B12-TiO2 hybrid catalyst for dehalogenation of organic halides

Article abstract of DOI:10.1246/cl.2009.468

A cobalamin derivative, cobyrinie acid, was effectively immobilized on TiO₂, and the hybrid TiO₂ was characterized by UV-vis, XPS, MALDI-TOFMS as well as TEM analysis. The hybrid TiO₂ exhibits high reactivity for dehalogenation of various organic halides such as phenethyl bromide, benzyl bromide, and 1,1-bis(4-chlorophenyl)-2,2,2-trichloroethane (DDT) under irradiation with UV light at room temperature. Copyright

Full text of DOI:10.1246/cl.2009.468

468
Chemistry Letters Vol.38, No.5 (2009)
B12–TiO2 Hybrid Catalyst for Dehalogenation of Organic Halides
Hisashi Shimakoshi,1 Emiko Sakumori,1 Kenji Kaneko,2 and Yoshio Hisaedaꢀ1
1Department of Chemistry and Biochemistry, Graduate School of Engineering, Kyushu University, Fukuoka 819-0395
2Department of Material Science, Graduate School of Engineering, Kyushu University, Fukuoka 819-0395
(Received February 2, 2009; CL-090117; E-mail: yhisatcm@mail.cstm.kyushu-u.ac.jp)
+
UV
A cobalamin derivative, cobyrinic acid, was effectively im-
HOOC
COOH
CH3
CH3
mobilized on TiO2, and the hybrid TiO2 was characterized by
UV–vis, XPS, MALDI-TOFMS as well as TEM analysis. The
hybrid TiO2 exhibits high reactivity for dehalogenation of vari-
ous organic halides such as phenethyl bromide, benzyl bromide,
and 1,1-bis(4-chlorophenyl)-2,2,2-trichloroethane (DDT) under
irradiation with UV light at room temperature.
H
Co
Co
HOOC
H3C
H3C
COOH
e−
CoI
Co
X
N
Co
H
N
e−
h+
H
H
CH3
N
H
N
Y
Co
HOOC
CH3
TiO2
Co
H3C
H
Co
CH3
COOH COOH
Cl−
Co
Co
X = CN,Y = H2O
[(CN)(H O)Cob(III)7COOH]Cl
=
Co
2
Cobalamin-dependent enzymes catalyze various molecular
transformations that are of particular interest from the viewpoint
of biological chemistry as well as organometallic and catalytic
chemistry.1 One of the most significant properties of cobalamin
is the high nucleophilicity toward various alkyl halides in its CoI
state to form an alkylated complex with dehalogenation.2 As
cobalamin derivatives are involved in the enzymatic reduction
of chlorinated organic compounds by a variety of anaerobic
bacteria,3 reductive dechlorination of organic halides, ubiquitous
pollutants such as polychlorinated alkenes and alkanes, cata-
lyzed by cobalamin derivatives has been reported.4 Recently,
we also reported the dechlorination of an environmental pollu-
tant, 1,1-bis(4-chlorophenyl)-2,2,2-trichloroethane (DDT) uti-
B12-TiO2
Cobalamin derivative (B12
)
ca. 2 nm
Figure 1. Schematic representation of cobalamin derivative
(B12)–TiO2 hybrid catalyst.
lizing
a
cobalamin derivative, heptamethyl cobyrinate
[Cob(II)7C1ester]ClO4, as catalyst under electrochemical and
photochemical conditions.5 The recent requirement of green
chemistry along with economical and environmental pressures
has led chemists to attempt to utilize a more efficient and clean
catalyst while using a simpler and more facile system. Therefore,
we tried to construct a new cobalamin derivative (B12)-depend-
ent dehalogenation system utilizing titanium dioxide (TiO2)
photosensitizer. It is well known that TiO2 particles generate
electron–hole pairs under band gap excitation with UV light
irradiation (eq 1).
Figure 2. Electronic absorption spectra of [Cob(II)7C1ester]-
ClO4 (8:7 ꢂ 10ꢁ5 M) in ethanol (solid line) containing TiO2
(P-25, 0.2 mg) and after irradiation with UV light (20 min,
broken line).
vis absorption spectrum in ethanol as shown in Figure 2 (solid
line). The solution then changed from brown to dark green and
showed a typical UV–vis spectrum for the CoI state of the coba-
lamin derivative2b with absorption maximum at 390 nm after
irradiation with UV light as shown in Figure 2 (broken line).
As oxidized product, acetaldehyde diethyl acetal was detected
by GC-MS. This UV–vis spectral change was not observed in
the absence of TiO2 and suggested that the cobalamin derivative
was reduced to a CoI species by photoinduced electron transfer
from TiO2.
The B12–TiO2 hybrid catalyst was prepared as follows: TiO2
powder (P-25; a mixture of rutile (20%) and anatase (80%) with
a BET surface area of 50 m2 gꢁ1, Japan Aerosil) was suspended
in ethanol containing 10 mM cobalamin derivative, cyanoaqua
cobyrinic acid [(CN)(H2O)Cob(III)7COOH]Cl (see the Support-
ing Information9) at room temperature. After stirring for 24 h,
red-purple TiO2 was filtered and washed with ethanol, then dried
under vacuum. The B12–TiO2 hybrid catalyst was characterized
hꢀ
ꢁ
TiO2 ꢁ! hVBþ þ eCB
ð1Þ
The conductive band electron (eCBꢁ) for anatase-type TiO2
has an Ered of ꢁ0:5 V vs. NHE in pH 7 aqueous solution.6 In
contrast, the redox potential for the CoII/CoI couple of cobala-
min derivatives is observed at ꢁ0:3 to ꢁ0:4 vs. NHE in various
media. Therefore, it is possible to generate the reactive CoI spe-
cies of cobalamin derivatives via electron transfer from TiO2
under irradiation with UV light. Thus in the present paper we
describe a novel catalytic system for dehalogenation of various
organic halides utilizing cobalamin derivative (B12) immobi-
lized on TiO2 photosensitizer as shown in Figure 1. Similar
hybrid TiO2 systems have been reported,7,8 and cobalt phthalo-
cyanine and cobalt porphyrin were used in these papers.
First, we examined the reductive formation of a CoI species
of a cobalamin derivative by UV–vis spectroscopy in the pres-
ence of TiO2 under irradiation with UV light (365 nm). The co-
balamin derivative, [Cob(II)7C1ester]ClO4, shows typical UV–
Copyright Ó 2009 The Chemical Society of Japan

Chemistry Letters Vol.38, No.5 (2009)
469
Table 1. Dehalogenation of various alkyl halides catalyzed by
B12–TiO2 hybrid catalysta
species of B12 via electron transfer from TiO2 is essential for this
reaction. Thus formed super-nucleophilic CoI species reacts with
organic halide with dehalogenation. Dehalogenation of various
organic halides also efficiently occurred as shown in Table 1.
In conclusion, a B12–TiO2 hybrid catalyst was readily pre-
pared from cobyrinic acid and TiO2. The hybrid catalyst showed
high reactivity for dehalogenation of various organic halides un-
der irradiation with UV light. We believe that this system would
be applicable to the design of new green catalysts for various
molecular transformations.
Entry
Substrate
Conversionb/%
Product (Yieldb/%)
EB (76), DB (4)
trace
1
PhCH2CH2Br
95
trace
0
2c PhCH2CH2Br
3d PhCH2CH2Br
—
4
PhCH2Br
99ꢃ
99ꢃ
toluene (2), bibenzyle (94)
DDD (13), TTDB(E/Z) (30/3),
DDA ethyl ester (12),
DDMU (16), DDMS (13)
DDMU (30), DDMS (35),
DCS (15)
5e DDT
6e DDD
82
We thank Prof. T. Ohno, Kyushu Insitute of Technology, for
useful discussions. This work was supported by a Grant-in-Aid
for Scientific Research on Priority Areas (417 and 460) and
Global COE Program ‘‘Science for Future Molecular Systems’’
from the Ministry of Education, Culture, Sports, Science and
Technology (MEXT) of Japan, and an Industrial Technology Re-
search Grant Program in 2005 from New Energy and Industrial
Technology Development Organization (NEDO) of Japan.
aConditions: [B12–TiO2] = 20 mg, [substrate] = 3:0 ꢂ 10ꢁ3 M, solvent:
30 mL of EtOH under N2, reaction time 24 h. bConversions were estimat-
ed by the recovery of the substrate. Yields were based on initial concen-
tration of the substrate. cUnmodified-TiO2 (P-25, 19 mg) was used as cat-
alyst. dSolvent, 30 mL of CH3CN. eDDD, 1,1-bis(4-chlorophenyl)-2,2-
dichloroethane; TTDB, 1,1,4,4-tetrakis(4-chlorophenyl)-2,3-dichloro-2-
butene; DDA ethyl ester, ethyl bis(4-chlorophenyl)acetate; DDMU, 1,1-
bis(4-chlorophenyl)-2-chloroethylene; DDMS, 1,1-bis(4-chlorophenyl)-
2-chloroethane; DCS, 3,6-dichlorophenanthrene (Chart S1).9
References and Notes
1
by high-resolution (HR)-TEM, UV–vis, and MALDI-TOF mass
spectrometry as well as XPS analysis. The UV–vis diffusion
reflectance analysis showed a reflectance maximum around
520 nm which is characteristic of the ꢁ-band of cyanoaqua
cobyrinic acid.1,2b XPS peaks at 780.4 (Co2p), 458.6 (Ti2p),
and 398.8 eV (N1s) were observed. The MALDI-TOF mass
spectrum (Figure S2)9 showed intense peaks at 964, 952, and
938, which are ascribed to [Mþ ꢁ H2O], [Mþ ꢁ CN], and
[Mþ ꢁ H2O ꢁ CN] of [(CN)(H2O)Cob(III)7COOH]þ, respec-
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of TiO2 was determined by UV–vis analysis to be 3:5 ꢂ
10ꢁ5 mol gꢁ1, and the apparent surface coverage with the coby-
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Dehalogenation of organic halide was carried out as shown
in Table 1. The starting phenethyl bromide almost disappeared,
and ethylbenzene (EB) and 2,3-diphenylbutane (mixture of rac-
emi and meso) (DB) were produced in 76% and 4% yields,
respectively (Entry 1, Table 1). The turnover number based on
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reaction, the hybrid catalyst was readily recovered by filtration.
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unmodified-TiO2 (Entry 2, Table 1) under the same conditions.
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change of a cobalamin derivative shown in Figure 2 did not oc-
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Supporting Information is available electronically on the CSJ-
Journal Web site, http://www.csj.jp/journals/chem-lett/index.html.
Published on the web (Advance View) April 11, 2009; doi:10.1246/cl.2009.468