Reductive dechlorination of DDT electrocatalyzed by synthetic cobalt porphyrins in N,N′-dimethylformamide

Article abstract of DOI:10.1142/S1088424611002957

Two cobalt porphyrins, (OEP)Co^II and (TPP)Co^II, where OEP and TPP are the dianions of octaethylporphyrin and tetraphenylporphyrin, respectively, were examined as electrocatalysts for the reductive dechlorination of DDT (1,1-bis(4-chlorophenyl)-2,2,2-trichloroethane) in N,N′- dimethylformamide (DMF) containing 0.1 M tetra-n-butylammonium perchlorate (TBAP). No reaction is observed between DDT and the porphyrin in its Co(II) oxidation state but this is not the case for the reduced Co(I) forms of the porphyrins which electrocatalyze the dechlorination of DDT, giving initially DDD (1,1-bis(4-chlorophenyl)-2,2-dichloroethane), DDE (1,1-bis(4-chlorophenyl)-2, 2-dichloroethylene) and DDMU (1,1-bis(4-chlorophenyl)-2-chloroethylene) as determined by GC-MS analysis of the reaction products. A further dechlorination product, DDOH (2,2-bis(4-chlorophenyl)ethanol), is also formed on longer timescales when using (TPP)Co as the electroreduction catalyst. The effect of porphyrin structure and reaction time on the dechlorination products was examined by GC-MS, cyclic voltammetry, controlled potential electrolysis and UV-visible spectroelectrochemistry and a mechanism for the reductive dechlorination is proposed.

Full text of DOI:10.1142/S1088424611002957

Journal of Porphyrins and Phthalocyanines
J. Porphyrins Phthalocyanines 2011; 15: 66–74
DOI: 10.1142/S1088424611002957
Published at http://www.worldscinet.com/jpp/
Reductive dechlorination of DDT electrocatalyzed by
synthetic cobalt porphyrins in N,N′-dimethylformamide
a
a
a
a
a
Weihua Zhu , Yuanyuan Fang , Wei Shen , Guifen Lu , Ying Zhang ,
a◊
b◊
Zhongping Ou* and Karl M. Kadish*
a
School of Chemistry and Chemical Engineering, Jiangsu University, Zhenjiang 212013, P. R. China
Department of Chemistry, University of Houston, Houston, TX 77204-5003, USA
b
Received 24 September 2010
Accepted 21 December 2010
II
II
ABSTRACT: Two cobalt porphyrins, (OEP)Co and (TPP)Co , where OEP and TPP are the dianions
of octaethylporphyrin and tetraphenylporphyrin, respectively, were examined as electrocatalysts
for the reductive dechlorination of DDT (1,1-bis(4-chlorophenyl)-2,2,2-trichloroethane) in N,N′-
dimethylformamide (DMF) containing 0.1 M tetra-n-butylammonium perchlorate (TBAP). No
reaction is observed between DDT and the porphyrin in its Co(II) oxidation state but this is not the
case for the reduced Co(I) forms of the porphyrins which electrocatalyze the dechlorination of DDT,
giving initially DDD (1,1-bis(4-chlorophenyl)-2,2-dichloroethane), DDE (1,1-bis(4-chlorophenyl)-2,
2
-dichloroethylene) and DDMU (1,1-bis(4-chlorophenyl)-2-chloroethylene) as determined by GC-MS
analysis of the reaction products. A further dechlorination product, DDOH (2,2-bis(4-chlorophenyl)
ethanol), is also formed on longer timescales when using (TPP)Co as the electroreduction catalyst. The
effect of porphyrin structure and reaction time on the dechlorination products was examined by GC-
MS, cyclic voltammetry, controlled potential electrolysis and UV-visible spectroelectrochemistry and a
mechanism for the reductive dechlorination is proposed.
KEYWORDS: cobalt porphyrins, electrochemistry, spectroelectrochemistry, DDT, electrocatalysis,
reductive dechlorination.
methods of dechlorination have attracted a great deal of
interest in recent years [5–15].
INTRODUCTION
DDT (1,1-bis(4-chlorophenyl)-2,2,2-trichloroethane)
was widely used for three decades as a potent pesticide
for controlling malaria, typhus and other insect borne
diseases but its use in agriculture has been banned in
many countries since the mid-1970s because its exposure
to human and wildlife can result in severe health prob-
lems [1–4]. Unfortunately, DDT is resistant to degrada-
tion in the environment and is still present in significant
quantities in soil and water at numerous locations around
the world. For this reason, research and development into
new techniques for the degradation of DDT by various
It has long been known that macrocycles containing
transition-metal ions can be used as catalysts for the
reductive dehalogenation of organohalides [16–31]. One
example is vitamin B12 [15] which has been examined
as a catalyst for the biomimetic reductive dechlorina-
tion of chlorinated pollutants such as DDT. The reduced
Co(I) form of this porphyrin-like macrocycle can act
as a supernucleophile which reacts with organohalides
to form alkylated Co(III) complexes while also accom-
plishing dehalogenation [15, 25–28]. Other examples of
cobalt macrocycles which have been studied as dehalo-
genation catalysts include phthalocyanines [19, 32] and
porphyrins [16, 33].

SPP full member in good standing
Most cobalt(III) and cobalt(II) porphyrins can be
reversibly reduced to their Co(I) forms under the applica-
tion of an applied potential [34–37] and the compound in
*
Correspondence to: Zhongping Ou, email: zpou2003@yahoo.
com, tel: +86 510-88791928, fax: +86 510-88791800 and Karl
M. Kadish, email: kkadish@uh.edu, tel: +1 (713)-743-2740
Copyright © 2011 World Scientific Publishing Company

RꢀDꢁCꢂIvꢀ DꢀCꢃꢄORIꢅꢆꢂIOꢅ Oꢇ DDꢂ ꢀꢄꢀCꢂROCꢆꢂꢆꢄꢈzꢀD ꢉꢈ SꢈꢅꢂꢃꢀꢂIC COꢉꢆꢄꢂ PORPꢃꢈRIꢅS
67
this oxidation state will react with alkyl halides (RX) to
are the dianions of octaethylporphyrin and tetraphenyl-
porphyrin, respectively (Chart 1a). Each porphyrin was
reduced to its cobalt(I) form and reacted with DDT in
solutions of N,N′-dimethylformamide (DMF) containing
0.1 M tetra-n-butylammonium perchlorate (TBAP). The
resulting dechlorination products are shown in Chart 1b
and were identified by GC-MS analysis. The effect of
porphyrin structure and reaction time on the dechlorina-
tion products were also examined by cyclic voltammetry,
controlled potential electrolysis and UV-visible spec-
troelectrochemistry and a mechanism for the reductive
dechlorination is proposed.
give the σ-bonded Co(III) porphyrin [31, 38] which can
be further reduced to its Co(II) form as shown by Equa-
tions 1–4 where Por represents the dianion of a given
porphyrin. The dechlorination step occurs in reaction 3.
iii
+
II
→
[
(Por)Co ] + e (Por)Co
(1)
(2)
(3)
(4)
←
ii
I -
→
Por)Co + e [(Por)Co ]
←
(
i -
iii
-
→
(Por)Co ] + RX (Por)Co (R) + X
←
→
Por)Co (R) + e [(Por)Co (R)]
←
[
iii
II
-
(
If the resulting Co(III) or Co(II) σ-bonded porphyrins
formed in Equations 3 and 4 are unstable, a cleavage of
the cobalt-carbon bond will occur to generate the mono-
dechlorinated axial ligand in its anionic or radical forms
ExpERImENTAl
-
•
III +
II
(
R or R ) and [(Por)Co ] or (Por)Co , the latter of which
is immediately reduced to its Co(I) form (Equation 2)
under the application of an applied potential sufficient to
reduce the homogeneously generated (Por)Co(R) (Equa-
Cheꢀicaꢁs
The cobalt porphyrins (OEP)Co and (TPP)Co and
the chlorinated hydrocarbons, 1,1-bis(4-chlorophenyl)-
I -
tion 4). The newly formed [(Por)Co ] can then further
2
2
,2,2-trichloroethane (DDT), 1,1-bis(4-chlorophenyl)-2,
-dichloroethane (DDD), 1,1-bis(4-chloro-phenyl)-2,2-
react with the original organohalide compound in solution
or it can react with its dechlorinated product if it also con-
tains a halogen group. This suggests that simple synthetic
cobalt(II) porphyrins might be used as efficient catalysts
for the electroreductive multiple dechlorination of DDT.
dichloroethylene (DDE) and 1,1-bis(4-chlorophenyl)-2-
chloroethylene (DDMU) were purchased from Sigma-
Aldrich and used as received. N,N-dimethylformamide
II
(DMF) was purchased from Sigma-Aldrich and used as
This is examined in the present paper using (OEP)Co
II
received or from Shanghai Guoyao Co. and freshly dis-
tilled before use. Tetra-n-butylammonium perchlorate
and (TPP)Co as electrocatalysts, where OEP and TPP
(TBAP) was purchased from Fluka Chemical Company
(a) Cobalt porphyrins
and used without further purification. Other solvents
were of analytical grade and obtained from Shanghai
Guoyao Co.
N
N
N
N
N
Co
N
Co N
Instruꢀentation
N
Cyclic voltammetry was performed in a three-electrode
cell using a BiStat or Chi-730C electrochemistry sta-
tion. A glassy carbon disk electrode was utilized as the
working electrode while a platinum wire and a saturated
calomel electrode (SCE) were employed as the counter
and reference electrodes, respectively. An “H” type cell
with a fritted glass layer to separate the cathodic and
anodic sections of the cell was used for bulk electroly-
sis. The working and counter electrodes were made from
platinum mesh and the reference electrode was an SCE.
Both the working and reference electrodes were placed
in one compartment while the counter electrode was in
other compartment of the cell.
Before GC-MS measurements, a pre-treatment proce-
dure for removing TBAP from the electrolysis solution
was utilized. The solution was transferred into a 150 mL
flask and evaporated to remove the DMF solvent under
vacuum at 45 °C in a roto-evaporator. Cyclohexane was
then added in the flask containing the residue to extract
DDT and its possible dechlorinated products. After stir-
ring and centrifuging, the TBAP supporting electro-
lyte was removed and the solution was subjected to the
GC-MS analysis.
(
OEP)Co
(TPP)Co
(
b) DDT and its dechlorinated products
DDT
Cl
CCl3
C
H
Cl
DDD
HCCl2
DDE
Cl
CCl2
C
C
H
Cl
Cl
Cl
Cl
DDMU
HCCl
C
DDOH
CH2OH
C
H
Cl
Cl
Cl
Chart 1. Structures of (a) cobalt porphyrins and (b) DDT along
with its dechlorinated products
Copyright © 2011 World Scientific Publishing Company
J. Porphyrins Phthalocyanines 2011; 15: 67–74

68
W. zꢃꢁ ꢀꢁ ꢂꢃ.
GC-MS system Model HP6890-GC and HP5975-
MSD was utilized to separate and identify the products of
catalytic reductive dechlorination. The GC was equipped
with an HP-5 5% phenyl methyl siloxane column (length
3
0 m, ID 250 μm, film 0.25 μm). Temperature pro-
gramming was applied to the GC analysis. The initial
temperature was 100 °C and increased at a rate of 15 °C/
min to 300 °C. An area-normalized method was applied
for analysis of the peaks. The molar response factor of
DDT was defined as 1.00 and the relative values for
DDE, DDD and DDMU were measured as 1.076 ± 0.039,
0
.934 ± 0.053, 1.003 ± 0.031, respectively.
UV-visible spectroelectrochemical measurements
were performed with a home-made optically transparent
thin-layer cell with Pt mesh as the working electrode. The
potential was applied using a BiStat or Chi-730C poten-
tiostat. Time-resolved UV-visible spectra were recorded
with a HP 8453A diode array spectrophotometer. All
electrochemical and spectroelectrochemical measure-
ments were carried out under a nitrogen atmosphere.
RESUlTS AND DISCUSSION
II
Cycꢁic voꢁtaꢀꢀetry of (por)Co in the absence and
ꢂresence of DDT
II
The electrochemical properties of (OEP)Co and
II
(
TPP)Co were previously characterized in DMF con-
taining 0.1 M TBAP [39, 40]. Both porphyrins undergo
a reversible one-electron reduction under these experi-
I -
mental conditions to give [(Por)Co ] at E = -0.78 V
1
/2
(
Por = TPP) or -0.99 V (Por = OEP) (see Equation 2).
A one-electron reduction is also seen in other non-aque-
ous solvents and this process may be followed by elec-
trogeneration of a cobalt(I) porphyrin π-anion radical at
potentials close to the negative limit of the non-aqueous
solvent [36, 41–49].
-3
Fig. 1. Cyclic voltammograms of (OEP)Co (1.15 × 10 M) with
added 0.0–1.0 eq DDT and DDT (0.02 M) in the absence of
catalyst in DMF containing 0.1 M TBAP. Scan rate = 0.10 V/s.
The peak with as asterisk is associated with a product of the
initial reduction
II
I
The Co /Co reaction of (OEP)Co and (TPP)Co
becomes irreversible in DMF when DDT is added to
the solution as shown by the cyclic voltammograms in
Figs 1 and 2. At the same time, the reduction peak cur-
σ-bonded Co(III) porphyrins which are generated
from the reaction of [(Por)Co ] and DDT. For example,
(TPP)Co (R) where R = CH , C H or CH Cl is reduced
at E ≈ -1.40 V in CH Cl [46] and this potential is about
I -
II
I
III
rent for the Co /Co process increases with increasing
concentration of DDT while the reverse anodic peak for
3
2
5
2
1
/2
2
2
I -
II
re-oxidation of [(Por)Co ] disappears. This is consis-
600 mV more negative than for reduction of (TPP)Co
tent with the occurrence of a catalytic process involving
DDT and the electrogenerated Co(I) porphyrin to give
a σ-bonded Co(III) derivatives as shown in Equation 3.
Similar oxidative addition reactions have been shown to
occur between simple alkyl or aryl halides and the elec-
trogenerated Co(I) form of different cobalt macrocycles
(E1/2 = -0.78 V) under the same solution conditions.
II
The increase in reduction peak current for the Co /
I
Co process of (Por)Co as a function of increasing DDT
concentration is shown in Fig. 3, where i and i are the
p
p0
measured cathodic peak currents obtained from cyclic
voltammograms of the porphyrin in the presence and
absence of DDT, respectively. The measured i /i ratio is
[
15, 25–30, 38, 46, 50].
p
p0
Additional irreversible reduction peaks are observed
larger for (OEP)Co than for (TPP)Co in DMF solutions
containing a given DDT concentration and suggests that
the rate to regenerate the corresponding catalyst is larger
for [(OEP)Co ] than [(TPP)Co ] . This observation is not
consistent with the thermodynamic ease for generating
II
I
II
following the Co /Co reduction of (Por)Co in the DMF
solutions containing DDT. These are indicated by aster-
isks in the cyclic voltammograms of Figs 1 and 2 and
are assigned to irreversible reductions of one or more
I -
I -
Copyright © 2011 World Scientific Publishing Company
J. Porphyrins Phthalocyanines 2011; 15: 68–74

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69
the Co(I) species since the first reduction of (OEP)Co
is located at E = -0.99 V while (TPP)Co is reduced to
1/2
I -
[(TPP)Co ] at -0.78 V, implying that factors other than
thermodynamic potentials dominate the catalytic process.
Controꢁꢁed-ꢂotentiaꢁ eꢁectroꢁysis and GC-mS
anaꢁysis
In order to determine the products of DDT dechlori-
nation, controlled-potential bulk electrolysis was carried
-
4
II
out in DMF solutions containing 2.0 × 10 M (Por)Co ,
0
-
3
.1 M TBAP and 15 eq DDT (3.0 × 10 M). The bulk
electrolysis potential was set at -1.70 V which is suffi-
I -
cient to generate the singly reduced species [(Por)Co ]
and also to reduce the expected homogeneously generated
σ-bonded Co(III) porphyrin products.
Because the TBAP supporting electrolyte and its
decomposition components formed during the high tem-
perature GC process will block the column and prevent
determination of the DDT dechlorination products, it was
necessary to first remove all TBAP from solution before it
could be examined by GC analysis. The procedure adopted
for solution “pre-treatment” in this present work is shown
in Scheme 1. After each hour of electrolysis at a given
reduction potential, 1.0 mL solution was removed from
the H-cell and pretreated for GC-MS analysis. Most of the
TBAP in solution was successfully removed after which the
product distribution of DDT and its dechlorinated products
was determined by GC-MS. Well-defined GC data from
these solutions were obtained as shown by the examples in
Figs 4 (OEP) and 5 (TPP). The distribution of DDT and its
dechlorinated products, DDD, DDE, DDMU and DDOH
were calculated from the chromatographic data and are
summarized in Table 1. The distribution as a function of
time is also graphically illustrated in Fig. 6.
As seen from Table 1 and Fig. 6, 15 eq DDT are com-
pletely removed from the solution within 6–8 h after start-
ing bulk electrolysis at an applied potential of -1.70 V.
Three main DDT dechlorinated products (DDD, DDE
and DDMU) are detected during the first 6 h of electroly-
sis, when using (OEP)Co as the catalyst and an additional
dechlorinated product, DDOH, is also seen in the case
of (TPP)Co. These four dechlorinated products were the
only ones observed after 4 h or longer of electrolysis time
Fig. 2. Cyclic voltammograms for the first reduction of (TPP)
-
4
Co (7.49 × 10 M) with added 0.0–2.0 eq DDT in DMF con-
taining 0.1 M TBAP. Scan rate = 0.10 V/s. The peak with an
asterisk is associated with a product of the initial reduction
4.5
(
OEP)Co
3.5
(
see Fig. 6b).
2.5
(
TPP)Co
The DDT distribution decreased as expected with
increased reaction time and no DDT remained in solu-
tion after 6–8 h of bulk-electrolysis. The measured dis-
tribution of DDD was 11.1% after one hour in solutions
of (OEP)Co and 8.7% in solutions of (TPP)Co. Both
solutions showed a maximum amount of DDD formation
1
.5
0
.5
(~30%) after 4 h and then this value leveled off as seen in
0.0
0.2
0.4
0.6
0.8
1.0
[
DDT]/[(Por)Co]
Table 1 and Fig. 6.
The DDE distribution was 6.1% after one hour of
electrolysis with the (OEP)Co catalyst, 13% with the
(TPP)Co catalyst and in both cases reached a maximum
of about 30% before decreasing at longer times. The
Fig. 3. Plot of ip/ip0 vs the ratio of [DDT]/[(Por)Co], where i and
p
ip0 are the measured cathodic peak currents obtained from cyclic
voltammograms for the first reduction of (Por)Co in the pres-
ence and absence of DDT in DMF containing 0.1 M TBAP
Copyright © 2011 World Scientific Publishing Company
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70
W. zꢃꢁ ꢀꢁ ꢂꢃ.
Similar changes in the DDD and DDE dis-
tribution were obtained for the two porphyrins
as a function of time but different distribution
are seen for DDMU and DDOH, the latter of
which is only seen for the (TPP)Co catalyst
and is accompanied by a decrease in the dis-
tribution of DDMU after 8 h of electrolysis.
It summary, three main dechlorinated
products are detected when DDT reacts with
singly reduced (OEP)Co but four products
are seen when using (TPP)Co as the catalyst.
Furthermore, the type, number and the distri-
bution of the dechlorinated products depends
not only on the catalyst used but also varies
with the reaction time.
Solution of
(
Por)Co, DDT, TBAP, DMF
Electrolysis
(Controlled-potential)
Solution
after electrolysis)
(
Rotating evaporation
o
(
45 C under vacuum)
Solvent
DMF)
Residue
(
(
(
i) Cyclohexane extraction
ii) Centrifuging
Sꢂectroeꢁectrocheꢀicaꢁ ꢀonitoring of the
reduction ꢂroducts and ꢂroꢂosed ꢀecha-
nisꢀ for dechꢁorination of DDT
Residue
Porphyrins,TBAP)
Solution for GC-MS analysis
(DDT and its dechlorinated products)
I -
(
The fate of [(Por)Co ] electrogenerated
from (Por)Co in the presence of DDT was
spectroelectrochemically monitored to pro-
vide a better understanding of the DDT reduc-
Scheꢀe 1. The procedure for reductive dechlorination of DDT catalyzed by
cobalt(II) porphyrins
tive dechlorination mechanism. The UV-visible spectra
n+
of [(Por)Co] where n = 3, 2 or 1 are known for Por =
OEP and TPP as are the spectra for (TPP)Co(R) where
R is a simple alkyl or aryl group. Thus the progress of
the catalytic reaction could be monitored by measuring
Fig. 4. GC analysis of the DDT and its dechlorinated products
during the controlled-potential electrolysis at -1.70 V using
(
OEP)Co as the catalyst
same time, the DDMU distribution increased continu-
ously over the 8 h of electrolysis and ranged from 4.1%
after one hour in the case of (OEP)Co to 44.7% after
6
h. This distribution data implies that DDE is initially
Fig. 5. GC analysis of the DDT and its dechlorinated products
during the controlled-potential electrolysis at -1.70 V using
(TPP)Co as the catalyst
formed after which DDMU is generated at longer times
of electrolysis.
Copyright © 2011 World Scientific Publishing Company
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71
Tabꢁe 1. Analysis data of DDT and its dechlorinated products obtained at different reaction time during the
controlled-potential electrolysis at -1.70 V
a
Compound
Rxn time, h
Distribution, %
DDT
DDD
DDE
DDMU
DDOH
(
OEP)Co
0.0
100.0
78.8
58.1
30.1
15.5
3.9
0.0
11.1
19.7
27.5
30.2
29.9
30.2
0.0
6.1
0.0
4.1
0.0
0.0
0.0
0.0
0.0
0.0
0.0
1
2
3
4
5
6
.0
.0
.0
.0
.0
.0
12.3
25.6
28.7
30.6
25.1
9.9
16.8
25.6
35.6
44.7
0.0
(
TPP)Co
0.0
100.0
72.0
55.4
32.2
15.2
4.1
0.0
8.7
0.0
13.0
16.9
23.2
27.8
29.6
13.4
0.0
6.2
0.0
0.0
1
2
3
4
5
8
.0
.0
.0
.0
.0
.0
16.8
24.4
27.7
29.0
30.6
10.9
20.2
26.3
33.5
28.9
0.0
0.0
3.0
3.7
0.0
27.2
a
Distribution (%) = [(moles of a specific compound)/(moles of all compounds)] × 100%.
(
a) (OEP)CoII
(b) (TPP)CoII
100
100
DDT
DDT
80
DDD
DDE
80
DDD
DDE
DDMU
DDOH
6
4
0
0
DDMU
6
4
2
0
0
0
0
2
0
0
0
1
2
3
4
5
6
0
1
2
3
4
5
6
7
8
time (h)
time (h)
Fig. 6. Distribution of DDT and its dechlorinated products obtained in DMF at different electrolysis time during controlled-potential
reduction of (a) (OEP)Co and (b) (TPP)Co at -1.70 V
the UV-visible spectra as a function of time during the
electroreduction.
the literature for reduction of related cobalt(II) porphy-
rins in other non-aqueous solvents [44, 47].
Neutral (OEP)Co in DMF has a Soret band at 393 nm
and two visible bands at 525 and 547 nm while (TPP)Co
is characterized by a Soret band at 414 nm and a broad
visible band at 520 nm. The two cobalt(I) porphyrins
both display a split Soret band and these spectra are
No spectral changes are seen when DDT is added to
II
II
solutions containing unreduced (OEP)Co or (TPP)Co ,
indicating the lack of a reaction with the neutral cobalt
porphyrins. However, the spectra of the electroreduced
compounds change significantly in solutions containing
DDT. As seen from Fig. 7, the Soret band of (OEP)Co
(393nm)and(TPP)Co(414nm) bothdecreasein intensity
during controlled potential reduction, but the split Soret
band characteristic of Co(I) formation is not observed in
II
I
observed during the Co /Co process in the absence of
DDT as shown in Fig. 7 where λmax = 354 and 414 nm
I -
I -
for [(OEP)Co ] and at 365 and 432 nm for [(TPP)Co ] .
Similar spectral transformations have been reported in
Copyright © 2011 World Scientific Publishing Company
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72
W. zꢃꢁ ꢀꢁ ꢂꢃ.
II
II
Fig. 7. Thin-layer UV-visible spectral changes obtained during the first reduction of (a) (OEP)Co and (b) (TPP)Co without or with
DDT in DMF containing 0.1 M TBAP
+
e
e
CoIII
CoII
CoI
(
(
1)
2)
Co = (Por)Co
Cl
H
Cl
Cl
Cl
Cl
Cl
CoI
CoIII
C
H
+
C
C
C
Cl
Cl-
DDT
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
CoIII
C
H
C
C
H
C
C
C
Cl
(
3)
Cl
H+
Cl
(
Por)CoII
Cl
Cl
DDE
III +
[
(Por)Co
]
+
e
Cl
Cl
Cl
Cl
Cl
Cl
C
CoII
C
H
C
H
H
C
H
C
Cl
C
Cl
Cl
H+
(
Por)CoII
Cl
Cl
Cl
DDD
Scheꢀe 2. Proposed mechanism of DDT dechlorination catalyzed by cobalt porphyrins
Copyright © 2011 World Scientific Publishing Company
J. Porphyrins Phthalocyanines 2011; 15: 72–74

RꢀDꢁCꢂIvꢀ DꢀCꢃꢄORIꢅꢆꢂIOꢅ Oꢇ DDꢂ ꢀꢄꢀCꢂROCꢆꢂꢆꢄꢈzꢀD ꢉꢈ SꢈꢅꢂꢃꢀꢂIC COꢉꢆꢄꢂ PORPꢃꢈRIꢅS
73
the solutions containing DDT. Instead the final spectrum
6. Gautam SK and Sumathi S. Environ. Technol. 2006;
27: 387–394.
7. Wang YS, Chiu TC and Yen J. Organohalogen
Compd. 2004; 66: 2232–2236.
8. Huang HJ, Liu SM and Kuo CE. J. Environ. Sci.
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obtained from the reduction of (OEP)Co or (TPP)Co
displays a Soret band at 390 or 410 nm, respectively,
and this spectrum is assigned to the transient formation
III
of a (Por)Co (R) species. A similar spectrum has been
III
reported for (TPP)Co (C H ) which has a Soret band at
2
5
4
07 nm [46].
In summary, the electrochemistry, spectroelectro-
9. Kitamura S, Yoshida M, Sugihara K and Ohta S.
J. Health Sci. 1999; 45: 217–221.
chemistry and GC analysis of the dechlorinated products
suggest the proposed catalytic cycle for reductive dechlo-
rination of DDT which is illustrated in Scheme 2. The
10. Sayles GD, You G, Wang MX and Kupferle MJ.
Environ. Sci. Technol. 1997; 31: 3448–3454.
11. Purnomo AS, Koyama F, Mori T and Kondo R.
Chemosphere 2010; 80: 619–624.
12. Gfrerer M and Lankmayr E. J. Chromatogr. A 2005;
1072: 117–125.
II
II
neutral (OEP)Co and (TPP)Co initially undergo a one-
electron reduction to yield the corresponding cobalt(I)
I -
complex, [(Por)Co ] which then reacts with DDT to
generate the organometallic intermediate with a Co-C
13. Gach PC, Mubarak MS, Karty JA and Peters DG.
J. Electrochem. Soc. 2007; 154: F1–F6.
14. Mubarak MS, Gach PC and Peters DG. Electro-
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15. Hisaeda Y and Shimakoshi H. In Handbook of Por-
phyrin Science, Vol. 10, Kadish KM, Smith KM
and Guilard R. (Eds.) World Scientific: New Jersey,
2010; Chapter 48, pp 313–370.
III
σ bond, (Por)Co (R). This intermediate can undergo a
photoinduced cleavage of the Co-C bond leading to DDD
II
III
or DDE and the unreduced Co or Co porphyrins which
would then be immediately reduced under the application
of an applied reducing potential. The σ-bonded Co(III)
porphyrin can also be reduced by one-electron to form a
II
-
[
(Por)Co (R)] species at the applied potential of -1.70 V
after which the electrogenerated σ-bonded Co(II) com-
pound will undergo a cleavage of the Co-C bond leading
to formation of DDD as shown in the last reaction step
of Scheme 2.
16. Alonso F, Beletskaya IP and Yus M. Chem. Rev.
2002; 102: 4009–4091.
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1
1
2
2
8. Davies TJ, Garner AC, Davies SG and Compton
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CONClUSION
1
995; 117: 1127–1134.
Cyclic voltammetry, controlled potential bulk elec-
trolysis and thin-layer UV-visible spectroelectrochemis-
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1. Maldotti A, Amadelli R, Bartocci C, Carassiti V,
Polo E and Varani G. Coord. Chem. Rev. 1993; 125:
II
II
try of (OEP)Co and (TPP)Co in the presence of DDT
show that the two singly reduced porphyrins, represented
I -
as [(Por)Co ] , will catalyze the dechlorination of DDT
1
43–154.
to generate DDD, DDE, DDMU as well as some less
chlorinated products on longer electrolysis timescales.
The Co(I) forms of the reduced porphyrins will also react
with DDT leading to the formation of intermediates hav-
ing a Co-C σ bond, but a cleavage of the bond will occur
with formation of the DDT dechlorination products.
2
2
2
2
2
2
2
2
2. Krone UE, Thauer RK, Hogenkamp HPC and Stein-
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1
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8. Pratt DA and Van der Donk WA. Chem. Commun.
Acknowꢁedgeꢀents
Support from the Robert A. Welch Foundation (KMK,
Grant E-680) and the Natural Science Foundation
(
BK2008226) of Jiangsu Province, P. R. China is grate-
fully acknowledged.
2
006; 558–560.
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