Highly efficient C–Cl bond cleavage and unprecedented C–C bond cleavage of environmentally toxic DDT through molecular electrochemical catalysis

Article abstract of DOI:10.1016/j.apcata.2017.07.026

The electrocatalytic properties of a Co(II)octaalkoxyphthalocyanine complex (Co(II)Pc) with eight strongly electron-donating substituents provide the first example of the complete dechlorination of DDT through molecular electrocatalysis, rather than the u

Full text of DOI:10.1016/j.apcata.2017.07.026

Accepted Manuscript
Title: Highly Efficient C−Cl Bond Cleavage and
Unprecedented C−C Bond Cleavage of Environmentally
Toxic DDT through Molecular Electrochemical Catalysis
Authors: Xu Liang, Tingting Huang, Minzhi Li, John Mack,
Martijn Wildervanck, Tebello Nyokong, Weihua Zhu
PII:
S0926-860X(17)30336-8
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.apcata.2017.07.026
APCATA 16331
To appear in:
Applied Catalysis A: General
Received date:
Revised date:
Accepted date:
22-1-2017
3-7-2017
15-7-2017
Please cite this article as: Xu Liang, Tingting Huang, Minzhi Li, John Mack,
Martijn Wildervanck, Tebello Nyokong, Weihua Zhu, Highly Efficient C−Cl
Bond Cleavage and Unprecedented C−C Bond Cleavage of Environmentally
Toxic DDT through Molecular Electrochemical Catalysis, Applied Catalysis A,
Generalhttp://dx.doi.org/10.1016/j.apcata.2017.07.026
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Highly Efficient C−Cl Bond Cleavage and Unprecedented C−C Bond Cleavage of Environmentally
Toxic DDT through Molecular Electrochemical Catalysis
Xu Liang*, a Tingting Huang,a Minzhi Li,a John Mack*, b Martijn Wildervanck,b Tebello Nyokong,b and
Weihua Zhu*, a
a School of Chemistry and Chemical Engineering, Jiangsu University, Zhenjiang 212013, P. R. China;
b Department of Chemistry, Rhodes University, Grahamstown 6140, South Africa
Corresponding Author
*E-mail: liangxu@ujs.edu.cn, Tel&Fax: +86-511-8879-1928 (to X.L.); E-mail: sayman@ujs.edu.cn,
Tel&Fax: +86-511-8879-1928 (to W.Z.); E-mail: j.mack@ru.ac.za, Tel&Fax: +27-46-603-7234 (to J.M).
Graphical abstract
ABSTRACT
The electrocatalytic properties of a Co(II)octaalkoxyphthalocyanine complex (Co(II)Pc) with eight
strongly electron-donating substituents provide the first example of the complete dechlorination of
DDT through molecular electrocatalysis, rather than the use of metal electrodes which had been
achieved previously. Interaction with a highly nucleophilic [Co(I)Pc]2− species results in rapid
cleavage of the C(sp3)−Cl, C(sp2)−Cl and aromatic C(sp2)−Cl bonds. Bis(p-chlorophenyl)methanone
(BPCl2) is detected in high yield along with its full dechlorination product, diphenylmethanone (BP)
and the conventional C−Cl bond cleavage products, due to an unprecedented C−C bond cleavage
reaction that is followed by the formation of a C−O bond. Theoretical calculations are used to
analyze trends in the electronic structure of the Co(II)octaalkoxyphthalocyanine complex that
account for the efficiency of the C−Cl bond cleavage reactions, and the reaction process and
mechanism are analyzed in depth.

KEYWORDS. C-Cl Bond Cleavage, C-C Bond Cleavage, Electrocatalysis, Phthalocyanine, Electronic
Structure.
1. Introduction
Over the past few decades, organochlorides, organic compounds with at least one covalent C−Cl
bond, such as 1,1-bis(4-chlorophenyl)-2,2,2-trichloroethane (DDT), pentachlorophenol (PCP),
polychlorinated biphenyls (PCBs) and hexachlorocyclohexane (HCH) have been used in a wide range
of applications [1-4]. Unfortunately, organochlorides often do not degrade in the environment, so
despite restrictions being placed on their use in agriculture in many countries, they are still present
around the world at significant concentrations as a ubiquitous contaminant in soils and waters [5-8].
There has been considerable interest in the development of new techniques to achieve their full
degradation through C−Cl bond cleavage reactions. This has included microbial methods [9-14],
along with chemical [15-19], photochemical [20-25] and electrochemical approaches [26-31].
There has been considerable interest in electrocatalytic processes due to their facile procedures
and low cost. Electrocatalysis usually makes use of macrocyclic transition metal complexes, such
as phthalocyanines (Pcs), porphyrins and cycloamines [32,33]. Typically, when a phthalocyanine is
used, a central divalent metal ion is reduced electrochemically to form a [MIPc(−2)]− species, so it
can react with alkyl halides (R−X) to form a σ-bond and a MIII-macrocycle, since this results in the
release of a chloride ion through the cleavage of a C−Cl bond [34,35]. The dissociation energy of
the bond is determined by the type of s-p hybridization that is involved. The values for C(sp3)−Cl
bonds lie in the 330−350 kJ/mol range, whereas those of C(sp2)−Cl and aromatic C(sp2)−Cl bonds lie in
the 370−390 kJ/mol range and close to 400 kJ/mol, respectively [36-42]. The efficiency of C−Cl
bond cleavage reactions differs significantly, therefore. For example, when iron(III)porphyrins
were used as electroreductive catalysts for the dechlorination of lindane, pentachlorocyclohexene
(PCCH), tetrachlorocyclohexadienes (TCDNs) and trichlorobenzenes (TCBs) were formed through the
activation of the C(sp3)−Cl bond, but there was no significant degradation of the more stable
aromatic C(sp2)−Cl bonds of the trichlorobenzenes [22,24].
One main challenge for researchers active in the field of electrocatalysis is to develop catalysts
that can enable the complete dechlorination of environmentally problematic organochlorides, such
as DDT. When CoII, FeIII and MnIIIporphyrins and CoIIphthalocyanines have previously been used as
electrocatalysts, DDT has been degraded to form 1,1-bis(4-chlorophenyl)-2,2-dichloroethane) (DDD),
(1,1-bis(4-chlorophenyl)-2,2-dichloroethylene) (DDE), and (1,1-bis(4-chlorophenyl)-2-chloroethylene)
(DDMU) since full C(sp3)−Cl bond cleavage occurs followed by partial cleavage of the C(sp2)−Cl bonds
[20,21,23]. Although NiIIsalen complexes have been reported to enhance the dechlorination
process, the activation yield for the C(sp2)−Cl bond is still relatively low [43]. In this study, an
electron-rich Co(II)phthalocyanine (1) with eight electron-donating alkoxy substituents,
Co(II)-α,α’-(OC5H11)8Pc, is used to provide the complete dechlorination of DDT in
controlled-potential bulk electrolysis experiments. To the best of our knowledge, there have

previously been no reports of the full carbon-chlorine bond cleavage of DDT through molecular
electrochemical catalysis, although this has been achieved previously with metal electrodes.
2. Experimental
2.1 Instrumentation
Bruker AVANCE 500 (500.13 MHz) or Bruker AVANCE 400 (400.03 MHz) spectrometers were used
to measure 1H NMR spectra. Residual solvent peaks were used to provide internal references for
1H (δ = 7.26 ppm for CDCl3, δ = 5.32 ppm for CD2Cl2). Reagent grade chemicals and solvents were
used as supplied unless otherwise noted. A three-electrode cell attached to a Chi-730D
electrochemistry station was used during cyclic voltammetry measurements. A saturated calomel
electrode (SCE), a glassy carbon disk, a platinum wire were the reference, working and counter
electrodes, respectively. Bulk electrolysis was carried out with platinum mesh working and
counter electrodes in an “H” type cell. The anodic and cathodic sections of the cell were separated
with fritted glass and an SCE was used as the reference electrode and was placed in the same
compartment as the working electrode. An optically transparent thin-layer cell with a Pt mesh
working electrode that was assembled in house was attached to a Chi-730D potentiostat to enable
the UV-visible spectroelectrochemical measurements. UV-visible spectra were recorded with a HP
8453A diode array spectrophotometer. A nitrogen atmosphere was used for all of the
measurements that are related to electrochemistry and spectroelectrochemistry. Products of the
electrolyses were identified with an Agilent HP6890 GC gas chromatography system equipped with a
HP5975-MSD detector. A JASCO J-815 spectrodichrometer with a JASCO electromagnet (1.6 tesla)
mounted in the sample compartment were used to record magnetic circular dichroism (MCD)
spectra. Parallel and antiparallel fields were used and the intensity mechanism conventions that
were recommended by Piepho and Schatz are used throughout [44-46].
2.2. Removal of supporting electrolyte
A pre-treatment procedure was required prior to the GC-MS measurements to remove the TBAP
supporting electrolyte. The DMF solvent was evaporated under vacuum at 45oC on a rotary
evaporator. Cyclohexane was then added to extract DDT and the dechlorinated products from the
residue. The TBAP was removed after stirring and centrifuging, prior to the GC-MS analysis on
HP6890-GC and HP5975-MSD systems with an HP-5 5% phenyl methyl siloxane column (length 30 m,
ID 250 μm, film 0.25 μm). The temperature was initially set at 100oC and was then programmed to
rise at 15oC/min until a maximum of 300oC was reached. The intensity of the peaks was analyzed
by area-normalized method. When the molar response factor for DDT was set as 1.00, the relative
values for DDE, DDD and DDMU were determined to be 1.076±0.039, 0.934±0.053, 1.003±0.031,
respectively. The molar response factors of some of the intermediate products could not be
determined, since the appropriate reagent is not commercially available.
2.3. Computational methods

Geometry optimizations for unsubstituted metal free phthalocyanine (H2Pc) and
ethoxy-substituted model complexes for 1 and H2-1 were carried out at the B3LYP/6-31G(d) level of
theory by using the Gaussian 09 software package [47]. Slightly saddled structures were predicted
with N−M−N angles in the central cavity of below 10o for 1. The CAM-B3LYP functional was used
for TD-DFT calculations, since it introduces a long-range correction at high interelectronic
separations that makes it better suited for compounds and complexes that have excited states with
significant charge transfer character.
2.4. Synthesis and Characterizations
2.4.1. Synthesis of 1,4-n-OC5H11-2,3-phthalonitrile. 1,4-(OH)2-2,3-phthalonitrile (1.6 g, 10 mmol)
was added to 20 mL of a dry acetone solution containing 1-iodopentane (4.4 g, 22 mmol, 2.2 eq.)
and K2CO3 (5.5 g, 40 mmol, 4.0 eq). The resulting mixture was gradually heated to 60°C, and the
temperature was maintained for 4 h. After removal of the solvent, the reaction mixture was
purified by silica gel column chromatography with CHCl3 as the eluent. Recrystallization from
CHCl3 and MeOH provided 1,4-n-OC5H11-2,3-phthalonitrile as a white solid compound in 88% yield
(2.64 g). 1H NMR (400 MHz, CDCl3): δ = 7.15 (s, 2H; β-phenyl), 4.04 (t, J = 8.0 Hz, 4H; -OCH2-), 1.84
(dd, J1=12.0 Hz, J2 = 8.0 Hz, 4H; -CH2-), 1.49−1.35 (m, 8H, -CH2CH2-), 0.93 (t, J = 8.0 Hz, 6H; -CH3).
2.4.2. Synthesis of H2-α,α’-n-(OC5H11)8-phthalocyanine (1-H2): lithium (56 mg, 8.0 mmol) was added
to 6 mL of freshly distilled 1-butanol, and the solution was stirred and heated at 150°C under an N2
atmosphere until the lithium was completely dissolved. 1,4-OC5H11-2,3-phthalonitrile (300 mg, 1.0
mmol) was then added and the resulting mixture was gradually heated at 60°C, and the
temperature was maintained for 2 h. After removal of the solvent, the reaction mixture was
purified by silica gel column chromatography with CHCl3:MeOH (100:3) as the eluent.
Recrystallization from CHCl3 and MeOH provided the target compound, 2, as a green solid in 63%
yield (2.64 g). 1H NMR (400 MHz, CDCl3): δ = 7.15 (s, 2H; β-phenyl), 4.04 (t, J = 8.0 Hz, 4H; -OCH2-),
1.84 (dd, J1=12.0 Hz, J2 = 8.0 Hz, 4H; -CH2-), 1.49~1.35 (m, 8H, -CH2CH2-), 0.93 (t, J = 8.0 Hz, 6H; -CH3).
MALDI-TOF-MS: m/z = 1203.83 (Calcd. for C72H98N8O8 [M]+ = 1203.60).
2.4.3. Synthesis of Co(II)-α,α’-n-(OC5H11)8Pc (1). Co(CH3COO)2∙4H2O (100 mg, 4.0 mmol, 4.0 eq) and
H2Pc (120 mg, 1.0 mmol) were dissolved in 4 mL of anhydrous DMF, stirred and heated at 180°C for
20 min under N2. The reaction mixture was poured into 30 mL of ice-water once it had cooled to
room temperature. After filtration, a green solid was collected and further purified by alumina gel
column chromatography with CHCl3:MeOH (100:5) as the eluent. The target compound 3b was
obtained in 90% yield. MALDI-TOF-MS: m/z = 1260.73 (Calcd. for C72H98N8O8 [M]+ = 1260.51).
3. Results and Discussion
3.1 Synthesis of the molecular catalyst
The literature procedure was used to prepare 1 [48], with a CoCl2 salt used during the metal
insertion step (Scheme 1). Trends that are identified in the electronic structures and optical
properties of metal phthalocyanine complexes can be used to rationalize their redox and

electrocatalytic properties [49-53].
3.2. Spectroscopic investigation and theoretical calculation
A perimeter model approach can be used to analyze trends in the electronic structures of
porphyrinoid metal complexes [49-56], by considering the effect that structural modifications
perturbations have on the frontier molecular orbitals (MOs) of a D16h symmetry C16H162− parent
perimeter that corresponds to the inner perimeter of the ligand. There is a ML = 0, ±1, ±2, ±3, ±4,
2−
±5, ±6, ±7, 8 sequence in the -MOs of the C16H16
.
The HOMO and LUMO have ML values of ±4
and ±5, respectively. This forms the basis of Gouterman’s 4-orbital model [57], which is comprised
of a forbidden lower energy Q (ΔML = ±9) transition and an allowed B (ΔML = ±1) transition at
higher energy. Spectral band deconvolution analyses have demonstrated that two intense
overlapping B1 and B2 bands are present in the B band region of phthalocyanine spectra, due to
configurational interaction with -MOs that are associated with the peripheral fused benzene rings
[58-60]. Michl [51-55] introduced a terminology in which π-MOs derived from the HOMO and
LUMO of the parent C16H162− species that have an angular node aligned with the y-axis as the a and
–a MOs, respectively, whereas analogous MOs that lie on antinodes are referred to as the s and –s
MOs (Fig. 2), since this facilitates a comparison of porphyrinoid derivatives of different symmetry,
such as 1 and H2-1.
Derivative-shaped Faraday A1 terms dominate the MCD spectra of metal phthalocyanine
complexes, such as 1, that have a four-fold axis of symmetry, since the excited states are orbitally
degenerate [44-46,54-56]. When the symmetry of the phthalocyanine ligand is lowered so there is
no three-fold or higher symmetry axis, as is the case with the free base ligand of 1 (H2-1), coupled
pairs of Gaussian-shaped oppositely-signed B0 terms become the dominant spectral feature. Since
the A1 term at 735 nm in the spectrum of 1 corresponds to the absorption band at 738 nm, this
band can be readily assigned to the Q transition. In a similar manner, the A1 term and absorption
band at 322 nm can be assigned to the B transition, slightly to the red of where they are predicted
to lie in the TD-DFT calculation (Fig. 1). Metal-to-ligand charge-transfer bands of 1 are predicted to
lie to the red of the Q band, and other charge-transfer bands are predicted in the B band region
(Figs. 1 and 2). There is a relative destabilization of the a MO (the HOMO), because there are
larger MO coefficients at the non-peripheral positions on the fused benzene rings than is the case
with the s, –a and –s MOs. This leads to a narrowing of the HOMO−LUMO gap of H2-1 relative to
the parent unsubstituted H2Pc (Fig. 2), and explains why the Q bands are red-shifted to beyond 700
nm (Fig. 1). The blue shift of the Q band of 1 compared to those of H2-1 (Fig. 1) is related to mixing
between the LUMO of the Pc π-system (Fig. 2) and the occupied 3dxz and 3dyz (3d) orbitals for
symmetry systems [57].
3.3. Electrochemical measurements
Electrochemical measurements were carried out in DMF to provide further insight into the
electronic properties of 1, since it is well established that the HOMO and LUMO energies of

phthalocyanine derivatives correlate closely with their first oxidation and reduction potentials when
the central metal is not electroactive [46,61-65]. The electron-donating alkoxy substituents were
expected to increase the electron density on the phthalocyanine π-system, which leads to more
difficult reduction and easier oxidation, and hence in the formation of stronger reducing agents
when reduced anionic species are formed. The cyclic voltammograms of 1 contain two reversible
oxidation steps at +0.54 and +0.91 V, and two reversible reduction processes at −0.42 and −1.53 V
that can be assigned, respectively, to metal-centered reduction [CoIIPc(−2)]/[CoIPc(−2)]− and π-anion
formation [CoIPc(−2)]−/[CoIPc(−3)]2− steps [66-69]. Additional irreversible reduction peaks are
observed following the [CoIIPc(−2)]/[CoIPc(−2)]− reduction step upon addition of 0.0−2.0 eq of DDT
as indicated by an asterisk in the cyclic voltammograms (Fig 3, left), which can be assigned to
irreversible reductions of one or more of the σ-bonded [CoIIIPc(−2)]+ species that are generated
from the reaction between [CoIPc(−2)]− and DDT. The irreversible peak potentials are about
500−600 mV more negaꢀve than the reversible [CoIIPc(−2)]/[CoIPc(−2)]− step at −0.42 V. The
increase in the cathodic peak current for the [CoIPc(−2)]−/[CoIPc(−3)]2− process upon addition of DDT
is more significant than for the CoIIPc(−2)]/[CoIPc(−2)]− process (Fig. 3, right). This indicates that
the reactivity of the [CoIPc(−3)]2− species towards DDT is significantly higher.
3.4. Spectroelectrochemical measurements
Spectroelectrochemistry can be used to monitor the chemical reactions between reduced
metalloporphyrinoids and organochlorides, which produce metal-carbon bond derivatives as the
key intermediates [70-72]. The spectral changes which occur during the first two reduction steps
of 1 in the absence and presence of DDT are shown in Fig. 4. Identical spectral changes are
observed during the first one-electron reduction step in the presence and absence of DDT. When
the potential was set at E = −1.80 V, the spectral changes in the absence of DDT exhibits a decrease
of the absorption intensity in both B and Q band regions, along with an increase in the intensity of
the absorption bands at 463 and 578 nm, which is characteristic of the formation of a [CoIPc(−3)]2−
species [73,74]. After 5.0 eq of DDT is added, significant differences are clearly observed. The
final UV-visible absorption spectrum displays similar decreases in the intensity in the B and Q band
regions, with increased absorbance observed in the visible region. This is consistent with the cyclic
voltammetric properties, since a catalytic reaction involving DDT and the electrogenerated
[Co(I)Pc(−3)]2− species occurs after the second reduction step. Controlled-potential bulk
electrolyses were carried out in 2.0 × 10−4 M DMF solutions of 1, 0.1 M TBAP, 15.0 eq of DDT (3.0 ×
10−3 M) and 50 μL of trimethylamine, in order to determine the C−Cl bond cleavage products during
electrocatalysis. The bulk electrolysis potential was set at E = −1.85 V, which is sufficient to
generate the doubly reduced [CoIPc(−3)]2− species and also to reduce the homogeneously generated
σ-bonded CoIIIPc intermediates. Since decomposition components of the TBAP supporting
electrolyte are formed during high temperature gas chromatography (GC), it was necessary to first
remove the supporting electrolyte to prevent blockage of the column by following procedures
described in the experimental section.

3.5 Electrochemical catalyzed carbon-chlorine bond cleavage
According to previously reported electrocatalysis results [23], the C−Cl bond cleavage of DDT is
initially achieved through C(sp3)−Cl bond cleavage with a roughly 1:1 ratio in the formation of
(1,1-bis(4-chlorophenyl)-2,2-dichloroethane) (DDD) and
(1,1-bis(4-chlorophenyl)-2,2-dichloroethylene) (DDE). Subsequently,
(1,1-bis(4-chlorophenyl)-2-chloroethylene) (DDMU) is slowly formed through C(sp3)−Cl bond cleavage
of DDD or C(sp2)−Cl bond cleavage of DDE. DDMU is usually the main ultimate degradation product
with only trace amounts of further C−Cl bond cleavage products being formed, due to the low
reactivity of the highly stable C(sp2)−Cl and aromatic C(sp2)−Cl bonds. In contrast, when 1 is used as a
catalyst, the initial C(sp3)−Cl bond cleavage reaction is extremely fast and DDE is obtained with high
selectivity (Fig. 5 and Scheme 2). DDE is subsequently completely converted to DDMU through
C(sp2)−Cl bond cleavage. The further C(sp2)−Cl bond cleavage of DDMU was fully complete within 4 h
and yields dichlorophenylethylene (DPECl2) and its stable aromatic C(sp2)−Cl bond cleavage products
(Fig. 5, middle). This C−Cl bond cleavage system provides the most saꢀsfactory result that has
been reported to date for DDT dechlorination, since the reaction does not stop at the DDMU step.
After electrolysis for 6 h, all of the products containing even a single aromatic C(sp2)−Cl bond were
completely degraded. Diphenylethylene (DPE), the full dechlorination product of DDT, was
obtained for the first time (Scheme 2) and DPE was completely reduced to diphenylethane (DPEA) in
ca. 57% yield. The obvious explanation for this is that the incorporation of electron-rich alkoxy
groups at the non-peripheral positions results in an unusually electron-rich Pc ligand and hence
forms a very strong reducing agent during electrocatalysis, since there is a significant interaction
between the LUMO of the phthalocyanine π-system and the 3dxz and 3dyz orbitals of the central
Co(II) ion (Fig. 2) [57].
Surprisingly, when 1 is used as the catalyst in DMF, products resulting from C−C bond cleavage
reactions are also observed (Fig. 5, right). The formation of dichlorophenylmethane (DPM) can
readily be explained, since the key intermediate, dichlorophenylcarbon anion, reacts with water
during the quenching of the reaction (Scheme 3). When heavy water (D2O) was used deuterated
dichlorophenylmethane-d (DPM-d) was identified by GC-MS analysis. The formation of
p,p’-dichlorobenzophenone (BPCl2) as a C−O bond formation product probably results from a
reaction between the p,p’-dichlorophenylcarbon anion and oxygen gas, since trace levels of oxygen
gas may enter from the working electrode compartment or may be present as a trace impurity in
the nitrogen gas that is used during the electrolysis. The C−C bond cleavage reaction is a rapid one,
whereas the C−O formation occurs on a longer timescale. It is noteworthy that
p-chlorobenzophenone (BPCl) and benzophenone (BP), the partial and full C−Cl bond cleavage
products for p,p’-dichlorobenzophenone (BPCl2) (Scheme 2), were also obtained in significant yields.
The use of Co(II)-α,α’-(OC5H11)8Pc, as a catalyst for the electroreduction of DDT, results in highly
efficient C−Cl bond cleavage and provides the first example of the full dechlorination of DDT
through molecular electrochemical catalysis. It should be emphasized, however, that similar results

for the electroreduction of carbon-halide bonds have recently reported by Gennaro et al. through
the use of a silver cathode [75] and that the complete dechlorination of DDT had already been
achieved previously through the use of other metal electrodes [19,31,76]. GC-MS analysis
identified the formation in high yield of bis(p-chlorophenyl)methanone (BPCl2), along with
diphenylmethanone (BP) upon full dechlorination, due to C−C bond cleavage followed by C−O bond
formation, which has not been reported previously during the electrocatalysis of DDT. The highly
efficient C−Cl bond cleavage reported in this study has a broad range of potential applications in
both chemistry and environmental remediation.
3.6. Reaction process and mechanism study
To confirm the reaction process and the mechanisms for the C−Cl and C−C bond cleavages,
electrochemical and spectroelectrochemical measurements and controlled-potential bulk
electrolyses of DDD, DDE and DDMU were carried out in the presence of 1 under identical
conditions. There are significant differences in the electrochemical measurements upon addition
of 0 − 2 eq of DDD, DDE and DDMU, Fig. 6. Irreversible reduction peaks are observed following the
[CoIIPc(−2)]/[CoIPc(−2)]− reduction step upon addition of DDD in DMF, but not for DDE and DDMU.
This demonstrates that the carbon atom s-p hybridization of the C−Cl bond and the local
environment of the chlorine atom significantly influence the electrochemical behavior, so the C−Cl
bond cleavage process can be separated into a series of different reactions. When 5 eq of DDD,
DDE and DDMU are added to solutions of 1 in DMF, the observed spectral changes (Fig. 7) of DDD
are similar to those obtained with DDT, but those observed with DDE and DDMU are not. In the
case of DDD, diagnostic decreases are observed in the B and Q band regions at E = −1.85 V, along
with an increase in absorbance at 470 nm. In contrast, for DDE and DDMU, an increase in intensity
at 572 nm was observed. This indicates that cleavage of the C(sp3)−Cl and C(sp2)−Cl bonds of DDT
can be readily distinguished by spectroelectrochemical measurements. Time-dependent GC
analysis of the controlled-potential bulk electrolysis of DDE (Fig. 8) demonstrates that a C(sp2)−Cl
bond cleavage reaction initially yields DDMU, and then an aromatic C(sp2)−Cl bond cleavage forms
DPECl2, DPECl and DPE (Scheme 2), which in turn are converted to diphenylethane (DPEA) in
approximately the same ratio as that observed during the electrolysis of DDT. At the same time,
the C−C bond cleavage process also results in the formation of chlorobenzophenone and
benzophenone (BP). In the case of DDMU (Fig. 9), the time-dependent changes in the C−Cl bond
cleavage reaction products are quite similar to those for DDT and DDE with the final conversion to
DPEA occurring within a shorter time, but in contrast the final product yields for C−C bond cleavage
decrease significantly to less than 4.0%. This can be readily explained, since DDMU is less
electronegative due to the lower number of chlorine atoms, making the initial nucleophilic attack
more difficult. These results confirm that the C−C bond cleavage reacꢀon mainly occurs due to the
C−Cl bond cleavage of DDE, and are consistent with the proposed mechanism for the C−Cl bond
cleavage of DDT (Scheme 2). On the other hand, both C−Cl and C−C bond cleavage products were
observed in the case of DDD (Fig. 10), but there is a significant difference in the product

distributions. Since the fraction of DDD formation is relatively small, the influence of DDD in this
system can usually be omitted from consideration.
4. Conclusions
In summary, the use of an electron-rich cobalt(II)phthalocyanine, Co(II)-α,α’-(OC5H11)8Pc, as a
catalyst for the electroreduction of DDT, results in highly efficient C−Cl bond cleavage and provides
the first example of the full dechlorination of DDT through molecular electrochemical catalysis,
rather than the use of metal electrodes which had been achieved previously. Time-dependent GC
analyses have been used to identify the degradation products so that the reaction kinetics and
mechanism can be studied in detail. This identified the formation in high yield of
bis(p-chlorophenyl)methanone (BPCl2), along with diphenylmethanone (BP) upon full dechlorination,
due to C−C bond cleavage followed by C−O bond formaꢀon, which has previously not been reported
during the electrocatalytic reduction of DDT. The incorporation of electron-rich alkoxy groups at
the non-peripheral positions results in an unusually electron-rich Pc ligand and hence forms a very
strong reducing agent during electrocatalysis, since there is a significant interaction between the
LUMO of the π-system and the 3d orbitals of the central metal ion. The highly efficient C−Cl bond
cleavage reported in this study has a broad range of potential applications in both chemistry and
environmental science. This study should also provide information that helps to guide rational
design of other porphyrinoid complexes that are suitable for use as electrocatalysts in organic
synthesis and in the remediation of environmentally problematic organochlorides.
Supplementary Material. Details of the TD-DFT calculations are provided.
Acknowledgements
Financial support in China were provided by National Natural Science Foundation of China (No.
21171076), Natural Science Foundation of Jiangsu Province, P. R. China (No. BK200160449) to X. L.
and W. Z., and NRF of South Africa CSUR grant (uid: 93627) to JM. The theoretical calculations were
carried out at the Centre for High Performance Computing in Cape Town.

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Schemes
Scheme 1. Synthesis of Co(II)-α,α’-(OC5H11)8Phthalocyanine 1.

e-
e-
[Co(I)Pc]-
[Co(III)Pc]+
[Co(I)Pc]- +
Co(II)Pc
Cl
Ph
Cl
Cl
Pc(III)Co
Cl
Cl
Ph
Cl
Cl
- Cl-
Cl
Ph
H
Ph
[Co(I)Pc]-
Cl Ph
Cl
Ph
Cl
Ph
Cl
Cl
Cl
Cl
Ph
DDT
Cl
Cl
Ph
Cl
Pc(II)Co
Cl
Ph
Cl
Pc(II)Co
Cl
Cl
Cl
- H+
+ e-
- Co(II)Pc
Ph
Cl
Ph
Cl
DDE
Cl
Cl
Ph
Cl
Ph
Cl
Ph
- [Co(III)Pc]+
+ H+
Cl
Cl
Cl
Pc(III)Co
Cl
[Co(I)Pc]-
- Cl-
H
Ph
Cl
Ph
Cl
Ph
Cl
DDMU
DDE
Cl
Ph
Cl
Ph
Cl
Ph
- [Co(III)Pc]+
+ H+
H
H
Cl
Pc(III)Co
H
- Cl-
[Co(I)Pc]-
Ph
Cl
Ph
Cl
Ph
Cl
H
DPECl2
DDE
H
Ph
Cl
Ph
Co(III)Pc
Ph
- [Co(III)Pc]+
H
H
H
H
H
- Cl-
[Co(I)Pc]-
[Co(I)Pc]-
+ H+
H
Ph
Cl
Ph
Cl
Ph
Cl
DPECl
DPECl
H
Ph
Cl
Ph
Co(III)Pc
Ph
- [Co(III)Pc]+
H
H
H
H
H
H
- Cl-
+ H+
Ph
H
Ph
Ph
H
Ph
H
Ph
+ 2e-
H
H
H
H
Ph
Ph
DPEA
H
H
Scheme 2. Proposed carbon-chlorine bond cleavage procedure of DDT in DMF during the
electrochemical reduction process with 1.

D
+ D2O
+ H2O
+ O2
Ph
Ph
Cl
Cl
DPM-d
Cl
Ph
Cl
Ph
Cl
Ph
C-C Bond
Clevage
H
Cl
Pc(III)Co
Cl
Cl
[Co(I)Pc]-
+
Ph
Ph
- [Co(III)Pc]+
Cl
Ph
Cl
Cl
Cl
Ph
Cl
Cl
Cl
Ph
Cl
DPM
O
Ph
Ph
BPCl2
O
O
O
O
- [Co(III)Pc]+
+ H+
- Cl-
[Co(I)Pc]-
Cl
Ph
Ph
Ph
Cl
Ph
Ph
Ph
Ph
Ph
Pc(III)Co
Cl
Cl
Cl
H
Cl
BPCl
O
O
O
- [Co(III)Pc]+
+ H+
- Cl-
[Co(I)Pc]-
Ph
Ph
Ph
Ph
Ph
Ph
Pc(III)Co
H
H
H
BP
Cl
H
Scheme 3. Proposed C−C bond cleavage and C−O bond formation in DMF during the
electrochemical reduction process with 1.

Figures
Fig. 1. Magnetic circular dichroism (TOP) and electronic absorption spectra (BOTTOM) of 1 (RIGHT)
and its non-metallated precursor (H2-1, LEFT) in CH2Cl2. A secondary axis is used for the TD-DFT
spectra. The Q and B1 bands that are associated with Gouterman’s 4-orbital model [56] are
highlighted with red diamonds. Smaller black, green, gray and purple diamonds are used to
differentiate the nπ* bands, the remaining ππ* bands, bands that are associated with π-MOs that
have large MO coefficients on the peripheral fused benzene rings, and transitions that are
associated with the 3d MOs of the central Co(II) ion, respectively.

Fig. 2. Angular nodal patterns of the a, s, -a and -s MOs of H2-1 (TOP) and 1 (CENTER) at an
isosurface value of 0.02 a.u. MO energies (BOTTOM) of the parent non-metallated phthalocyanine
with no peripheral substituents (H2Pc), 1 and its non-metallated precursor (H2-1) at the B3LYP/SDD
level of theory (bottom). The occupied MOs are highlighted with black diamonds. The a, s, −a
and −s MOs are highlighted with thicker gray lines, while green lines denote MOs that are largely
associated with the 3d atomic orbitals of the Co(II) ion. Blue lines denote π-MOs that have large
MO coefficients on the peripheral fused benzene rings, and hence are most likely to be destabilized
through a mesomeric interaction with the electron-donating alkoxy substituents. The
HOMO−LUMO gaps are ploꢁed against a secondary axis and are highlighted with gray diamonds.

Fig. 3. Left: Reductive electrochemical measurements for 1 upon addition of 0.0−2.0 eq of DDT in
DMF containing 0.1 M TBAP. Additional irreversible reduction peaks are observed following the
addition of DDT that are indicated with an asterisk. Right: Plots of ip−i vs. the ratio of [DDT]/[1] in
DMF containing 0.1 M TBAP.

Fig. 4. Reductive spectroelectrochemical measurement for 1 at an applied potential at E = −1.85 V in
the presence (top) or absence (bottom) of DDT in DMF containing 0.1 M TBAP.

Fig. 5. The time-dependent distribution of DDT and its electrocatalyzed C−Cl bond cleavage products
identified by GC during controlled-potential electrolysis with 1 at E = −1.85 V in DMF containing 0.1
M TBAP.

Fig. 6. Reductive cyclic voltammetry measurements for 1 in the presence of 0.0−2.0 eq of DDD (leꢂ),
DDE (middle) and DDMU (right) in DMF containing 0.1 M TBAP. Additional irreversible reduction
peaks are observed following the addition of DDT and these are indicated by asterisks.

Fig. 7. In situ spectroelectrochemistry of 1 in the absence and presence of 5.0 eq of DDT (purple),
DDD (blue), DDE (pink) and DDMU (green) in DMF containing 0.1 M TBAP.

Fig. 8. The time-dependent distribution of DDE and its electrocatalyzed C−Cl bond cleavage
products identified by GC during controlled-potential electrolysis with 1 at E = −1.85 V in DMF
containing 0.1 M TBAP.

Fig. 9. The time-dependent distribution of DDMU and its electrocatalyzed C−Cl bond cleavage
products identified by GC during controlled-potential electrolysis with 1 at E = −1.85 V in DMF
containing 0.1 M TBAP.

Fig. 10. The time-dependent distribution of DDD and its electrocatalyzed C−Cl bond cleavage
products identified by GC during controlled-potential electrolysis with 1 at E = −1.85 V in DMF
containing 0.1 M TBAP.