Synthesis, electrochemistry, spectroelectrochemistry and catalytic properties in DDT reductive dechlorinationin of iron(II) phthalocyanine, 2,3-and 3,4-tetrapyridinoporphyrazine complexes

Article abstract of DOI:10.1142/S1088424613500077

Iron(II) 2,3-and 3,4-tetrapyridinoporphyrazine complexes (2,3-PyD and 3,4-PyD) were synthesized and characterized as to their electrochemistry, UV-visible spectroelectrochemistry and catalytic properties towards the reductive dechlorination of 1,1-bis(4-chlorophenyl)-2,2,2-trichloroethane (p,p′-DDT) in pyridine, dimethyl sulfoxide (DMSO), N,N′- dimethylacetamide (DMA) and N,N′-dimethylformamide (DMF). These properties were compared with those of the unsubstituted iron(II) phthalocyanine ((Pc)Fe). Electrochemistry indicates that there are up to three reductions and one oxidation in the three investigated derivatives. The easiest reduction takes place for 3,4-PyD while the most difficult one occurs for (Pc)Fe in all of the solvents investigated. The first reduction is metal-centered corresponding to the formation of [P(-2)Fe(I)]^- while the second and third reductions are ring-centered leading stepwise to the generation of [P(-3)Fe(I)] ^2- · and [P(-4)Fe(I)]^3-, where P = phthalocyanine or tetrapyridinoporphyrazine rings. Aggregation exists in the solutions of all three iron complexes and its extent depends upon the nature and concentration of the iron compounds and the binding property of each solvent. The order of the extent of aggregation for the three iron derivatives is 3,4-PyD > 2,3-PyD > (Pc)Fe. Stronger binding solvents such as pyridine and DMSO do not favor the aggregation. The singly and doubly reduced species of investigated complexes, [P(-2)Fe(I)]^- and [P(-3)Fe(I)]^2- ·, are active in DDT reductive dechlorination, the latter of which has better catalytic performance. As a result, three products, 1,1-bis(4-chlorophenyl)-2,2- dichloroethane (p,p′-DDD), 1,1-bis(4-chlorophenyl)-2,2-dichloroethylene (p,p′-DDE), and 1,1-bis(4-chlorophenyl)-2-chloroethylene (p,p′-DDMU), were obtained after the dechlorination of DDT catalyzed by each iron complex. The increasing order of catalytic performance is 3,4-PyD < 2,3-PyD < (Pc)Fe in pyridine, which is superior to DMSO and DMA for the DDT dechlorination reaction. An overall electrocatalytic mechanism is proposed for DDT reductive degradation based on the electrochemical and UV-visible spectroelectrochemical results.

Full text of DOI:10.1142/S1088424613500077

Journal of Porphyrins and Phthalocyanines
J. Porphyrins Phthalocyanines 2013; 17: 317–330
DOI: 10.1142/S1088424613500077
Published at http://www.worldscinet.com/jpp/
Synthesis, electrochemistry, spectroelectrochemistry and
catalytic properties in DDT reductive dechlorinationin of
iron(II) phthalocyanine, 2,3- and 3,4-tetrapyridinoporphyrazine
complexes
◊
a
a
a
b
Jianguo Shao * , Kema Richards , Dwayne Rawlins , Baocheng Han and
a
Christopher A. Hansen*
a
Department of Chemistry, Midwestern State University, Wichita Falls, Texas 76308, USA
Department of Chemistry, University of Wisconsin-Whitewater, Whitewater, Wisconsin 53190, USA
b
Received 22 December 2012
Accepted 23 January 2013
ABSTRACT: Iron(II) 2,3- and 3,4-tetrapyridinoporphyrazine complexes (2,3-PyD and 3,4-PyD)
were synthesized and characterized as to their electrochemistry, UV-visible spectroelectrochemistry
and catalytic properties towards the reductive dechlorination of 1,1-bis(4-chlorophenyl)-2,2,2-
trichloroethane (p,p′-DDT) in pyridine, dimethyl sulfoxide (DMSO), N,N′-dimethylacetamide (DMA)
and N,N′-dimethylformamide (DMF). These properties were compared with those of the unsubstituted
iron(II) phthalocyanine ((Pc)Fe). Electrochemistry indicates that there are up to three reductions and
one oxidation in the three investigated derivatives. The easiest reduction takes place for 3,4-PyD while
the most difficult one occurs for (Pc)Fe in all of the solvents investigated. The first reduction is metal-
-
centered corresponding to the formation of [P(-2)Fe(I)] while the second and third reductions are ring-
2
-
3-
centered leading stepwise to the generation of [P(-3)Fe(I)] ∙ and [P(-4)Fe(I)] , where P = phthalocyanine
or tetrapyridinoporphyrazine rings. Aggregation exists in the solutions of all three iron complexes and
its extent depends upon the nature and concentration of the iron compounds and the binding property
of each solvent. The order of the extent of aggregation for the three iron derivatives is 3,4-PyD > 2,3-
PyD > (Pc)Fe. Stronger binding solvents such as pyridine and DMSO do not favor the aggregation.
-
2-
The singly and doubly reduced species of investigated complexes, [P(-2)Fe(I)] and [P(-3)Fe(I)] ∙,
are active in DDT reductive dechlorination, the latter of which has better catalytic performance. As a
result, three products, 1,1-bis(4-chlorophenyl)-2,2-dichloroethane (p,p′-DDD), 1,1-bis(4-chlorophenyl)-
2
,2-dichloroethylene (p,p′-DDE), and 1,1-bis(4-chlorophenyl)-2-chloroethylene (p,p′-DDMU), were
obtained after the dechlorination of DDT catalyzed by each iron complex. The increasing order of
catalytic performance is 3,4-PyD < 2,3-PyD < (Pc)Fe in pyridine, which is superior to DMSO and
DMA for the DDT dechlorination reaction. An overall electrocatalytic mechanism is proposed for DDT
reductive degradation based on the electrochemical and UV-visible spectroelectrochemical results.
KEYWORDS: electrochemistry, spectroelectrochemistry, DDT dechlorination, iron phthalocyanine,
iron 2,3-tetrapyridinoporphyrazine, iron 3,4-tetrapyridinoporphyrazine.

SPP full member in good standing
*
Correspondence to: Jianguo Shao, email: jianguo.shao@
mwsu.edu, tel: +1 (940)-397-4463, fax: +1 (940)-397-4831;
Christopher A. Hansen, email: chris.hansen@mwsu.edu, tel:
+
1 (940)-397-4285
Copyright © 2013 World Scientific Publishing Company

3
18
J. SHAO ET AL.
tetrapyridinoporphyrazines — towards the dechlorination
of DDT under electroreduction atmosphere.
INTRODUCTION
1,1-bis(4-chlorophenyl)-2,2,2-trichloroethane
(p,p′-
Iron(II) 2,3- and 3,4-tetrapyridinoporphyrazine
complexes (2,3-PyD and 3,4-PyD) were synthesized
and investigated for their electrochemistry, UV-visible
spectroelectrochemistry and electrocatalytic properties
towards DDT dechlorination in pyridine, dimethyl
sulfoxide (DMSO), N,N′-dimethylacetamide (DMA)
and N,N′-dimethylformamide (DMF). These properties
were then compared with those obtained from the
unsubstituted iron(II) phthalocyanine under the same
solution conditions. The skeletal structures of three iron
macrocyclic complexes are shown in Chart 1. Each
derivative undergoes up to three reductions and one
oxidation in the four solvents. The E1/2 values of the redox
reactions depend upon the nature of the iron compounds
and the solvents utilized. The species electrogenerated
after the first and second reductions are active towards
DDT dechlorination and are able to catalytically convert
DDT into three dechlorinated products, DDD, DDE and
DDMU, where 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. The structures of all these compounds
are shown in Chart 2.
DDT) was utilized worldwide during the 1940s as a
broad-spectrum pesticide in agriculture and its use has
been banned since the early 1970s due to its detrimental
environmental impact. DDT is known as an endocrine
disruptor and a potential carcinogen for human beings and
other animals [1–7]. It has a low biodegradation rate in
the environment and tends to accumulate in soil, water and
animal tissues [8]. Research in finding effective catalysts to
accelerate the degradation of DDT is of great importance.
Macrocyclic compounds containing various transition
metal ions such as Fe, Co and Ni have been investigated
towards DDT reductive dechlorination [9]. Hematin,
cobalamin, coenzyme F430 [9, 10], cobalt porphyrins [11],
cobalt phthalocyanines and tetrapyridinoporphyrazines
[12] were confirmed as effective catalysts for DDT
dechlorination in reductive environments. Central metal
ions in the derivatives above can be reduced chemically
or electrochemically into metal ions with lower oxidation
states. These species are strong nucleophiles and are
capable of reacting with DDT to form alkylated complexes
with dechlorination [10, 13]. Similarly, this paper
examines the catalytic properties of three iron macrocyclic
complexes — the unsubstituted phthalocyanine and two
N
N
N
N
N
N
N
N
N
N
N
N
N
N
Fe
N
N
N
Fe
N
N
N
Fe
N
N
N
N
N
N
N
N
N
N
N
N
(Pc)Fe
2,3-PyD
3,4-PyD
Chart 1.
H
C
Cl
Cl
Cl
C
Cl
CCl3
CCl2
p, p'-DDT
p, p'-DDE
H
C
Cl
Cl
Cl
C
Cl
CHCl2
CHCl
p, p'-DDD
p, p'-DDMU
Chart 2.
Copyright © 2013 World Scientific Publishing Company
J. Porphyrins Phthalocyanines 2013; 17: 318–330

SYNTHESIS, ELECTROCHEMISTRY, SPECTROELECTROCHEMISTRY AND CATALYTIC PROPERTIES IN DDT
319
Chemicals and reagents
EXPERIMENTAL
N,N′-dimethylacetamide (DMA, 99.5+%) and tetra-n-
butylammonium perchlorate (TBAP) were obtained from
Alfa Aesar and Fluka Chemical Company respectively
and used without further purification. Anhydrous
pyridine (99.8%), dimethyl sulfoxide (DMSO, 99.9%),
N,N′-dimethylformamide (DMF, 99.9%), 1,1-bis(4-
chlorophenyl)-2,2,2-trichloroethane (p,p′-DDT, 98%),
Instrumentation
Cyclic voltammetry was carried out at room
temperature by using a Pine Instrument Company
AFCBP1 potentiostat. A three-electrode system was
used and consisted of a platinum or glassy carbon disk
working electrode, a platinum wire counter electrode,
and a saturated calomel reference electrode (SCE). The
SCE was separated from the bulk of the solution by a
fritted-glass bridge of low porosity that contained the
solvent/supporting electrolyte mixture. All potentials
are referenced to the SCE. Sonication was utilized
to obtain a suitable solution for the electrochemical
measurement.
UV-visible spectroelectrochemical measurements
were performed with an in-house developed optically
transparent thin-layer cell with Pt mesh as the working
electrode [14]. The potential was applied using a Pine
Instrument Company AFCBP1 potentiostat. The time-
resolved UV-visible spectra were recorded with a HP
1
9
,1-bis(4-chlorophenyl)-2,2-dichloroethane (p,p′-DDD,
7%), 1,1-bis(4-chlorophenyl)-2,2-dichloroethylene
(p,p′-DDE, 99%) and 1,1-bis(4-chlorophenyl)-2-
chloroethylene (p,p′-DDMU, 99%) were purchased from
Sigma-Aldrich and used as received (see Chart 2).
The unsubstituted iron phthalocyanine ((Pc)Fe, >
9
7%),wasobtainedfromSigma-Aldrichandusedwithout
further purification. Two iron tetrapyridinoporphy-
razine derivatives (2,3-PyD and 3,4-PyD) were
synthesized based on a common method utilized
for the synthesis of metal phthalocyanines and/or
porphyrazines described in the literature [15–18].
The electronic absorption spectra of the three iron
compounds in pyridine were illustrated in Fig. 1 and
8
453 diode array spectrophotometer.
1
the UV-visible, infrared and H NMR spectral data
The electrocatalytic dechlorination of DDT was
summarized in Table 1.
investigated using controlled potential bulk electrolysis.
A “H” type cell with a fritted glass layer to separate
the cathode and anode portions was used for bulk
electrolysis. The working and counter electrodes were
made from platinum mesh, and the reference electrode
was an SCE. Both the working electrode (area ≈ 2.5
In the cases of 3,4-PyD and (Pc)Fe, the strongest
Q-band are the same in wavelength, both of which are
located at 654 nm (shown in Fig. 1). However, a 23 nm
blue-shift in the corresponding Q-band was observed
for 2,3-PyD. The positions of the Soret band (at ~340
nm) in three complexes do not differ significantly under
the same solution conditions (Fig. 1 and Table 1) and
the wavelengths of this band are located in a narrow
range between 342 and 345 nm. The spectral results
shown in Fig. 1 resemble those of other compounds with
similar macrocycles containing different metal ions,
where the Q-band wavelengths in the 3,4-isomers and
phthalocyanines are very close to each other, both of
which are red-shifted by about 20–50 nm as compared to
the 2,3-isomers [12, 19, 20].
2
cm ) and the reference electrode were placed in one
compartment while the counter electrode was in the
other compartment. The concentration of each iron
-
4
complex in the bulk electrolysis was 2.0 × 10 M. The
potential used for the bulk electrolysis in the catalytic
reduction was -1.20 and -1.60 V for (Pc)Fe while the
potential applied for both 2,3-PyD and 3,4-PyD was
-
1.40 V.
A GC-MS system (HP6890-GC and HP5973-
MSD) was utilized to monitor DDT catalytic reductive
dechlorination reactions. The GC was equipped with
a Phenomen 79973 ZB-5 column (length 30 m; ID
2
50 mm, film 0.25 mm). The area-normalized method
was applied for the analysis of GC peaks. The relative
molar response factor of DDT was defined as 1 while
the values for DDE, DDD and DDMU were measured
as 0.827 (± 0.011), 0.948 (± 0.010) and 0.979 (± 0.012),
respectively.
The conversion in DDT and the yield of each
dechlorination product are defined below:
DDT conversion (%) = (mole of DDT converted) ÷ (mole
of DDT in total) × 100
-5
Yield of a product (%) = (mole of a specific product) ÷
(mole of DDT in total) × 100.
Fig. 1. UV-visible spectra of 10 M (Pc)Fe, 2,3-PyD and 3,4-
PyD in pyridine
Copyright © 2013 World Scientific Publishing Company
J. Porphyrins Phthalocyanines 2013; 17: 319–330

3
20
J. SHAO ET AL.
1
Table 1. UV-visible, infrared and H NMR spectral data of (Pc)Fe, 2,3-PyD and 3,4-PyD
a
-1 b
c
Cpd
lmax, nm
nmax, cm
d, ppm
2
,3-PyD
345, 422, 576
1670, 1581, 1515, 1394, 1252, 1194, 1132, 1102,
922, 809, 791, 759, 746, 653
9.72 (m, 4H), 9.45 (m, 4H)
7.83 (m, 4H)
6
06, 631
345, 414, 595
20, 654
342, 413, 594,
30, 654
3,4-PyD
1670, 1605, 1584, 1514, 1405, 1248, 1163, 1108,
1024, 914, 836, 755, 686, 652, 584
10.89 (m, 4H), 9.38 (m, 4H)
9.31 (m, 4H)
6
(
Pc)Fe
1727, 1653, 1609, 1559, 1491, 1472, 1378, 1331,
1304, 1182, 1118, 1080, 892, 778, 751, 728
9.60 (m, 4H), 8.01 (m, 4H),
7.89 (m, 4H), 7.61 (m, 4H)
6
a
b
c
Pyridine as solvent, molar absorptivity not measured due to aggregation of the compounds. KBr pelleting. d-pyridine as solvent.
V for 2,3-PyD and at -0.76, -1.11 and -1.64 V for 3,4-
RESULTS AND DISCUSSION
PyD. Because of the limited redox potential window in
pyridine, the first oxidation in 2,3- and 3,4-isomers and
the third reduction in (Pc)Fe were not observed in the
cyclic voltammograms (Fig. 2).
Electrochemistry and spectroelectrochemistry of (Pc)
Fe, 2,3-PyD and 3,4-PyD in pyridine
Half-wave potentials of the first two reductions in
,3-PyD exhibit a positive shift compared with those of
Pc)Fe in pyridine (Fig. 2). A 240-mV and a 150-mV
The unsubstituted iron phthalocyanine and two iron
tetrapyridinoporphyrazine compounds have very low
solubility in non- or weakly binding organic solvents.
However, their solubility becomes much higher in
strongly binding solvents such as pyridine, DMSO,
DMA, and DMF. These will be the solvents used
in this study to investigate the electrochemical and
spectroscopic properties of the three complexes. Cyclic
voltammograms of the investigated compounds in
pyridine are illustrated in Fig. 2 and the corresponding
redox potentials summarized in Table 2.
2
(
anodic shift were seen in the first and second reductions,
respectively. A further positive potential shift by 50–60
mV was observed in the first and second reductions
from 2,3-PyD to 3,4-PyD (Fig. 2 and Table 2). Among
the three investigated complexes, the easiest reduction
takes place in 3,4-PyD while the most difficult one
occurs in (Pc)Fe. The easiest oxidation was seen in (Pc)
Fe and the oxidation in 2,3- and 3,4-isomers became too
positive and were not observed under our experimental
conditions. This trend in redox potential of three iron
derivatives is the same as the literature results for
complexes with similar macrocyclic ligands containing
different central metal ions [20]. The same trend was
also observed for (Pc)Fe, 2,3-PyD and 3,4-PyD in other
non-aqueous solvents as discussed in the following
section.
(
Pc)Fe exhibits two reversible reductions at E1/2 = -1.06
and -1.31 V and one quasi-reversible oxidation at E1/2
.72 V in pyridine containing 0.1 M TBAP (Fig 2a).
=
0
These potential values are very close to the previous
literature results (Table 2). Meanwhile, both 2,3-PyD
and 3,4-PyD undergo three reversible reductions under
the same solution conditions (Figs. 2b and 2c) and their
half-wave potentials are located at -0.82, -1.16 and -1.75
The first reduction of (Pc)Fe is metal-centered
[
22], corresponding to the conversion of Fe(II) to
Fe(I) while the second reduction is ring-centered
22, 23]. As seen below, 2,3-PyD and 3,4-PyD
[
share a similar electroreduction mechanism to (Pc)
Fe based on the spectroelectrochemical results
obtained during the first and second reduction
processes.
The thin-layer UV-visible spectroelectro-
chemistry of (Pc)Fe, 2,3-PyD and 3,4-PyD were
examined in pyridine containing 0.2 M TBAP. The
time-resolved spectral changes upon the first and
second reductions are illustrated in Figs 3 and 4.
There are two major steps in spectral change during
the first reduction of each derivative (Fig. 3). On
the first step in the case of (Pc)Fe (Fig. 3a), the
strongest Q-band at 654 nm and the Soret band
at 342 nm decrease in intensity along with the
appearance of a broad visible band centered at 518
Fig. 2. Cyclic voltammograms of (Pc)Fe (a), 2,3-PyD (b) and 3,4-
PyD (c) in pyridine containing 0.1 M TBAP
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SYNTHESIS, ELECTROCHEMISTRY, SPECTROELECTROCHEMISTRY AND CATALYTIC PROPERTIES IN DDT
321
Table 2. Half-wave potentials (V vs. SCE) of (Pc)Fe, 2,3-PyD and 3,4-PyD in pyridine, DMSO, DMA and
DMF containing 0.1 M TBAP
Cpd
Solvent
Red
2nd
Ox
1st
Reference
a
(DN)
1st
3rd
(
Pc)Fe
pyridine
33.1)
-1.06
-1.07
-0.82
-0.76
-0.76
-0.71
-0.54
b
-1.31
-1.32
-1.16
-1.11
-1.15
-1.15
-0.98
-0.90
-1.17
-0.93
-0.85
-1.17
-0.95
-0.86
b
—
—
0.72
0.66
—
tw
22
tw
tw
tw
22
tw
tw
22
tw
tw
tw
tw
tw
(
2
3
,3-PyD
,4-PyD
-1.75
-1.64
—
—
(
Pc)Fe
DMSO
29.8)
0.44
0.46
0.66
—
(
—
2
3
,3-PyD
,4-PyD
-1.74
-1.65
—
(
Pc)Fe
DMA
-0.55
b
0.38
—
2
3
,3-PyD
,4-PyD
(27.8)
-1.77
-1.67
—
b
—
(
Pc)Fe
DMF
-0.58
b
0.24
—
2
3
a
,3-PyD
,4-PyD
(26.6)
-1.76
-1.63
b
—
DN = donor number; data taken from reference 21. Ill-defined due to aggregation. tw = this work.
nm and a weak near-IR band at 802 nm. After a longer
electrolysis, the second step takes place (Fig. 3b) which
involves a slight enhancement in intensity for two bands
located at 347 and 520 nm, along with the disappearance
of the near-IR peak at 802 nm.
The first step has been observed in previous studies
in the case of (Pc)Fe and assigned as the iron-centered
reduction [22, 23]. The visible band located at 518 nm
corresponds to the charge transfer from singly reduced
Fe(I) to Pc(-2) ligand, a MLCT process, indicating
that the electron is added to the Fe(II) center to
generate the Fe(I) anion in this process. The Fe(II)
center in the neutral (Pc)Fe is bound axially with two
pyridine molecules to form a six-coordinated complex
Figures 3c–3f demonstrate that 2,3-PyD and 3,4-PyD
also undergo two separated spectral changes during the
first reduction. The MLCT bands appearing in the first
step are located at 536 and 518 nm, respectively for 2,3-
and 3,4-isomers. The weak bands very close to the near-IR
range centered at 773 nm for 2,3-PyD and at 794 nm for
3,4-PyD were also observed in the first step, but disappear
in the second step. These patterns in spectral change are
very similar to those of (Pc)Fe as demonstrated in Figs 3a
and 3b. Thus, an identical electrode reaction mechanism
can be postulated for 2,3- and 3,4-isomers during the first
reduction as shown in Equations 1 and 2 where P = either
2,3- or 3,4-tetrapyridinoporphyrazine ring.
The spectral changes of the three investigated
complexes on the second reduction (Fig. 4) are not so
characteristic as compared with those occurring in the
first reduction. The major MLCT peak located at 520
nm in the case of (Pc)Fe slightly decreases in intensity
in the second reduction and another weak visible band at
[
23]. After the first reduction, one axial ligand is lost
leading to the formation of a five-coordinated species
on the Fe(I) center of (Pc)Fe. This process can be
expressed in Equation 1 where P = phthalocyanine
macrocycle.
6
94 nm (Fig. 4a) appears. This reduction of (Pc)Fe has
-
-
(
py) P(-2)Fe(II) + e ⇌ [(py)P(-2)Fe(I)] + py
(1)
been assigned as the electron addition to the macrocycle
of phthalocyanine to generate an anionic radical [22, 23]
as shown in Equation 3 where P = phthalocyanine ring.
2
The second step shown in Fig. 3b, however, has never
been reported in the literature. This step is possibly
associated with a slow chemical reaction following the
electron-addition process during the first step. It might
be assigned as a loss of the axially bound pyridine
molecule from the five-coordinated Fe(I) center of (Pc)
Fe to generate a planar four-coordinated Fe(I) complex
as shown in Equation 2.
-
-
2-•
[P(-2)Fe(I)] + e ⇌ [P(-3)Fe(I)]
(3)
Similar spectral changes were also observed for
both 2,3- and 3,4-tetrapyridinoporphyrazine derivatives
during the second reduction. The MLCT band located
at 538 nm in the case of 2,3-PyD and at 519 nm in the
case of 3,4-PyD becomes lower in intensity. Meanwhile,
a broad visible band appears for both complexes
-
-
[
(py)P(-2)Fe(I)] ⇌ [P(-2)Fe(I)] + py
(2)
Copyright © 2013 World Scientific Publishing Company
J. Porphyrins Phthalocyanines 2013; 17: 321–330

3
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J. SHAO ET AL.
Fig. 3. UV-visible spectral changes observed during the first reduction of (Pc)Fe (a and b) at -1.20 V, 2,3-PyD (c and d) at -1.00 V
and 3,4-PyD (e and f) at -1.00 V in pyridine, 0.2 M TBAP
(
Figs 4b and 4c). This reduction in the cases of 2,3-PyD
ions such as Mg(II), Zn(II) and 2H(II) within the same
reduction potential range as examined in our study [23].
and 3,4-PyD is also associated with the electron addition
to the macrocyclic ring of each complex to generate an
anionic radical as shown in Equation 3 where P = 2,3- or
Electrochemistry of (Pc)Fe, 2,3-PyD and 3,4-PyD in
different solvents and electroreduction mechanism of
3
,4-tetrapyridinoporphyrazine ring.
The spectral changes upon the third reduction of 2,3-
2
,3-PyD and 3,4-PyD
and 3,4-isomers were not examined. But this reaction can
be easily assigned to the second ring-centered reduction.
This assignment is based on the fact that there are
generally two reversible macrocycle-centered reductions
for phthalocyanines containing non-reducible central
The cyclic voltammetric measurements were
performed for (Pc)Fe, 2,3-PyD and 3,4-PyD in DMSO,
DMA and DMF (Table 2 and Figure 5). Figure 5 shows
the reduction properties of 2,3-PyD in four different
solvents. Two types of reduction behavior can be
Copyright © 2013 World Scientific Publishing Company
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SYNTHESIS, ELECTROCHEMISTRY, SPECTROELECTROCHEMISTRY AND CATALYTIC PROPERTIES IN DDT
323
The Epc of the first reduction disappears in DMA and
DMF. This phenomenon could not be improved by either
the sonication of the solution or the addition of more 2,3-
PyD to the system. This result can be attributed to the
severe aggregation of this compound in DMA and DMF.
In addition, both (Pc)Fe and 2,3-PyD also undergo one
reversible oxidation in DMSO with the E1/2 located at
0
.44 and 0.66 V, respectively (Table 2).
Phthalocyanines and porphyrazines tend to aggregate
in many solvents and the extent of aggregation depends
upon the nature, the concentration of the compound and
the solution conditions [24]. Among the four solvents
utilized in this study, pyridine and DMSO have a relatively
strongly binding ability with donor numbers of 33.1 and
2
9.8, respectively [21]. In contrast, DMA and DMF are
relatively weaker binding solvents with donor numbers of
7.8 for DMA and 26.6 for DMF. Stronger coordinating
2
solvents, pyridine and DMSO, tend to axially bind to the
iron(II) center in (Pc)Fe, 2,3-PyD and 3,4-PyD to form
five- or even six-coordinated complexes, thus decreasing
aggregation. As a result, well-defined reductions
were observed in the two solvents. This is different in
weakly binding solvents, DMA and DMF, which have
a lower tendency to coordinate with iron(II) center than
pyridine and DMSO, leading to more severe aggregation.
Accordingly, the reversibility of the three reductions of
2
,3-PyD become ill-defined in the two solvents.
The extent of aggregation also depends upon the
nature of the investigated compounds. This difference
can be seen through a simple comparison in the
reversibility of the first reduction in the three compounds
under different solvent conditions (Table 2). The first
reduction of (Pc)Fe is reversible in the four solvents and
this result differs from those of 2,3-PyD and 3,4-PyD.
The first reduction for 2,3-PyD is only reversible in both
pyridine and DMSO while a well-defined first reduction
can only be detected in pyridine for 3,4-PyD. From
this comparison, it can be concluded that the extent of
aggregation among the three derivatives increases in the
order of: (Pc)Fe < 2,3-PyD < 3,4-PyD, consistent with
the results seen in the case of cobalt phthalocyanine and
cobalt tetrapyridinoporphyrazines [12].
As shown in Table 2, the reduction potentials of each
iron complex depend upon the solvent properties. The
first and second reductions of 2,3-PyD shift by 280 mV
and 180 mV anodically from pyridine to DMSO (Figs 5a
and 5b), indicating that both reductions become easier in
DMSO. An additional positive shift for the first reduction
of the same complex was seen when the solvent changed
from DMSO to either DMA or DMF. However, there is
not much difference in the E1/2 of the second reduction in
these three solvents (Figs 5b–5d). Meanwhile, the half-
wave potentials of the third reduction of 2,3-PyD do not
vary significantly in all of the four solvents used, and
range narrowly from -1.74 to -1.77 V (Fig. 5).
Fig. 4. UV-visible spectral changes observed during the second
reduction of (Pc)Fe (a) at -1.60 V, 2,3-PyD (b) at -1.60 V and
3
,4-PyD (c) at -1.40 V
observed in Fig. 5. The first type shown in Figs 5a and 5b
involves three well-defined reductions in both pyridine
and DMSO. The second type was seen in DMA and
DMF solutions (Figs 5c and 5d), where 2,3-PyD also
undergoes three reductions, but each reduction decreases
in current and the first reduction becomes ill-defined
compared with the next two reductions (Figs 5c and 5d).
The positive potential shift in the first and second
reductions, when the solvent changes from pyridine to
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3
24
J. SHAO ET AL.
pyridine to DMSO, DMA and DMF. But this
trend is not so clear for 3,4-PyD because its first
reduction is ill-defined due to a severe aggregation
of 3,4-PyD in DMSO, DMA and DMF. The E1/2
values of the second reduction for (Pc)Fe and
the second and third reductions for 3,4-PyD do
not change remarkably under DMSO, DMA and
DMF.
Additionally, the E1/2 values of the second
reductions of the three complexes follow the
order of 3,4-PyD > 2,3-PyD > (Pc)Fe, in DMSO,
DMA and DMF, with 3,4-PyD having the easiest
reduction (Table 2). This order is the same as the
result previously observed in pyridine (Fig. 2).
Based on the discussion above, it can be
concluded that the site of electron transfer for
the reductions in 2,3- and 3,4-isomers are very
similar to that of (Pc)Fe under different solvents.
The first reduction is iron-centered while the
second and third reductions are ring-centered.
However, the number of solvent molecules bound
axially to the Fe(II) in the neutral complexes or
Fe(I) in the singly-reduced species may vary with
solvent conditions due to their different binding
abilities. An overall mechanism showing the site
of electron transfer for the electroreductions of 2,3-PyD
and 3,4-PyD is proposed in Scheme 1.
Fig. 5. Cyclic voltammograms of 2,3-PyD in various solvents
containing 0.1 M TBAP
DMSO, DMA and DMF, can be accounted for by the
stronger binding property of pyridine as compared to the
other solvents. As discussed above, pyridine molecules are
able to axially bind with iron centers in both neutral Fe(II)
and singly reduced Fe(I) species to form more stable penta-
or even hexa-coordinated complexes as compared with the
weakerbindingsolvents, DMAandDMF.Asaconsequence,
both the neutral and the singly reduced species of 2,3-PyD
can be stabilized in pyridine, leading to more difficult
reductions than in other solvents (Fig. 5 and Table 2).
However, the potentials of the second and third reductions
in DMSO, DMA and DMF do not vary significantly,
indicating that these solvents have little interaction with the
iron centers in the singly and doubly reduced species of 2,3-
PyD. The site of electron addition for the second and third
reductions in the DMSO, DMA and DMF solutions should
be the same, both of which are ring-centered.
Electrochemistry of (Pc)Fe, 2,3-PyD and 3,4-PyD in
the presence of DDT
Three iron complexes undergo two to three reductions
in the four different solvents as discussed in the previous
sections. The neutral Fe(II) species of each derivative can
-
be reduced stepwise into the Fe(I) anion, [P(-2)Fe(I)] ,
2
-·
and the Fe(I) anionic radical, [P(-3)Fe(I)] , upon the
first two reductions as displayed in Scheme 2a. Both
electrogenerated Fe(I) species can serve as catalysts for
DDT dechlorination as discussed below.
The interaction between DDT and each iron complex
after reduction was first investigated in pyridine
containing 0.1 M TBAP. Figure 6 compares the cyclic
voltammograms of 2,3-PyD and 3,4-PyD in the absence
and presence of DDT. Two well-defined reductions
were seen within the potential range from 0 to -1.70
V for both complexes in the absence of DDT (Fig.
6b and b′). Upon stepwise addition of DDT to both
solutions (Figs 6c–6f and 6c′–6f′), the first reduction
It is worth noting that the potential changes seen above
for 2,3-PyD under different solvents have also been
observed for (Pc)Fe. As shown in Table 2, an evident
positive shift was also observed for the first and second
reductions of (Pc)Fe when the solvent changes from
e-
+e-
+e-
+
-
[P(-3)Fe(I)]2-.
[P(-4)Fe(I)]3-
(
S) P(-2)Fe(II)
[(S) P(-2)Fe(I)]
x
y
(
x = 1 or 2)
(y = 0 or 1)
P = 2,3- or 3,4-tetrapyridinoporphyrazine macrocycle; S = solvent
Scheme 1.
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SYNTHESIS, ELECTROCHEMISTRY, SPECTROELECTROCHEMISTRY AND CATALYTIC PROPERTIES IN DDT
325
+ e-
+ e-
(
a)
-
2-.
FeII
FeI
FeI
1st red
2nd red
=
phthalocyanine or porphyrazine rings
Solvent molecules bound with Fe(II) and Fe(I) are omitted for clarity.
-
FeIII
Cl
(
(
b)
c)
Cl
Cl
Cl
Cl
2
-.
FeI
Cl-
+
+
Cl
Cl
Cl
Cl
(
DDT)
2
-
FeII
-
FeIII
Cl
Cl
2
-.
+
e-
Cl
Cl
Cl
FeI
+
Cl
Cl
Cl
Cl
Cl
Cl
Cl
(
d)*
Cl
Cl
Cl
Cl
β-elimination
Cl
Cl
Cl
Cl
(
DDE)
+
e-
H
Cl
Cl
Cl
Cl
Cl
H
+
H+
base
Cl
Cl
Cl
Cl
Cl
Cl
(
DDD)
(DDMU)
*
Step d taken from Ref. 28
Scheme 2.
does not exhibit a remarkable change in the cathodic
implies that the doubly reduced anionic radical, [P(-2)
2
-
peak current (i ). However, this is not the case for the
Fe(I)] · participates in the electrocatalytic process of
DDT dechlorination. The third reduction in the case of
2,3- and 3,4-isomers was not investigated in our studies
because its potential is too close to the electroreduction
of DDT itself (Fig. 6a).
Similar results were also seen for (Pc)Fe in pyridine
and/or (Pc)Fe, 2,3-PyD and 3,4-PyD in DMSO and
DMA, indicating that a similar electrocatalytic reaction
occurs for DDT reductive dechlorination over three iron
macrocyclic complexes.
pc
second reduction of both complexes, where the cathodic
peak current (i ) of this electrode reaction is enhanced
pc
significantly with DDT addition and this reduction
becomes irreversible at higher DDT concentrations in
both solutions. This increase in reduction current of the
second process cannot be simply attributed to the direct
reduction of DDT, because DDT alone does not show
any reductions in the potential range from 0 to -1.60
V under the same solution conditions (Fig. 6a). This
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3
26
J. SHAO ET AL.
-
4
-4
Fig. 6. Cyclic voltammograms of 2,3-PyD (2.5 × 10 M) and 3,4-PyD (2.5 × 10 M) in the presence of various DDT eqs. (b–f and
b′–f′) and cyclic voltammogram of DDT alone (a) in pyridine, 0.1 M TBAP
It has been known [13] that the low valent transition
metal ions, such as Fe(I), Co(I), Rh(I) and Ir(I), in
the metalloporphyrins are able to react with alkyl or
aryl halides (RX) to form s-bonded M(III) porphyrin
complexes (where M = Fe, Co, Rh, Ir, etc.). Some carbon-
metal s bonds in the intermediates generated above are
unstable and can be broken upon electroreduction or in
the presence of light to recover the original low valent
metal centers [13, 25–27]. Dehalogenation takes place
if the processes stated above keep cycling between RX
and the low valent metal ions in metalloporphyrins at an
applied negative potential. During the process, the central
metal ions with low oxidation states can serve as the
catalysts for the dehalogenation of alkyl or aryl halides
such as DDT.
and 2,3- and 3,4-tetrapyridinoporphyrazine derivatives
investigatedinthisstudyexhibitsimilarproperties(Fig.6).
The doubly reduced Fe(I) species in these complexes are
also effective in catalyzing DDT reductive dechlorination
to generate the three dechlorinated products, DDD, DDE
and DDMU, which will be demonstrated below.
Similar to the reaction mechanism of DDT
dechlorination postulated in the literature [28], the first
step in DDT reaction over the three iron complexes may
be the formation of carbon-metal s-bonded complexes
generated between Fe(I) centers in the doubly reduced
(Pc)Fe, 2,3-PyD and 3,4-PyD complexes and DDT
molecules as shown in Scheme 2b. A cleavage of this
s bond takes place when an electron is added to the
s-bonded complex, leading to the recovery of Fe(I)
anionic species and the generation of a chlorine-
containing intermediate radical (Scheme 2c). Through
further chemical and electroreduction steps, the chlorine-
containing radical intermediates are eventually converted
As reported in the literature, cobalt and iron porphyrins
and cobalt phthalocyanine and porphyrazines have been
confirmed to be effective catalysts towards the reductive
dechlorination of DDT [11, 12]. Iron phthalocyanine
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SYNTHESIS, ELECTROCHEMISTRY, SPECTROELECTROCHEMISTRY AND CATALYTIC PROPERTIES IN DDT
327
by their different half-wave potentials during
the second reduction. The highest activity of
(
Pc)Fe in DDT dechlorination corresponds
to the most negative potential of its second
reduction at E1/2 = -1.31 V, implying that
a more negative reduction potential will
be beneficial for the formation of higher
reactive Fe(I) anionic radicals. The catalytic
activity order shown in Fig. 7a is also
consistent with the results obtained in the
controlled-potential electrolysis displayed in
the following section.
Figure 7b compares the ip/ip0 ratios of
(
Pc)Fe in three different solvents: pyridine,
DMSO and DMA. In the examined [DDT]/
Fe] molar ratio range, there are different
[
trends in the three solvents. In the case
of pyridine, the ratio ip/ip0 continuously
increases as [DDT]/[Fe] increases, but this is
not the case for the other two solvents, where
a constant i /i ratio is obtained after [DDT]/
p
p0
[Fe] is equal to 6 for DMSO and about 3 for
DMA. Pyridine gives the highest catalytic
current compared to DMSO and DMA,
again due to the fact that (Pc)Fe has a more
negative potential in its second reduction in
pyridine. In addition, as shown in Fig. 7b, the
catalytic current in DMA is much lower than
that in DMSO, although the second reduction
potential of -1.17 V in DMA is slightly more
negative than -1.15 V in DMSO. This result
may be attributed to the stronger aggregation
of (Pc)Fe in DMA than in DMSO, as shown
in Table 2. This results in the conclusion that
pyridine is the best solvent towards DDT
reductive dechlorination comparing with
DMSO and DMA.
Fig. 7. Plot of ip/ip0 vs. [DDT]/[Fe] ratio where i and i are the measured
p
p0
cathodic peak currents obtained from cyclic voltammograms for the second
reduction of each Fe catalyst in the presence and absence of DDT. The results
of (Pc)Fe, 2,3-PyD and 3,4-PyD in pyridine shown in (a) while those of (Pc)
Fe in pyridine, DMSO, DMA illustrated in (b)
Controlled-potential electrolysis of (Pc)Fe, 2,3-PyD
and 3,4-PyD in the presence of DDT
into dechlorinated products, DDE, DDD and DDMU as
shown in Scheme 2d. The Fe(I) anionic radical recovered
in Scheme 2c is able to recombine with DDT to catalyze
DDT reductive dechlorination.
The reductive dechlorination of DDT catalyzed by
the three iron derivatives was examined in pyridine
using controlled-potential electrolysis in the presence of
10 or 100 eq. DDT. The potential applied for the bulk
electrolysis is slightly more negative than the E1/2 of the
second reduction of each compound, so that the neutral
Fe(II) species can be electroreduced into Fe(I) anionic
radicals, the latter of which will serve as the catalyst for
DDT dechlorination. Figure 8 exhibits the conversion
(%) of DDT and the yield (%) of dechlorinated products
at various bulk electrolysis time and the corresponding
results are given in Table 3.
The increase in cathodic peak current (i ) for the
pc
second reduction of each investigated iron macrocycle
as a function of increasing DDT concentration in
the pyridine solution is illustrated in Fig. 7a, where ip
and ip0 represent the measured cathodic peak currents
collected from cyclic voltammograms of the three
macrocyclic derivatives in the presence and absence of
DDT, respectively. The i /i ratio in pyridine for (Pc)Fe
p
p0
complex is the highest among the three iron complexes
while similar ratios of i /i are observed for 2,3-PyD
p
p0
and 3,4-PyD when the [DDT]/[Fe] molar ratio is below
. However, the ratio of ip/ip0 in the case of 2,3-PyD
becomes larger than that of 3,4-isomer when the [DDT]/
Fe] ratio is greater than 4. This can be readily explained
To better understand the performance of each catalyst
after the second reduction, it is necessary to know the
reactivity of the Fe(I) anion electrogenerated at the first
reduction. A separate bulk electrolysis was therefore
4
[
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3
28
J. SHAO ET AL.
Fig. 8. DDT conversion (●) and the yield of dechlorination products (∆ for DDE; □ for DDD and ○ for DDMU) over (Pc)Fe (a and
b), 2,3-PyD (c) and 3,4-PyD (d) in pyridine, 0.1 M TBAP upon the second reduction. The molar ratio between DDT and the iron
catalyst is 10:1 in (a), (c) and (d), and is 100:1 in (b)
-
carried out for (Pc)Fe in pyridine containing 10 eq. DDT
at an applied potential of -1.20 V, where only the singly
reduced Fe(I) anion, [P(-2)Fe(I)] , is formed.
for the singly reduced species [P(-2)Fe(I)] . However,
the decomposition of these s-bonded intermediates to
generate dechlorinated products might be driven by visible
light and not by electroreduction as seen in the case of
-
GC-MS of the products obtained after dechlorination
-
2-
of DDT electrocatalyzed by the [P(-2)Fe(I)] revealed
[P(-3)Fe(I)] · (Fig. 6). The reactivity between the singly
reduced [P(-2)Fe(I)] and DDT is also similar to literature,
where DDT is able to react with the singly reduced Co(I)
anions in cobalt phthalocyanine [12]. It should be noted
that the catalytic activity of [P(-2)Fe(I)] is much lower
-
that 22.8% DDT was dechlorinated after two hours. Two
products, DDD and DDE, were detected and their yields
were 2.4% and 20.4% respectively. The more extensive
dechlorination product, DDMU, was not detected under
the experimental conditions. These results clearly indicate
-
2-
than that of the doubly reduced [P(-3)Fe(I)] · in DDT
dechlorination, as demonstrated below.
-
that the singly reduced [P(-2)Fe(I)] is active toward DDT
dechlorination, although this process was not observed
from the cyclic voltammograms of (Pc)Fe in the presence
Figure 8 and Table 3 demonstrate that the catalytic
2
-
activity of the doubly reduced [P(-3)Fe(I)] in (Pc)
Fe towards DDT dechlorination is notably higher than
of DDT. It is not surprising to observe a reaction between
-
-
[
P(-2)Fe(I)] and DDT because it is known that Fe(I)
that of the singly reduced [P(-2)Fe(I)] under identical
porphyrin anions can also react with alkyl and aryl halides
to form s-bonded complexes [13]. This is also the case
experimental conditions. After two hours of electrolysis,
DDT conversion (%) seen in the case of [P(-3)Fe(I)] in
2
-·
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SYNTHESIS, ELECTROCHEMISTRY, SPECTROELECTROCHEMISTRY AND CATALYTIC PROPERTIES IN DDT
329
Table 3. DDT conversion (%) and product yield (%) over three catalysts, (Pc)Fe, 2,3-Fe and 3,4-Fe in pyridine. e molar ratio of
DDT to each catalyst is 10:1
Cpd
Time, h Conversion, %
Yield, %
Cpd
Time, h Conversion, %
Yield, %
DDD DDE DDMU
DDD DDE DDMU
a
(
Pc)Fe
1
2
3
4
5
1
2
3
4
5
38.3
90.2
12.7
22.5
9.7
0
22.9
49.1
56.5
53.2
51.4
46.1
68.0
70.8
65.3
65.2
2.7
18.6
33.8
46.8
48.6
6.5
(Pc)Fe
1
2
3
4
5
1
2
3
4
5
19.4
32.4
51.5
62.3
79.5
51.5
68.9
83.8
97.3
97.5
6.8
11.2
17.9
22.2
32.5
11.9
15.4
17.3
16.0
15.8
10.2
16.1
22.8
24.1
25.7
32.2
41.4
50.2
59.8
59.7
2.4
5.1
100
10.8
15.9
21.2
7.4
100
100
0
2
,3-Fe
58.4
98.1
100
5.8
8.0
0
3,4-Fe
22.1
29.2
34.7
34.8
12.1
16.3
21.5
22.0
100
100
0
0
a
The molar ratio between DDT and (Pc)Fe is 100:1.
(
Pc)Fe is 90.2%, which is almost four times higher than
No DDD can be detected in the solutions of (Pc)Fe and
2,3-PyD after four hours of electrolysis (Figs 8a and 8c),
indicating that DDD is a very reactive product, which
can also be dechlorinated through a similar mechanism
shown in Scheme 2, to convert into DDMU [12]. A
similar result has been reported These results contrast to
those observed in the 3,4-PyD solution, where DDD still
exists in the solution even after five hours of electrolysis,
again indicating that 3,4-PyD has a lower catalytic ability
than (Pc)Fe and 2,3-PyD in DDT dechlorination.
Figures 8a, 8c and 8d further disclose that the yield
of DDMU is enhanced in each catalytic system with
increasing electrolysis time. DDMU is stable under our
experimental conditions and does not convert into other
dechlorinated products during electrolysis. The yields of
DDMU (Figs 8a, 8c and 8d) after five hours are 48.6%
for (Pc)Fe, 34.8% for 2,3-PyD and 22.0% for 3,4-PyD,
with an order of (Pc)Fe > 2,3-PyD > 3,4-PyD. This
order is identical to the results predicted by the cyclic
voltammograms shown in Fig. 7a. Consequently, (Pc)
Fe has the best catalytic performance towards DDT
reductive dechlorination among the three investigated
iron derivatives.
The effect of DDT concentration on the dechlorination
results was also investigated in the case of (Pc)Fe in
pyridine containing 100 eq. DDT as shown in Fig. 8b.
A distinct linear relationship was seen between DDT
conversion (%) and the electrolysis time (Fig. 8b). This
result reveals clearly that the turnover rate of DDT
conversion over the catalyst is almost constant during the
electrolysis period. There is no obvious deactivation in
catalytic property of (Pc)Fe containing excessive DDT
within five hours of reaction. Three products, DDD, DDE
and DDMU were detected, but their yields were very
close to each other. This differs from the results shown
-
that of the corresponding [P(-2)Fe(I)] . Both DDD and
DDEwerealsodetectedinthedechlorinatedproductswith
yields of 22.5% and 49.1%, respectively. Meanwhile, the
more extensive dechlorinated product, DDMU, was also
formed, which was not produced for the catalyst [P(-2)-
Fe(I)] , confirming that the doubly reduced [P(-3)Fe(I)] ·
is more efficient in catalyzing DDT dechlorination than
the corresponding singly reduced [P(-2)Fe(I)] .
-
2-
-
Three separate bulk electrolyses for (Pc)Fe, 2,3-PyD
and 3,4-PyD upon the second reduction in pyridine
containing 10 eq. DDT were performed (Figs 8a, 8c and
8
d). DDT conversion (%) changing with the reaction
time for(Pc)Fe (Fig. 8a) and 2,3-PyD (Fig. 8c) are very
similar to each other, where a complete DDT conversion
was seen for both catalysts after three hours. In contrast,
DDT dechlorination rate for 3,4-PyD (Fig. 8d) is much
slower than those of (Pc)Fe and 2,3-PyD (Figs 8a and
8
c), where DDT can still be detected in the 3,4-PyD
solution even after five hours of electrolysis (Table 3).
These results indicate that 3,4-PyD is the least effective
among the three catalysts studied.
The yields of the three dechlorinated products DDD,
DDE and DDMU analyzed by GC-MS are shown in
Figs 8a, 8c and 8d and revealed that DDE is the major
product in each solution during the electrolysis period.
It is quickly formed at the beginning of electrolysis and
reaches its maximum value around three hours in the case
of (Pc)Fe and 2,3-PyD. After that, DDE yield decreases
slightly with electrolysis time, implying that DDE
itself can also be dechlorinated under the experimental
conditions [12]. The second dechlorinated product,
DDD, has a very low yield in every reaction system.
Its concentration is relatively high during the first two
hours and then gradually decreases with reaction time.
Copyright © 2013 World Scientific Publishing Company
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3
30
J. SHAO ET AL.
in Fig. 8a, where significant differences were seen in the
yields of DDD, DDE and DDMU in the presence of 10
eq. DDT.
10. Hisaeda Y and Shimakoshi H. In Handbook of
Porphyrin Science, Vol. 10, Kadish KM, Smith KM
and Guilard R. (Eds.) World Scientific: New Jersey,
2
010; Chapter 48, pp 313–370.
1
1. Zhu W, Fang Y, Shen W, Lu G, Zhang Y, Ou Z and
CONCLUSION
Kadish KM. J. Porphyrins Phthalocyanines 2011;
1
5: 66–74.
(Pc)Fe, 2,3-PyD and 3,4-PyD exhibit up to three
1
1
1
1
1
1
1
2. Shao J, Thomas A, Han B and Hansen CA. J.
Porphyrins Phthalocyanines 2010; 14: 133–141.
3. Guilard R and Kadish KM. Chem. Rev. 1988; 88:
reductions and one oxidation in pyridine, DMSO, DMA
and DMF. The reversibility and half-wave potentials of
these redox reactions are dependent upon the nature of the
iron derivatives, the solvents and the extent of aggregation
in solutions. The most difficult reduction occurs for
1
121–1146.
4. Lin XQ and Kadish KM. Anal. Chem. 1985; 57:
498–1501.
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(
Pc)Fe while the easiest one takes place for 3,4-PyD in
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6. Achar BN, Fohlen GM, Parker JA and Keshavayya
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different solvents. The first reduction is metal-centered
and the second and third reductions are ring-centered.
Both the singly and doubly reduced Fe(I) species in
each complex are active towards DDT dechlorination,
but the latter displays stronger catalytic activity. Three
products, DDD, DDE and DDMU were detected after
DDT dechlorination and the conversion (%) of DDT and
the yields of dechlorinated products depend upon the
nature of the catalysts, the solvents and the molar ratio of
1
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2
9. Kobayashi N. In Phthalocyanines-Properties
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Kadish KM, Smith KM and Guilard R. (Eds.)
Academic Press: New York, 2003; Chapter 100,
pp 161–262.
[
DDT]/[Fe]. The best DDT dechlorination results can be
obtained when (Pc)Fe is used as the catalyst and pyridine
as the solvent. A possible electrocatalytic mechanism in
DDT dechlorination over three iron complexes is given
in Scheme 2.
Acknowledgements
The financial support of the Robert Welch Foundation
A0-0001) is gratefully acknowledged. This work is also
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