90
41
39
5
0
0
11
4
[d]
[e]
B12, TiO2
TiO2
4
–
0
0
[f]
B12-TiO2
B12-TiO2
[c,g]
[a] UV irradiation with black light (UVP), lmax =365 nm, 1.76 mWcmÀ2
.
[b] Turnover numbers (TON) were based on B12. [c] [B12-TiO2]=10 mg (B12,
5.71ꢀ10À5 m), [EDTA·2Na]=0.1m, solvent H2O 6 mL under N2 at room
temperature. Reaction time=10 h. [d] [B12-TiO2]=8.9 mg (B12, 5.71ꢀ
10À5 m), [EDTA·2Na]=0.1m, solvent H2O 6 mL under N2 at room tempera-
ture. Reaction time=10 h. [e] Heptamethyl cobyrinate perchlorate
(0.39 mg, 5.71ꢀ10À5 m), TiO2 =10 mg, [EDTA·2Na]=0.1m, solvent H2O
6 mL under N2 at room temperature. Reaction time=10 h. [f] In the dark.
[g] In the presence of DMPO (1.0m).
The B12-TiO2 was separated from the solution easily after the
reaction by filtration or centrifugation (Figure S2). The turnover
number of the B12 complex was 100 per hour. The hydrogen
evolution started after the consumption of all the alkene (Fig-
ure S5). This suggests that the same intermediate is involved
during hydrogen production and alkene reduction. Notably,
the alkene reduction did not proceed with the platinum-
loaded TiO2 as well as the bare TiO2. The B12 complex effective-
ly mediates the reaction. The source of the hydrogen in the re-
duced product was determined by using D2O as the solvent.
Two deuterium atoms were incorporated into the 2-phenyl
propionic acid, PhCD(CH2D)COOH, as confirmed by 1H NMR
and MS analyses (Figures S3b and S4b). Most hydrogenations
by transition-metal systems are conducted under H2, and
sometimes under high pressure, whereas the reaction by the
B12-TiO2 was achieved without H2, with H2O used as the H
source under mild conditions (room temperature, ordinary
pressure, and water as the solvent).
Figure 2. Time course for photocatalytic H2 evolution by B12-TiO2 from
EDTA·2Na aqueous solution (0.1m) at room temperature under N2. B12-TiO2
(anatase): red circles; B12-TiO2 (rutile): blue squares; B12 (heptamethyl cobyri-
nate perchlorate)+TiO2 (anatase): green triangles.
The generality of the reaction was then examined by using
a variety of substrates. The CÀC double bonds of unsaturated
esters, that is, alkylacrylates (3a–5a), were selectively reduced
to form alkyl propionates (3b–5b) in MeOH with moderate
yields, but required a long reaction time [Eq. (2)]. Here, MeOH
could work as a solvent and sacrificial reagent to consume the
hole of TiO2.[16b] In contrast, nonconjugated alkenes such as al-
lylbenzene or vinylcyclohexane were not reduced.
conduction band electron is more negative in anatase (Ered
À0.5 V vs. NHE in pH 7 aqueous solution) than in rutile TiO2
(Ered =À0.3 V vs. NHE in pH 7 aqueous solution).[19] This is an
advantage for B12 reduction to form reactive CoI species, be-
cause the redox potential for the CoII/CoI couple of the cobala-
min derivatives is observed at À0.3 to À0.4 V versus NHE in
various media.[2a] The hydrogen evolution is inhibited in the
presence of a spin-trapping reagent, 5,5-dimethyl-1-pyrroline-
N-oxide (DMPO) (entry 6 in Table 1), which suggests the exis-
tence of an intermediate having radical character during the
reaction.
=
From the time course of the hydrogen evolution, the turn-
over number for the B12 complex was estimated to be about
one per hour for the B12-TiO2 (anatase) system. The hydrogen
evolution by a cobalamin derivative, cobinamide, combining
the use of Eosin Y as a photosensitizer, was reported recently,
and the turnover number of the cobalt complex for hydrogen
evolution was only around 4.4ꢀ10À3 per hour.[20] The turnover
number frequency for hydrogen evolution by the B12-TiO2
system was superior to that of the reported system, probably
because of the efficient production of the CoI species.
With styrene (6a) as the substrate, 2,3-diphenylbutane (6b)
(racemi and meso) was obtained as the main product, and
a small amount of ethylbenzene (6c) was also produced
[Eq. (3)]. A similar product distribution was also seen for a-
methylstyrene (7a), with 2,3-dimethyl-2,3-diphenylbutane (7b)
and isopropylbenzene (7c) obtained as the main and minor
products, respectively. Note that the reductive dimerization of
arylalkenes catalyzed by B12/reducing agents such as Zn or TiIII
citrate was reported by Donk and co-workers.[21] The reported
photosensitizing system has further advantages in terms of its
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use of clean light energy and the fact that no additional chem-
ical reagents are required.
Interestingly, the B12-TiO2 catalyst showed dual reactions of
dehalogenation and hydrogenation. A b-bromostyrene (8) was
converted to 2,3-diphenylbutane (6b) (racemi and meso) and
ethylbenzene (6c), though substitution at the a position of
styrene lowered the reactivity so a long reaction time was re-
quired [Eq. (4)]. As the product distribution was similar to that
of styrene, the debromination of b-bromostyrene afforded the
styrene, and the subsequent reaction of styrene formed the
same products according to Equation (3). In contrast, the reac-
tion with a-bromostyrene (9) afforded a different product, and
2,3-diphenylbutane (10) (cis/trans ratio 1:1) was produced effi-
ciently within 2 h [Eq. (5)]. The position of the Br substituent
should change the reaction mechanism dramatically.
Figure 3. Mechanism for reduction of alkenes by the cobalt complex.
As for the hydrogen evolution, if the cobalt-hydride complex
has a radical character, a bimetallic mechanism for the CoIII-H
complex occurs significantly, as shown in Figure 4. However,
the redox potential of the CoIII-H/CoII-H couple is thought to
be more positive than that of the CoII/CoI couple in the cobalt
complex,[23a,b] so the CoIII-H complex may be reduced to the
CoII-H complex under the applied conditions. The more elec-
tron-rich CoII-H complex might be nucleophilic and react with
a proton to form hydrogen through a hydride mechanism
(monometallic mechanism).[24] Determination of the detailed
mechanism of hydrogen evolution by B12, that is, the cobala-
min derivative, is now in progress in our group.
On the basis of these results, the mechanism for alkene re-
duction by the B12-TiO2 is proposed as shown in Figure 3. The
CoI species of B12 first reacts with a proton to form the cobalt-
hydride intermediate, CoIII-H. It is likely that the CoIII-H complex
has radical character, and hydrogen radical attack occurs at the
b-position of the alkene avoiding the steric hindrance of the
phenyl group to form the a-radical intermediate (11). If R is H
or a methyl group, radical coupling occurs significantly to form
the dimerized product. If R is an electron-withdrawing (EWD)
group, further reduction by TiO2 to form the carbanion inter-
mediate (12) and subsequent protonation to form the simply
reduced product as 2-phenylpropionic acid (2) with R=COOH
occurs. If R=Br, the carbanion may lead to a carbene (13) with
elimination of the bromide ion.[12b,22] The electrophilic carbene
may react with the carbanion (12) to form 2,3-diphenylbutane
(10).
Figure 4. Mechanism for hydrogen evolution by the cobalt-hydride complex.
In conclusion, we have described the catalysis of the B12-TiO2
hybrid catalyst for hydrogen evolution and hydrogenation. The
synergistic effect of the naturally occurring cobalamin deriva-
tive, cobyrinic acid, and inorganic TiO2 provides the unique cat-
alysis of the metal complex-semiconductor hybrid system. Co-
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used as the hydrogen source for the hydrogenation. The heter-
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would be readily applicable for the design of an ecofriendly
catalyst, and the development of such bioinspired catalysis will
play an important role in next-generation science and technol-
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metal complex-semiconductor system including the B12 com-
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Experimental Section
The B12-TiO2 hybrid catalyst was prepared according to a method
reported previously.[16d] The content of the B12 complex on the sur-
face of anatase TiO2 was 3.43ꢀ10À5 molgÀ1, and the apparent sur-
face coverage by the B12 complex was 7.0ꢀ10À11 molcmÀ2; the B12
complex content on the surface of rutile TiO2 was 3.84ꢀ
10À5 molgÀ1, and the apparent surface coverage by the B12 com-
plex was 7.8ꢀ10À11 molcmÀ2. For the alkene reduction, 100 equiva-
lents (mol) of alkenes (5.7ꢀ10À3 m) versus the B12 complex on the
TiO2 were dissolved in the solvent. The reactions were performed
in MeOH except that for atropic acid (1) owing to their poor solu-
bilities in H2O. After the photoreaction, the products were identi-
fied by GC-MS or NMR spectroscopy.
Acknowledgements
This study was supported by a Grant-in-Aid for Scientific Research
on Priority Areas (No. 20031021) and Innovative Areas
(No. 25105744) from the Ministry of Education, Culture, Sports,
Science and Technology (MEXT) of Japan, a Grant-in-Aid for Sci-
entific Research (C) (No. 23550125) from the Japan Society for
the Promotion of Science (JSPS), an Industrial Technology Re-
search Grant Program in 2005 from New Energy and Industrial
Technology Development Organization (NEDO) of Japan, and the
General Sekiyu Foundation.
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Received: March 24, 2014
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H. Shimakoshi,* Y. Hisaeda*
&& – &&
B12-TiO2 Hybrid Catalyst for Light-
Driven Hydrogen Production and
Hydrogenation of CÀC Multiple Bonds
Hybrid catalyst: TiO2 modified with the
a variety of alkenes under mild condi-
tions (room temperature, ordinary pres-
sure, and water or alcohol solvent), and
the turnover number of the B12 complex
is a maximum of 100 per hour.
B
12 complex (cobyrinic acid), B12-TiO2,
mediates hydrogen evolution under UV
irradiation (see figure). B12-TiO2 also effi-
ciently catalyzes the hydrogenation of
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