Photooxidation of phenol derivatives using μ-(dihydroxo)dipalladium(II) bisporphyrin complex

Article abstract of DOI:10.1142/S108842461000174X

μ-(dihydroxo)dipalladium(II) complex with N²¹, N ²²-etheno bridged tetraphenylporphyrin ligand was employed in the catalytic photooxidation of phenol derivative in aerated homogeneous solution with visible light irradiation. The Pd complex promoted the degradation of p-tert-butylphenol as well as Cu phthalocyanine under basic conditions and it worked as a photosensitizer even in neutral conditions in contrast with Cu phthalocyanine. Some phenol derivatives afforded corresponding quinones selectively in this photooxidation. The present Pd complex coordinated by the bidentate porphyrin ligand showed higher photosensitizing ability than tetraphenylporphyrin free-base and ordinary tetradentate tetraphenylporphyrin palladium(II) complex for photooxidation of 2,6-di-tert-butylphenol. This bidentate Pd complex showed higher light durability even under the conditions where photodegradation of porphyrin free-base and tetradentate Pd porphyrin were observed by extensive irradiation.

Full text of DOI:10.1142/S108842461000174X

Journal of Porphyrins and Phthalocyanines
J. Porphyrins Phthalocyanines 2010; 14: 64–68
DOI: 10.1142/S108842461000174X
Published at http://www.worldscinet.com/jpp/
Photooxidation of phenol derivatives using
µ-(dihydroxo)dipalladium(II) bisporphyrin complex
a◊
a
a
a
Yuko Takao* , Toshinobu Ohno , Kazuyuki Moriwaki , Fukashi Matsumoto and
b◊
Jun-ichiro Setsune*
a
Osaka Municipal Technical Research Institute, Joto-ku, Osaka 536-8553, Japan
Department of Chemistry, Graduate School of Science, Kobe University, Nada-ku, Kobe 657-8501, Japan
b
Received 24 October 2008
Accepted 10 February 2009
2
1
22
ABSTRACT: µ-(dihydroxo)dipalladium(II) complex with N ,N -etheno bridged tetraphenylporphyrin
ligand was employed in the catalytic photooxidation of phenol derivative in aerated homogeneous
solution with visible light irradiation. The Pd complex promoted the degradation of p-tert-butylphenol
as well as Cu phthalocyanine under basic conditions and it worked as a photosensitizer even in neutral
conditions in contrast with Cu phthalocyanine. Some phenol derivatives afforded corresponding quinones
selectively in this photooxidation. The present Pd complex coordinated by the bidentate porphyrin ligand
showed higher photosensitizing ability than tetraphenylporphyrin free-base and ordinary tetradentate
tetraphenylporphyrin palladium(II) complex for photooxidation of 2,6-di-tert-butylphenol. This
bidentate Pd complex showed higher light durability even under the conditions where photodegradation
of porphyrin free-base and tetradentate Pd porphyrin were observed by extensive irradiation.
KEYWORDS: bridged porphyrin, µ-(dihydroxo)dipalladium complex, photooxidation, phenol
derivative.
as a bidentate ligand, we have been investigating the
INTRODUCTION
coordination chemistry and organometallic chemistry of
palladium porphyrin complexes [4], and showed that the
Palladium complexes with weakly coordinating
anions have been known to activate substrates in organic
transformations [1]. As an example, µ-(dihydroxo) and
µ-(dialkoxo)dipalladium(II) complexes have been draw-
ing much interest and their usage as catalyst for substitu-
tion, carbonylation and Mannich-type reactions is known
21
22
N ,N -bridged porphyrin ligand strongly coordinated to
Pd and influenced the dynamic behavior of the coexisting
21
22
ligand [5, 6]. It is of interest to make use of N ,N -bridged
porphyrins considering a role of palladium catalysts with
bidentate ligands in modern organic synthesis, though
there is concern about palladium porphyrins with regard
to their photophysical and electrochemical properties [7].
In this paper we report the application of the
µ-dihydroxo complex of dimeric palladium(II) porphy-
rin for photooxidation of phenol derivatives. Though
the oxidation of several substrates with porphyrin ana-
logs as photosensitizers have been developed [8, 9],
the oxidation of phenol derivatives is of special inter-
est because the oxidized products of phenol derivatives
such as quinone derivatives are known to be important
chemicals in the pharmaceutical and dye industries [10],
and because some phenol derivatives such as chlorinated or
p-alkylated phenols are known to be pollutants that accu-
mulate in the environment and thus sustainable methods
[
2]. We have reported the synthesis and the reactions of
unique µ-(dihydroxo) dipalladium(II) bisporphyrin (2)
that was obtained by the reaction of dicationic bis(aqua)
2
1
22
Pd(II) complexes of N ,N -etheno bridged porphyrin
1) with NaOH or triethyamine (Scheme 1) [3]. Since
(
palladium(II)porphyrinisacoordinativelysaturatedmetal
complex having a tetradentate ligand, it is not useful for
metal-catalyzed reactions that require coordination sites
2
1
22
for substrates. By using N ,N -etheno bridged porphyrin
š
SPP full member in good standing
*
Correspondence to: Yuko Takao, email: yuktakao@omtri.
or.jp, fax: +81 06-6963-8049 and Jun-ichiro Setsune, email:
setsunej@kobe-u.ac.jp, fax: +81 078-803-5770
Copyright © 2010 World Scientific Publishing Company

PHOTOOXIDATION OF PHENOL DERIVATIVES USING µ-(DIHYDROXO)DIPALLADIUM(II) BISPORPHYRIN COMPLEX
65
bridged TPP (1) was also prepared accor-
ding to a previously reported procedure [4].
Tetradentate (tetraphenylporphyrinato)Pd(II)
[
(TPP)Pd] was prepared as follows: A solution
of tetraphenylporphyrin [TPPH ] (0.1 mmol)
2
in CHCl (14 mL) was added to a solution
3
of Pd(OAc) (0.2 mmol) in propionic acid
2
(
10 mL). The mixture was refluxed for 6 h,
then evaporated, followed by column chroma-
tography on silica gel with dichloromethane.
The product was recrystallized from dichlo-
romethane and n-hexane to afford (TPP)Pd as
a red solid (yield: 71%). Solvents were puri-
fied prior to use by conventional methods.
Cupper(II) phthalocyanine-3,4′,4″,4′″-tetra-
sulfonic acid, tetrasodium salt [(Pc)Cu] and
Scheme 1. Preparation of µ-(dihydroxo)dipalladium(II) complex with
2
1
22
N ,N -(diphenyletheno) bridged porphyrin dimer
other chemicals were of reagent grade. CDCl was
3
for their decomposition are needed [11–13]. So oxi-
dation of several phenol derivatives has been studied
using porphyrins or phthalocyanines in the presence
of various oxidants, for example, oxygen [8b, 14],
hydrogen peroxide [11, 12, 15] or potassium mono-
peroxysulfate [11, 15]. Furthermore, photooxidations
using a singlet oxygen photosensitizer are regarded
as a friendly process for environment by considering
minimum usage of toxic chemicals, use of oxygen from
air and visible light which has the largest energy den-
sity in solar radiation as unlimited natural resources. A
number of examples have been reported for the use of
classic dyes such as Methylene Blue and Rose Bengal
as photosensitizers in these reactions. However, there is
a problem of photodegradation of sensitizer upon exten-
sive irradiation. Thus it is important to develop efficient
and stable photosensitizers [10, 16]. With this purpose
in mind, effective photooxidations using new phthalo-
cyanine and porphyrin derivatives have been developed
passed through basic Al O before use.
2 3
Measurement
1
H NMR spectra were recorded on JEOLAL-300 spec-
trometer. Chemical shifts were referenced with respect
to (CH ) Si (0 ppm) as an internal standard. Gas
3
4
chromatography was carried out using a HiCap-CBP1
25 m × 0.53 mm × 1 µm) Shimadzu capillary column
(
on a Shimadzu-14B chromatography. GC analysis was
run at 140°C; detector temperature 250°C, injector tem-
perature 250°C. Absorption spectra were measured on
Shimadzu UV-3100B instrument.
Photoreactivity study
The solution containing a substrate and a sensitizer
was placed into a 1 cm optical cell with external water
cooling and filled with oxygen gas, and then irradiated
by a 500 W xenon lamp through cut-filters (350 nm < λ
< 800 nm). In the case of measurement by GC, 3 mL of
acetonitrile solution containing substrate (0.5 mM) and a
[
8, 13, 17].
Here, the photooxidations of some phenol derivatives
-5
using µ-dihydroxo complex of dimeric palladium(II)
porphyrin are described, and the oxidation activity and
photostability of the bidentate porphyrin complex are
compared with those of a tetradentate normal metal por-
phyrin and a free-base porphyrin.
sensitizer (5 × 10 M) was irradiated. A 0.05 mL amount
of the solution was diluted 3.5 times and passed through
a little silica gel, and then analyzed. In the case of obser-
1
vation by H NMR spectrometry, a mixture of a substrate
(3 mM) and a sensitizer (0.03–0.3 mM) in a deuterated
solvent was irradiated. A 0.5 mL amount of sample solu-
tion was analyzed and content of product was calculated
from integral values of signals.
EXPERIMENTAL
Materials and synthesis
Photostability study
2
1
22
µ-(dihydroxo)dipalladium(II) complex with N ,N -
To study the photostability of sensitizers, 0.3 mL of
-5
(
diphenyletheno) bridged TPP (tetraphenylporphyrin)
dimer (2) was prepared by the reaction of dicationic
bis(aqua)Pd(II) complexes of N ,N -etheno bridged
porphyrin with NaOH or triethyamine according to the pre-
viously reported procedures [3]. Bis(perchlorato)bis(aqua)
the CH Cl solution containing a sample (2.5 × 10 M)
2
2
was placed into a 1 mm optical cell with external cool-
ing, and irradiated by a xenon lamp. The intact optical
cell was set in the spectrophotometer, and the absorbance
at the Soret band was measured to monitor the change of
concentration of samples.
2
1
22
2
1
22
palladium(II) complex of N ,N -(diphenyletheno)
Copyright © 2010 World Scientific Publishing Company
J. Porphyrins Phthalocyanines 2010; 14: 65–68

66
Y. TAKAO ET AL.
RESULTS AND DISCUSSION
The photocatalytic activities of the compound 1, 2,
(
TPP)Pd, TPPH and copper phthalocyanine ((Pc)Cu)
2
were examined in the photooxidation of some phenol
derivatives such as 3 and 5 as shown in Scheme 2. At
first, the solution of p-tert-butylphenol (3a; 0.5 mM) in
3
:5 (v/v) water-acetonitrile binary mixture containing
photosensitizers (0.05 mM) was irradiated by a xenon
lamp (350 nm < λ < 800 nm). The concentration of 3a
was measured by GC analysis to monitor the photodegra-
dation of 3a (Fig. 1).
Addition of complex 2 without photoirradiation
resulted in little change in the concentration of 3a after
3
h (filled square, Fig. 1). But under photoirradiation the
concentration of 3a decreased rapidly, especially in the
presence of NaOH (open circle, Fig. 1). The efficiency
of complex 2 was compared with that of (Pc)Cu and
they similarly promoted photodegradation of 3a under
basic conditions (open triangle, Fig. 1). On the other
hand, it is noteworthy that complex 2 showed 94% of
photodegradation of 3a (filled circle, Fig. 1) though
Fig. 1. GC analysis of photosensitized degradation of 3a (5 ×
-4
1
0 M) in MeCN-H O with irradiation with a 500W xenon lamp
2
(
Pc)Cu showed only 20% of photodegradation (filled
-5
at 80 cm distance. [photosensitizer] = 5 × 10 M; [NaOH] =
triangle, Fig. 1) after 3 h under neutral conditions. This
result means that 2 works as a photosensitizer without
adding some bases and is suitable for water treatment;
e.g. photodegradation of phenols in normal water.
-3
5
× 10 M
In the case of photooxidation of 3a oxidized products
were of complex mixtures, while in the photooxidation of
2
,6-di-tert-butylphenol (3b) the conversion of substrate
1
to products was confirmed by monitoring with H NMR
spectroscopy. Figure 2 shows the H NMR spectrum on
1
the photooxidation process of 3b in acetonitrile after 15
min irradiation in the presence of 2. Then it was proved
that almost single product was formed selectively as an
1
Fig. 2. H NMR spectrum in CD CN of the reaction mixture
3
-3
after irradiating 3b (3 × 10 M) with a 500 W xenon lamp for
5 min in acetonitrile in the presence of 2 (0.1 molar equiv.)
1
initial product under such conditions. Although 2,6-di-
tert-butyl-1,4-benzoquinone (4b) and 3,3′,5,5′-tetra-
tert-butyl-4,4′-diphenoquinone were formed as oxidized
products in thermal oxidation of 3b with porphyrin com-
plexes [13], only one product was formed in this photo-
oxidation and identified as 4b by comparing the chemical
shifts of the product with those of the standard 2,6-di-tert-
butyl-1,4-benzoquinone. The influence of the amount of
complex 2 as a photosensitizer on the conversion of 3b
to 4b was also examined after 60 min of irradiation in
acetonitrile. When 2 was added to the solution of 3b in
the molar ratio of 0.1, 0.01 and 0.001 to 3b, the formation
of 4b was 94%, 73% and 25%, respectively.
The photooxidation efficiency of sensitizers was evalu-
ated by the measurement of formation of 4b in the photo-
oxidation of 3b. Table 1 shows the yield of 4b determined
Scheme 2. Photooxidation of some phenol derivatives
Copyright © 2010 World Scientific Publishing Company
J. Porphyrins Phthalocyanines 2010; 14: 66–68

PHOTOOXIDATION OF PHENOL DERIVATIVES USING µ-(DIHYDROXO)DIPALLADIUM(II) BISPORPHYRIN COMPLEX
67
a
Table 1. Dependence of the photooxidation efficiency on the
sensitizer and solvent
Table 2. Photooxidation of some phenol derivatives
a
Substrate
Solvent
Time, min
Product yield, %
Por.
Molar ratio
Solvent
Yield of
4b, %
b
b
3b
MeCN
MeCN
MeCN
CHCl3
DMSO
MeCN
CHCl3
DMSO
MeCN
DMSO
40
3
0
(
Por./3b)
c
3
3
3
3
3
3
3
5
5
b
b
b
b
c
40
2
2
2
0.05
0.005
0.005
0.01
0.01
0.01
0.05
0.05
CH CN
90
68
56
39
24
44
0
3
60
120
40
96
48
6
CH CN
3
CH CN(7%CHCl )
3
3
(
TPP)Pd
CH CN(7%CHCl )
3 3
40
15
13
0
TPPH2
CH CN(7%CHCl )
3 3
c
120
120
5
1
CH CN(7%CHCl )
3 3
c
c
2
CH CN(20%H O)
3 2
a
b
96
25
d
2
CH CN(33%CHCl )
0
3
3
120
a
-3
[
3b] = 3 × 10 M, irradiation with a 500-W xenon lamp at
a
-3
-4
[
Substrate] = 3 × 10 M, [2] = 1.5 × 10 M, irradiation with a
b
c
d
8
1
0 cm distance, after 40 min, NaN was added, [3b] =
× 10 M, β-carotene was added.
3
b
c
5
00 W xenon lamp at 80 cm distance, not irradiated no
-
3
sensitizer.
1
by H NMR spectrum after 40 min irradiation. The best
result was obtained (90%) under the conditions in which
those of TPPH and (TPP)Pd under the same conditions.
As shown in Fig. 3, complex 2 was the most photostable
and remained unchanged after 100 min irradiation, while
2
1
0 mol.% of Pd atom in the form of 2 was added into
acetonitrile solution. For the purpose of comparison of
the photooxidation efficiency with other related porphy-
rin derivatives, the molar ratio of the sensitizers to 3b was
decreased to 0.01 and chloroform was added to the solu-
tion (7%) by considering the solubility of other sensitiz-
TPPH was almost completely photodecomposed under
2
the same conditions. This remarkable difference in the
photostability among 2, TPPH and (TPP)Pd was not
2
reflected in the photoactivity examined in Table 1, because
the yield data were taken while photosensitizers are alive.
Thus, the photooxidation of phenol derivatives pro-
ceeded under mild conditions in neutral solution at room
temperature using complex 2. It is noteworthy that it works
without a base as a photosensitizer probably because the
OH ligand of the Pd (OH) core of 2 acted as a base, which
ers such as TPPH . The conditions resulted in decrease of
2
yield of 4b from 56% by using 2 to 39, 24, and 44% yield
by using (TPP)Pd, TPPH and 1, respectively, while 3b
2
did not react in the absence of sensitizers under otherwise
the same conditions. When aqueous NaN or β-carotene
3
2
2
was added to the acetonitrile solution as a singlet oxy-
gen scavenger, the photooxidation of 3b was inhibited.
These experiments might suggest that singlet oxygen is
involved in the present oxidation reactions.
was confirmed from previous experiments that 2 reacted
with an acidic substrate such as methyl 3-butenoate to give
(
p-methoxycarbonylallyl)Pd complexes by deprotonation
under neutral conditions [3].
The photooxidation of some other phenol derivatives
such as 1,2,4-trimethylphenol (3c) and naphthols (5) were
also carried out because the corresponding quinones 4c and
6
are regarded as intermediates for the syntheses of bio-
chemicals, physiologically active substances or starting
materials for preparing antiviral and antitumor agents [10a].
The results of photooxidation in the presence of 5 mol.% of
2
under some conditions are shown in Table 2. The oxida-
tion products, 4c, 6a and 6b, were identified on the basis of
the reported spectroscopic data [10]. These photooxidation
yields were dependent on the structures of substrates and
reaction solvents. It was found that reactions in MeCN were
much faster than those in CHCl and DMSO by monitoring
3
1
with H NMR spectroscopy, while 3b did not react in the
absence of sensitizers in both MeCN and CHCl under the
3
same irradiation conditions. 3b and 5a could be converted
to the corresponding quinones in excellent yields (96%)
though the conversion of 3c and 5b was low.
The photostability of porphyrin sensitizers was
evaluated by measuring the relative absorbance at the Soret
band with irradiation time. The decreasing of the relative
absorbance of 2 by irradiation was also compared with
Fig. 3. The change in the absorbance at the Soret bands of
-5
porphyrins (2.5 × 10 M) with time through irradiation with a
500 W xenon lamp at 80 cm distance in CH Cl
2
2
Copyright © 2010 World Scientific Publishing Company
J. Porphyrins Phthalocyanines 2010; 14: 67–68

68
Y. TAKAO ET AL.
4
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Fig. 4. UV-vis absorption spectrum of 2 in MeCN
Photooxidation of phenol derivatives was promoted by
the Pd complex of the bidentate porphyrin using visible
light energy. Moreover, complex 2 showed a slightly more
superior efficiency and much higher photodurability as a
photosensitizer than tetradentate (TPP)Pd and free-base
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2
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to 700 nm in the Q-band region. This spectral pattern is
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2
photophysical properties should be of interest in relation
to the present photooxidation reactions. We are planning
to study the photophysical properties of 2.
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