Photopromoted oxidative cyclization of an o-phenylene-bridged Schiff base via a manganese(III) complex, leading to a fluorescent compound, 2-(2-hydroxyphenyl)benzimidazole

Article abstract of DOI:10.1039/a802405g

Visible light photolysis of [N,N′-o-phenylenebis-(salicylideneaminato)]diaquamanganese(III) resulted in the fluorescent compound 2-(2-hydroxyphenyl)benzimidazole by a one electron redox reaction between Mn^III and the Schiff base ligand, followe

Full text of DOI:10.1039/a802405g

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Photopromoted oxidative cyclization of an o-phenylene-bridged Schiff base via
a manganese(iii) complex, leading to a fluorescent compound,
2
-(2-hydroxyphenyl)benzimidazole
a
a
b
a
a
a
Takashi Fukuda, Fuminori Sakamoto, Minoru Sato, Yoshiharu Nakano, Xiang Shi Tan and Yuki Fujii* †
a
Department of Chemistry, Faculty of Science, Ibaraki University, Mito, 310-8512, Japan
Department of Chemistry and Engineering, Ibaraki National College of Technology, Hitachinaka 344-1200, Japan
b
Visible light photolysis of [N,NA-o-phenylenebis-
solvated Mn–O distances to be longer than chelated ones.14 No
hydrogen bond between the phenolic oxygen atoms of the
Schiff base ligand and apical water or methanol was ob-
served.6
(
salicylideneaminato)]diaquamanganese(III) resulted in the
fluorescent compound 2-(2-hydroxyphenyl)benzimidazole
III
by a one electron redox reaction between Mn and the
Schiff base ligand, followed by the base-hydrolysis of the
oxidized Schiff base, and then the cyclization of the base-
hydrolyzed product.
Fig. 2 shows the time courses of UV–VIS (320 nm),
fluorescence (432 nm), and EPR spectra of 1 in water at initial
pH = 6.4 under irradiation with visible light (150 W tungsten
halogen lamp with UV cut filter L-39). The UV–VIS spectral
change exhibited an isosbestic point at 223 nm, and the half life
(t1/2) of 1 was 11–15 h at pH 6.4.∑ A similar spectral change also
occurred in the dark but was very slow (t1/2 ≈ 55 h). Our
preliminary experiments revealed that the photopromotion
occurs by the irradiation into the CT band (400 ± 20 nm), but
hardly for the p �? �p * (335 ± 20 nm) and d ? d (540 ± 20 nm)
bands. Further, the reaction becomes faster with increase of
solution pH (t1/2 ≈ 9 h at pH 7.5, ≈ 5 h at pH 8.6).
The photoactivation of manganese complexes is an attractive
theme both in the modeling of PSII and in the development of
a new photoreaction.1 It has been known that, upon visible
light irradiation, in the presence of p-quinone, manganese(iii)
complexes with tetradentate Schiff base ligands having the
salen-type skeleton generate molecular oxygen;5 in the
absence of p-quinone, rearrangement of the coordinated Schiff
–4
–7
8
base ligand occurs. Recently, we examined the photolysis of
[
N,NA-o-phenylenebis(salicylideneaminato]diaquamanganese-
On the other hand, the fluorescence and EPR intensities of the
photoreaction solution increased with the decay of 1, indicating
that the fluorescent compound, assigned as 2-(2-hydroxy-
phenyl)benzimidazole 2, was formed at early stages of the
(
iii) 1 and observed a photopromoted oxidative cyclization of
the coordinated Schiff base ligand, and the effective formation
of the fluorescent compound 2-(2-hydroxyphenyl)benzimida-
zole 2, which is useful as a laser dye, high-energy radiation
II
reaction. The EPR spectra show typical six-line Mn signals (g
III
II
detector, molecular energy storage system, and fluorescent
= 2.013), which proves that the Mn was reduced to Mn
probe.9
–11
2 is usually prepared from salicylic acid and
during this reaction. Since neither molecular oxygen nor
hydrogen peroxide were detected in the photoreaction solution,
it is strongly suggested that intramolecular one electron redox
o-phenylenediamine in polyphospholic acid at 190 °C in 14%
yield.12 This photoreaction thus provides a new and effective
synthetic method for the fluorescent compound 2 and its
derivatives.‡
III
occurred between Mn and the coordinated Schiff base ligand.
A similar photoreaction also occurred under vacuum, however,
no EPR signal corresponding to an organic radical was
observed.
13
Cation 1 was prepared by a literature method, and isolated
II
as its perchlorate salt.§ Absence of Mn was confirmed by EPR
spectroscopy. Slow evaporation of methanol solution of the
complex produced crystals suitable for single crystal X-ray
structure analysis.¶ An ORTEP drawing of 1 is shown in Fig. 1.
The structure of the complex cation comprises a planar
tetradentate Schiff base ligand tightly bound to the Mn center
via two Mn–N bonds [1.981(2), 1.978(2) Å] and two Mn–O
From the photoreaction solution, 2 and salicylaldehyde 3
could be separated both in about 40% yield by extraction with
ethyl acetate, followed by silica-gel column separation using
ethyl acetate–n-hexane (1:2) as eluent (R
f f
= 0.6 for 2, R = 0.8
III
for 3).** In addition to 2 and 3, under argon, N,NA-
bonds [1.859(1), 1.875(1) Å], and by axial methanol and water
III
molecules which complete an octahedron around the Mn ion.
Longer Mn–O distances to the apical ligands [2.272(2) Å for
2
Mn–H O and 2.291(2) Å for MeOH] may be partly attributed to
the Jahn–Tellar effect for the d ion, although it is common for
1.0
0.5
4
O(4)
C(18)
C(17)
C(19)
C(14)
C(16)
C(13)
N(1)
C(9)
C(3)
C(5)
C(8)
C(20) C(15)
Mn
C(2)
C(6)
N(2)
0.0
0.3
O(1)
C(1)
0
5
10
15
20
25
30
C(10)
C(7)
O(2)
C(4)
t / h
C(11)
C(12)
Fig. 2 Time courses of UV–VIS, fluorescence and X-band EPR spectra of
the photolysis solution of 1 at pH 6.4 and 25 °C. The complex
concentration is 2.0 3 10 mol dm for UV–VIS (5) and fluorescence
O(3)
=
2
5
23
C(21)
2
3
23
(
2) and 1.0 3 10 mol dm for EPR (8). Fluorescence: lex = 325 nm,
em = 432 nm, EPR: microwave power = 5 mW, microwave frequency =
9.238 GHz, 0.5 mT modulation amplitude, 25 °C.
Fig. 1 An ORTEP drawing of cation 1 with thermal ellipsoids at 50%
probability. Hydrogen atoms are omitted for clarify.
l
Chem. Commun., 1998
1391

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hydrolysis of B is the formation of salicylaldehyde G and rate
enhancement with increase of solution pH. A study of the
detailed reaction mechanism is under way.
This work was partly supported by a Grant-in-Aid for
Science Research (No. 09440224 and No. 97370) from the
Ministry of Education, Science and Culture of Japan.
H
hn
• +
N
O
N
MnIII
N
O
N
MnII
O
O
A
B
OH –
Notes and References
•
† E-mail: yuki@mito.ipc.ibaraki.ac.jp
NH
‡
This method is also applicable for the preparation of the cationic
N
O
fluorescent compound 2-(2-hydroxy-5-trimethylammoniomethylphenyl)-
benzimidazole chloride.
§ This complex contains methanol as apical ligand in the crystal state,
however the methanol is thought to be substituted by water in aqueous
solution.
MnII
O
O
¶
Crystal data: C21
P2 /c, Z = 4, a = 11.804(2), b = 13.435(2), c = 14.358(2) Å, b =
109.108(9)°, U = 2151.5(5) Å , D
20 2 8
H N O MNCl, M = 518.79, monoclinic, space group
C
1
H
N
3
23
H
O
MnII
c
= 1.601 g cm , Mo-Ka radiation (l
= 0.032 for 3585 observed reflections
= 0.710 73 Å), R = 0.033 and Rw
with I > 3s(I). CCDC 182/878.
∑ Since the reaction occurs with irregular induction time, t is variable.
*
N
•
O
1/2
OH
1
* 2: H NMR [(CD
.664 (d, 2 H), 7.400 (t, 1 H), 7.303 (br, 2 H), 7.074–7.019 (m, 2 H).
SO], d 158.07, 151.68, 140.93, 133.14, 126.19, 123.27,
3 2
) SO, TMS standard], d 13.18 (br, 1 H), 8.057 (d, 1 H),
D
E
13
7
C
O2
H+
NMR [(CD
1
)
3 2
+
22.38, 119.11, 117.93, 112.58, 111.51. EI-MS: m/z 210 (M )
H
N
O
MnII
1 A. Harriman, Coord. Chem. Rev., 1979, 28, 147.
2 V. L. Pecoraro, Photochem. Photobiol., 1988, 48, 247.
+
N
HO
3
K. Wieghardt, Angew. Chem., Int. Ed. Engl., 1989, 28, 1153.
4 V. L. Pecoraro, M. J. Baldwin and A. Gelasco, Chem. Rev., 1994, 94,
07.
OH
F
G
8
Scheme 1
5
F. M. Ashmawy, C. A. McAuliffe, R. V. Parish and J. Tames, J. Chem.
Soc., Dalton Trans., 1985, 1391.
6
N. Aurangzeb, C. E. Hulme, C. A. McAuliffe, R. G. Pritchard, M.
Watkinson, M. R. Bermejo, A. Garcia-Deibe, M. Rey, J. Sanmartin and
A. Sousa, J. Chem. Soc., Chem. Commun., 1994, 1153.
o-phenylenebis(salicylideneamine) 4 was isolated in ca. 10%
yield as a photoreaction product of 1.
On the basis of these facts, the reaction mechanism (Scheme
7 M. Watkinson, A. Whiting and C. A. McAuliffe, J. Chem. Soc., Chem.
Commun., 1994, 2141.
8 A. Garcia-Deibe, A. Sousa, M. R. Bermejo, P. P. MacRory, C. A.
McAuliffe, R. G. Pritchard and M. Helliwell, J. Chem. Soc., Chem.
Commun., 1991, 728.
III
1
) is suggested as follows: (i) Mn –Schiff base complex A is
II
reduced to Mn complex B with partially oxidized Schiff base
ligand, for which visible light promotes the reaction (the
reaction has been shown to be enhanced by excitation at the CT
band of 1, probably by LMCT). B is thought to be a transient
radical and a similar radical has been proposed by Floriani and
coworkers.15 (ii) B is base-hydrolyzed to form radical C,
followed by cyclization of the partially oxidized Schiff base
9
D. L. Williams and A. Heller, J. Phys. Chem., 1970, 74, 4473.
1
0 A. U. Acuna, F. Amat, J. Catalan, A. Costela, J. M. Figuera and J. M.
Munoz, Chem. Phys. Lett., 1986, 132, 567.
1
1 E. L. Roberts, J. Dey and I. M. Warner, J.Phys. Chem., A, 1997, 101,
5
296.
II
ligand to produce radical D and Mn complex E, D is oxidized
12 C. M. Oelando Jr., J. G. Wirth and D. R. Heath, J. Org. Chem., 1970, 35,
3147.
13 L. J. Boucher and C. G. Coe, Inorg. Chem., 1975, 14, 1289.
4 C. A. McAuliffe, A. Nabhan, R. G. Pritchard, M. Watkinson, M.
Bermejo and A. Sousa, Acta Crystallogr., Sect. C, 1994, 50, 1676.
5 E. Gallo, E. Solari, N. Re, C. Floriani, A. Chiesi-Villa and C. Rizzoli,
Angew. Chem., Int. Ed. Engl., 1996, 35, 1981.
2
by O to produce F. These processes are very fast, since final
II
products F and Mn appear from the start of the reaction with no
organic radical being detected in the timescale of EPR
measurement. No detection of radical D under vacuum may be
due to a redox reaction between A and D, which results in the
1
1
II
formation of the corresponding Mn –Schiff base complex and
F. This is partly supported by the fact that the Schiff base ligand
4
is detected under argon but not under air. Evidence for base-
Received in Cambridge, UK, 30th March 1998; 8/02405G
1392
Chem. Commun., 1998