Photosensitizing catalysis of the B12 complex without an additional photosensitizer

Article abstract of DOI:10.1039/c1cc12482j

A cobalamin derivative, heptamethyl cobyrinate perchlorate, was activated by UV light irradiation to form a Co(i) species in the presence of triethanolamine and used for a dechlorination reaction, and this photochemical reaction was accelerated in an ionic liquid.

Full text of DOI:10.1039/c1cc12482j

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Photosensitizing catalysis of the B complex without an additional
photosensitizerw
12
Hisashi Shimakoshi, Li Li, Masashi Nishi and Yoshio Hisaeda*
Received 28th April 2011, Accepted 31st May 2011
DOI: 10.1039/c1cc12482j
A cobalamin derivative, heptamethyl cobyrinate perchlorate,
was activated by UV light irradiation to form a Co(I) species
in the presence of triethanolamine and used for a dechlorination
reaction, and this photochemical reaction was accelerated in an
ionic liquid.
The cobalamin derivative (B ) is a cobalt complex with a
2
1
tetrapyrrole ring system (corrin ring) that has emerged in a
variety of enzymes such as methylmalonyl CoA mutase,
1
methionine synthase and reductive dehalogenase. Inspired
by the unique functions of these enzymes, various catalytic
reactions have already been reported using a B12 model
complex such as the 1,2-migration of a functional group,
dechlorination of organic chlorides, methylation of heavy
2
metals and thiols, etc. Among the B model complexes, the
1
2
corrinoid compound was mostly developed in the B12 mimic
reaction due to its structure and physicochemical properties
being similar to natural cobalamin.
Most of the catalytic reactions by the corrinoid compound
were achieved using the supernucleophilicity of the reduced
Fig. 1 Catalytic cycle of the B12 complex by UV light irradiation.
3
form of the Co(I) species. Therefore, all of the catalytic
property of the cobalamin derivative and determined the
unique photosensitizing property of the B12 complex, especially
in an ionic liquid. The ionic liquid is a unique ion pair
solvent having various properties such as high polarity,
reactions were coupled with a reducing system such as a
4
chemical reductant (NaBH , Zn, Na amalgam and so on),
electrochemical reduction and combined use with a photo-
2
sensitizer. Recently, we reported the dechlorination of
5
good conductivity and negligible vapor pressure. In this
communication, the unique photosensitizing property of the
1
,1-bis(4-chlorophenyl)-2,2,2-trichloroethane (DDT) catalyzed
by the B12 derivative, heptamethyl cobyrinate perchlorate
Co(II) form), with a [Ru(II)(bpy) ]Cl photosensitizer by
B
12 complex in the ionic liquid and its application for DDT
degradation without any photosensitizer are reported (Fig. 1).
The heptamethyl cobyrinate perchlorate was first synthe-
(
3
2
4
irradiation with visible light. The B complex was reduced
1
2
6
7
sized by Eschenmoser’s group and used by many groups. It
shows a high solubility toward not only a variety of organic
solvents, but also ionic liquids such as N-methyl-N-propyl-
pyrrolidinium bis(trifluoromethanesulfonyl)amide ([P13][TFSA])
and 1-butyl-3-methyl imidazolium bis(trifluoromethanesulfonyl)-
amide ([bmim][TFSA]) (Chart 1).
to form the reactive Co(I) species by electron transfer from the
ruthenium photosensitizer in the presence of a sacrificial
reductant. Although the B12 complex showed a high catalytic
efficiency in the reaction, a photosensitizer was required for
the reaction. If the B12 complex itself showed dual properties,
photosensitization and catalysis, the catalytic reaction was
achieved without any additional photosensitizer. To alleviate
this problem, we started to explore the photosensitizing
When heptamethyl cobyrinate perchlorate was dissolved in
an ionic liquid ([P13][TFSA]) or methanol containing 0.1 M
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; Tel: +81-92-802-2826
w Electronic supplementary information (ESI) available: Details of
experimental procedures. See DOI: 10.1039/c1cc12482j
Chart 1
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Chem. Commun., 2011, 47, 10921–10923 10921

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ꢁ4
Fig. 3 ESR spectra of the B12 complex (5.0 ꢀ 10 M) in the presence
of 0.1 M TEOA in [P13][TFSA] at 100 K under anaerobic conditions.
(
a) Before UV light irradiation, (b) after 2 h UV light irradiation.
ꢁ5
Fig. 2 UV-vis spectral change of the B12 complex (2.9 ꢀ 10 M) in
the presence of TEOA (0.1 M) by UV light irradiation (lmax = 365 nm)
under anaerobic conditions; (a) in [P13][TFSA], (b) in MeOH.
triethanolamine (TEOA) and subsequently irradiated by UV
light (lmax = 365 nm)z under anaerobic conditions, the color
of the solution changed from brown to dark green. This
photoreaction was well monitored by UV-vis spectroscopy.
The UV-vis spectrum of the starting Co(II) form of the
B12 complex having absorption maxima at 314 nm and 468 nm
was changed to a new spectrum with absorption maxima at
3
90 and 560 nm which is typical for the Co(I) state of the
8
B12 complex by UV light irradiation as shown in Fig. 2a
(
in [P13][TFSA]) and 2b (in MeOH).y
From these UV-vis spectral changes, the rate constant for
ꢁ4
ꢁ1
the Co(I) formation was determined to be k = 2 ꢀ 10
s
Fig. 4 Proposed photosensitizing mechanisms of the B12 complex by
ꢁ
5
ꢁ1
and 3 ꢀ 10
s in [P13][TFSA] and MeOH, respectively
UV light irradiation in the presence of TEOA.
(ESIw).z This photochemical reaction was ca. 7 times enhanced
in the ionic liquid. In [bmim][TFSA], the B12 complex showed
a poor UV-vis change (Fig. S1, ESIw). Because the imidazolium-
type ionic liquid has a relatively acidic C–H bond at the
1
2
in the gas phase was reported by Shafizadeh et al. The decay
is interpreted as resulting from a ring to metal charge transfer.
A similar excited state is predicted in the present case, and
such a charge-separated (CS) state (Co(I) cation radical) could
be stabilized in an ion-pair solvent, ionic liquid having a high
polarity, and by subsequent electron transfer from TEOA
to form the reduced form of B12, the Co(I) state (Fig. 4a).
Another possibility is forming an encounter complex between
the excited B12 complex and TEOA. In this model, electron
transfer from TEOA to the B12 complex in the encounter
9
imidazole ring, it should quench the Co(I) species of the B
1
complex.
1
2
0
The photoreduction of the B12 complex was also investigated
by ESR spectroscopy. The heptamethyl cobyrinate perchlorate
showed the typical Co(II) low-spin signal (g
Co
1
= 2.51, g
2
= 2.25,
Co
g
3
= 2.00, A
2
= 63 G, A
3
= 134 G) in [P13][TFSA]
1
1
containing 0.1 M TEOA as shown in Fig. 3a. After UV
light irradiation, this signal almost disappeared to form the
diamagnetic Co(I) species (Fig. 3b). This ESR spectral change
also indicated that the B12 complex was reduced to the Co(I)
form by the UV light irradiation.
+
ꢂ
complex should form the CS state (Co(I)–TEOA ). This
charge-separated state could be stabilized in a polar ionic
liquid to prevent a back-electron transfer process (Fig. 4b).
To further understand the photosensitizing property of the B12
complex, ultrafast transition spectroscopy for the Co(II) state
of the corrinoid compound is needed.
Two plausible mechanisms are shown in Fig. 4. Once the
Co(II) form of the B12 complex is excited, a ligand (corrin ring)
to the metal charge transfer state may be formed. While the
oxidation state of B12 was Co(III), an ultrafast electronic
relaxation of the excited state vitamin B12 (cyanocobalamin)
The catalytic reaction was carried out using DDT as a
substrate.8 The results are shown in Table 1. After a 2 h UV
1
0922 Chem. Commun., 2011, 47, 10921–10923
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Table 1 Photocatalytic dechlorination of DDT catalyzed by the B12
complex
Notes and references
a
z Black light (UVP, XX-15BLB) was used for the UV light irradiation
2
1.76 mW cm at 12 cm distance).
(
y The photochemical reaction did not proceed under visible irradiation
using a 200 W tungsten lamp with a 420 nm cut-off filter (Sigma Koki,
4
2L) and a heat cut-off filter (Sigma Koki, 30H).
z In MeOH, the photochemical reaction slowly proceeded without
TEOA. Therefore, MeOH also acts as a weak sacrificial reductant.
8 General procedure: A 4 mL [P13][TFSA] solution of the B12
b
Yields (%)
c
Entry
Solvent
Irradiation
TON
ꢁ5
ꢁ3
complex (4.4 ꢀ 10 M), DDT (2.2 ꢀ 10 M) and triethanolamine
1
2
3
[P13][TFSA]
[P13][TFSA]
MeOH
UV
Dark
UV
72
0
9
35
0
4
(0.1 M) was degassed by freeze–pump–thaw cycles. The solution was
then stirred at room temperature under irradiation by 365 nm
UV light.z After 2 h, the product was extracted with ether–hexane
a
ꢁ5
ꢁ3
(
1 : 3 v/v). The product, DDD, was identified by NMR and GC-MS
Conditions: [B12] = 4.4 ꢀ 10
M, [DDT] = 2.2 ꢀ 10
M,
comparison with the purchased authentic sample. The B12 complex
remaining in the ionic liquid was recycled after being dried under
reduced pressure for 24 h.
[
at room temperature. Reaction time, 2 h. Yields were based on initial
triethanolamine] = 0.1 M. lmax = 365 nm under degassed conditions
b
c
concentration of the substrate. Turnover numbers (TON) were based
on [B12].
1 (a) W. Buckel and B. T. Golding, Chem. Soc. Rev., 1996, 25, 329;
(
¨
b) Vitamin B12 and B12-Protein, ed. B. Krautler, D. Arigoni and
B. T. Golding, Wiley-VCH, Weinheim, 1998; (c) Chemistry and
Biochemistry of B12, ed. R. Banerjee, Wiley-Interscience, New
York, 1999; (d) R. Banerjee and S. W. Ragsdale, Annu. Rev.
Biochem., 2003, 72, 209; (e) T. Toraya, Chem. Rev., 2003,
light irradiation, DDT was converted to a monodechlorinated
compound, 1,1-bis(4-chlorophenyl)-2,2-dichloroethane (DDD),
in 72% yield (entry 1 in Table 1). The reaction did not proceed
in the dark (entry 2 in Table 1). In MeOH, the yield of DDD
was only 9% which reflects the Co(I) formation efficiency. As
for the mechanism, nucleophilic attack of the Co(I) species on
DDT could form an alkylated complex as an intermediate as
1
¨
03, 2095; (f) B. Krautler and S. Ostermann, in The
Porphyrin Handbook, ed. K. M. Kadish, K. M. Smith and
R. Guilard, Academic Press, 2003, vol. 11, pp. 229–276.
(a) K. L. Brown, Chem. Rev., 2005, 105, 2075; (b) Y. Hisaeda and
H. Shimakoshi, in Handbook of Porphyrin Science, ed.
K. M. Kadish, K. M. Smith and R. Guilard, World Scientific,
2010, vol. 10, pp. 313–370.
2
4
,13
shown in Fig. 1.
This alkylated complex homolysis by UV
3
4
(a) G. N. Schrauzer and E. Deutsch, J. Am. Chem. Soc., 1969,
91, 3341; (b) A. Fischli and J. J. Daly, Helv. Chim. Acta, 1980, 63, 1628.
(a) H. Shimakoshi, M. Tokunaga, T. Baba and Y. Hisaeda, Chem.
Commun., 2004, 1806; (b) H. Shimakoshi, S. Kudo and Y. Hisaeda,
Chem. Lett., 2005, 34, 1096.
light irradiation or thermolysis forms a substrate radical.
Hydrogen abstraction from the bulk formed DDD as a
dechlorinated product. After the photoreaction with workup,8
most of the B12 catalyst remained in the ionic liquid and was
quantitatively recovered as confirmed by UV-vis spectra
5
(a) T. Welton, Chem. Rev., 1999, 99, 2071; (b) J. Dupont, R. F.
de Souza and P. A. Z. Suarez, Chem. Rev., 2002, 102, 3667;
(Fig. S2, ESIw). Therefore, the B12 complex could be reused for
(
c) J. P. Hallett and T. Welton, Chem. Rev., 2011, 111, 3508.
the successive reaction. In fact, the DDT dechlorination reaction
proceeded with almost the same efficiency in the second and third
runs (yields of DDD, 2nd 70% and 3rd 68%).
6 (a) L. Werthemann, R. Keese and A. Eschenmoser, unpublished
results; (b) See: L. Werthemann, Dissertation, ETH Zu¨rich (No.
4
¨
097), Juris Druck and Verlag, Zurich, 1968.
7
(a) Y. Murakami, Y. Hisaeda and A. Kajihara, Bull. Chem. Soc.
Jpn., 1983, 56, 3642; (b) C. W. Exl, T. Darbre and R. Keese, Org.
In summary, the photosensitizing property of the B12
complex under anaerobic conditions was investigated. The
reductive quenching of the excited state of the B12 complex
by a sacrificial reductant provided the reduced form of the B12
complex, Co(I) species. This photosensitizing property of the
¨
Biomol. Chem., 2007, 5, 2119; (c) C. Mannel-Croise, B. Probst and
´
F. Zelder, Anal. Chem., 2009, 81, 9493; (d) H. A. Hassanin,
M. F. Ei-Shahat and M. S. A. Hamza, J. Coord. Chem., 2010,
6
3, 2431.
8
H. Shimakoshi, E. Sakumori, K. Kaneko and Y. Hisaeda, Chem.
Lett., 2009, 38, 468.
B
12 complex was used for dechlorination of the pollutant,
DDT. Therefore, the results reported here will open a door to
the new use of the corrinoid compound in photocatalytic
chemistry.
9 (a) A. G. Avent, P. A. Chaloner, M. P. Day, K. R. Seddon and
T. Welton, J. Chem. Soc., Dalton Trans., 1994, 3405;
(
b) E. Baciocchi, C. Chiappe, T. D. Giacco, C. Fasciani,
O. Lanzalunga, A. Lapi and B. Melai, Org. Lett., 2009, 11, 1413.
10 G. N. Schrauzer, Angew. Chem., Int. Ed. Engl., 1976, 15, 417.
11 (a) J. R. Pilbrow, EPR of B12-Dependent Enzyme Reactions and
Related Systems, in B12, ed. D. Dolphin, Wiley, New York, 1982,
vol. 1, p. 431; (b) S. V. Doorslaser, G. Jeschke, B. Epel,
This work was partially supported by a Grant-in-Aid for
Scientific Research on Priority Areas No. 452, ‘‘Science of
Ionic Liquids’’, Innovative Areas No. 2204, ‘‘Molecular
Activation toward Straightforward Synthesis’’ and the Global
COE Program ‘‘Science for Future Molecular Systems’’ from
the Ministry of Education, Culture, Sports, Science and
Technology (MEXT) of Japan and a Grant-in-Aid for
Scientific Research (A), No. 21245016, from the Japan Society
for the Promotion of Science (JSPS).
¨
D. Goldfarb, R.-A. Eichel, B. Krautler and A. Schweiger, J. Am.
Chem. Soc., 2003, 125, 5915; (c) H. Shimakoshi, A. Nakazato,
M. Tokunaga, K. Katagiri, K. Ariga, J. Kikuchi and Y. Hisaeda,
Dalton Trans., 2003, 2308.
1
1
2 N. Shafizadeh, L. Poisson and B. Soep, Chem. Phys., 2008, 350, 2.
3 H. Shimakoshi, M. Tokunaga and Y. Hisaeda, Dalton Trans.,
2004, 878.
This journal is c The Royal Society of Chemistry 2011
Chem. Commun., 2011, 47, 10921–10923 10923