Reductive dechlorination of atrazine using sodium-borohydride catalysed by cobalt(II) phthalocyanines

Article abstract of DOI:10.1016/j.tetlet.2011.10.080

A series of functional metallophthalocyanines have been synthesized to study their role as a catalyst towards the reductive dechlorination of atrazine using sodium borohydride as a mild reducing agent. The cobalt(II) phthalocyanine bearing nitro groups at the peripheral position is the most efficient catalyst with exceptionally high catalytic activity in comparison to other functional cobalt(II) phthalocyanines.

Full text of DOI:10.1016/j.tetlet.2011.10.080

Tetrahedron Letters 52 (2011) 7083–7086
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Reductive dechlorination of atrazine using sodium-borohydride catalysed
by cobalt(II) phthalocyanines
⇑
Poonam, Pratibha Kumari, Sohail Ahmad, Shive Murat Singh Chauhan
Bio-organic Research Laboratory, Department of Chemistry, University of Delhi, Delhi 110007, India
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of functional metallophthalocyanines have been synthesized to study their role as a catalyst
towards the reductive dechlorination of atrazine using sodium borohydride as a mild reducing agent.
The cobalt(II) phthalocyanine bearing nitro groups at the peripheral position is the most efficient catalyst
with exceptionally high catalytic activity in comparison to other functional cobalt(II) phthalocyanines.
Ó 2011 Elsevier Ltd. All rights reserved.
Received 23 September 2011
Revised 13 October 2011
Accepted 16 October 2011
Available online 23 October 2011
Keywords:
Metallophthalocyanines
Atrazine
Sodium borohydride
Reductive dechlorination
Degradation of highly persistent pesticides into environmen-
tally compatible products has been a worldwide environmental
challenge. The widespread use of pesticides has led to public health
concerns, increases the resistance of pests against various pesti-
cides and results in the contamination of water, air and soil. Atra-
zine (2-chloro-4-ethylamino-6-isopropylamino-triazine) is used as
a selective herbicide to control the grassy and broadleaf weeds in
corn, rangeland, sorghum and sugarcane crops.1 Atrazine is a well
recognized recalcitrant pollutant due to its slow degradation by
physico-chemical or biological methods. The presence of atrazine
even at very low concentrations is highly toxic for mammalian as
well as aquatic organisms and is identified as a mammalian carcin-
ogen and endocrine disrupter.2
In recent years, major efforts have been investigated in the
development of effective methods to detoxify this contaminant
present in the environment. The oxidative degradation of atrazine
has been studied by Fenton’s systems3,4 UV, UV–H2O2,5 UV–O3,6
UV–TiO2,7,8 metal-catalysed ozonation. Biodegradation of atrazine
via microbial9 and enzyme-catalysed10,11 reactions are another
way to detoxify it. Reductive degradation involving the use of
zero-valent iron12,13 is an additional approach for dechlorination
of atrazine. Porphyrin and phthalocyanine complexes are also
known to degrade atrazine by using titanium citrate as reducing
agent,14 and by photocatalytic15 and electrocatalytic methods.16
Although all the above methods transform atrazine into less toxic
dechlorinated or dealkylated nonherbicidal products, similar trans-
formations to detoxify these harmful chlorinated agrochemicals
with mild reducing agents by metallophthalocyanines are still
unexplored.
As far as environmental applications are concerned, metallopht-
halocyanines and metalloporphyrins have been used as a catalyst
for a variety of eco-friendly reactions17–20 which are effective in
transforming harmful environmental pollutants into biodegrad-
able moieties. The use of metallophthalocyanines as reduction pro-
moting catalysts20 under milder conditions stimulated us to
investigate their activity towards the reductive dechlorination of
atrazine in various conditions. Among the numerous available
metallophthalocyanines, those bearing cobalt have been demon-
strated to be effective catalysts for reductive mechanistic ap-
proach. Accordingly, the aims of the present study were to
examine the catalytic reductive transformation of atrazine by dif-
ferent functional metallophthalocyanines, and to compare the
use of mild, cheap and commercially available reducing agent so-
dium borohydride and metallophthalocyanines on the reaction
rate and yield of the product.
The reaction of atrazine (1) with sodium borohydride in metha-
nol catalysed by metallophthalocyanines (3) was performed at room
temperature under different reaction conditions (Scheme 1). The
functional metallophthalocyanines (MPcs) (3a–3h) were synthe-
sized by the cyclocondensation of the corresponding phthalonitriles
in the presence of metal salts under different reaction conditions in
ionic liquids.21 The reduction of atrazine (1) with NaBH4 in the ab-
sence of MPcs (3) did not yield any product even after stirring the
reaction mixture for 24 h at room temperature (Table 1, entry 1).
However, the reaction of 1 (1 mmol) with NaBH4 (4 mmol) catalysed
by (3a) (0.1 mmol) in methanol gave the dechlorinated product, N2-
ethyl-N4-isopropyl-1,3,5-triazine-2,4-diamine (2) in a 98% yield
⇑
Corresponding author. Tel./fax: +91 11 27666845.
E-mail address: smschauhan@chemistry.du.ac.in (S.M.S. Chauhan).
0040-4039/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2011.10.080

7084
Poonam et al. / Tetrahedron Letters 52 (2011) 7083–7086
Cl
N
N
N
N
MPc,NaBH4
MeOH,RT
NH
NH
HN
C2H5
HN
C2H5
N
N
CH(CH3)2
CH(CH3)2
1
2
R
R
N
a)R=H,M=Co(II)
b) R=H,M=Fe(III)Cl
N
N
N
N
N
c)
R=H,M=Ni(II)
d) R=CH3,M=Co(II)
e)
M
N
R=OCH3,M=Co(II)
f) R=NO2,M=Co(II)
g) R=OC8H17,M=Co(II)
h)
N
R=CH(CH3)3,M=Co(II)
R
R
3(MPc)
Scheme 1. Dechlorination reaction of atrazine catalysed by MPc using NaBH4.
Table 1
Dechlorination of 1 to 2 with NaBH4 catalysed by cobalt(II) phthalocyanine (3a) under different reaction conditionsa
Entry
System
1: 3a: NaBH4
Time (min)
Conversion (%)b
Yieldc (%)
1
2
3
4
5
6
7
8
9
1/3a/NaBH4
1/3a/NaBH4
1/3a/NaBH4
1/3a/NaBH4
1/3a/NaBH4
1/3a/NaBH4
1/3a/NaBH4
1/3a/NaBH4
1/3a/NaBH4
1:0:8
360
30
30
30
30
30
10
60
30
0
64
14
0
59
11
42
95
96
28
98
95
1:0.01:4
1:0.1:1
1:0.1:2
1:0.1:4
1:0.1:8
1:0.1:4
1:0.1:4
1:0.1:4
46
>99
100
34
100
>98
d
a
b
c
Reaction conditions: reactions were performed at room temperature in methanol.
Based on substrate (1).
Isolated yields.
d
The reaction carried out in dark.
(Scheme 1) (Table 1, entry 8). The product was separated by column
chromatography.
entries 2 and 5). The yields of the dechlorinated product also de-
pend upon the reaction time. As the time duration of the reaction
increased the yield of the product also increased. Only a 28% yield
of 2 was obtained when reaction was carried out for 10 min, while
the yield of the same increased to 95% and 98% when the time in-
creased to 30 and 60 min, respectively (Table 1, entries 5, 7 and 8).
The rate of the dechlorination reaction was observed to be same
when the reaction was carried out in dark (Table 1, entry 9).
The effects of metal atom present at the core of the phthalocy-
anine ring also govern the yield of the reaction. The change of the
central metal atom of phthalocyanine from Co to Ni and Fe showed
decrease in the catalytic activities giving the 95%, 64% and 48%
yields, respectively (Table 2, entries 1–3). This could be explained
on the basis of difference in the redox potential of MPcs with the
change in central metal atom.23 The central metal changes the
electronic state of the Pc ring, thereby causing a change in the oxi-
dation potential of MPc. Further, the interaction with groups above
and/or below the Pc plane produces significant changes in redox
potentials and this may be responsible for the least catalytic activ-
ity of Fe(III)Cl-Pc (3b).23b The effect of nature of substituents at-
tached to phthalocyanines ring on the course of reductive
dechlorination process was also examined. The cobalt(II) phthalo-
cyanines bearing electron-withdrawing nitro groups (3f) exhibited
better catalytic potential than that bearing electron donating
groups (3d) (Table 2, entries 4 and 6). However, the poor yield of
2 in the reduction reaction using 3g as the catalyst could be attrib-
uted to self-aggregation tendencies of functionalized phthalocya-
nines in the solution phase24 (Table 2, entry 7). Exceptionally
The melting point of 2 (181 °C) was compared with the melting
point of N2-ethyl-N4-isopropyl-1,3,5-triazin-2,4-diamine (181–
182 °C) reported in the literature.22 In the 1H NMR spectrum, the
appearance of new peak as a singlet at d 7.93 ppm corresponding
to the aromatic proton and no major changes in the resonance
for other protons confirmed the dechlorination of atrazine to aro-
matic C–H bond in 2. In 13C spectrum, the appearance of peaks
for aromatic carbons in the region d 164.89, 164.89, 165.54 and
the peaks for ethyl and isopropyl group carbons attached to –NH
in the aliphatic region at resonance d 14.68, 22.58, 22.89, 35.35,
42.18 again confirmed no changes in the aliphatic region. The
ESI–MS spectrum of 2 showed peak corresponding to its molecular
mass m/z 181.1654 further confirmed the structure of the dechlo-
rinated product (see Supplementary data).
To study the effect of concentration of sodium borohydride on
the course of reaction, the reduction of 1 was carried out with dif-
ferent molar ratio of sodium borohydride using 3a as catalyst in
methanol at room temperature for 4 h, shown in Table 1. The in-
crease in the amount of NaBH4, that is, the substrate: catalyst:
NaBH4 ratio from 1:0.1:1 to 1:0.1:4, increases the yield of product
from 11% to 95%. On further increasing the ratio up to 1:0.1:8, no
major change in the yields was observed (Table 1, entries 3–6).
The concentration of the catalyst also played a major role in the
yield of the dechlorinated product, with the increase in molar ratio
of substrate: catalyst: NaBH4 from 1:0.01:4 to 1:0.1:4, a substantial
increase in the yield from 59% to 95% was observed (Table 1,

Poonam et al. / Tetrahedron Letters 52 (2011) 7083–7086
7085
Table 2
reaction. The structure of 4 was differentiated from the atrazine
by the Rf difference in the TLC (thin layer chromatography) and
confirmed by 1H NMR spectroscopic and mass spectrometric anal-
ysis. In 1H NMR spectrum, the absence of peak at d 7.93 for aro-
matic proton clearly indicates the presence of deuterium at
aromatic ring. The LC–MS spectrum of structure 4 showed the peak
at molecular mass m/z 182.2 for [M+] which further confirmed the
replacement of hydrogen by deuterium (Fig. 5 refer Supplemen-
tary data).
Dechlorination of 1 to 2 with NaBH4 catalysed by different functional metallopht-
halocyanines (3) and metal salts under different reaction conditionsa
Entry
Catalyst
Time (min)
Conversion (%)b
Yieldc (%)
1
2
3
4
5
6
7
8
9
3a
3b
3c
3d
3e
3f
3g
3h
30
30
30
30
30
30
30
30
30d
60
60
60
98
55
70
84
49
>99
39
100
94
58
48
95
48
64
77
42
98
32
98
90
54
42
26
The reductive dehalogenation of atrazine was initiated by elec-
tron transfer from sodium borohydride to the central metal of the
phthalocyanine, due to its ability to take electron reversibly in the
D2z orbitals, leading to the formation of [Co(I)Pc] which is charac-
terized by UV–vis spectral studies of Co(II) Pc on addition of NaBH4
in methanol. In UV–vis spectral analysis, on addition of methanolic
solution of sodium borohydride to the solution of Co(II) Pc in meth-
anol, the Q band of Co(II) Pc showed a red shift from 662 to 705 nm
with the appearance of new band at 464 nm, specifying the forma-
tion of [Co(I)Pc] in the reaction.20,26 The reaction of iron porphyrins
with tetrabutylammonium borohydride has been known to gener-
ate covalently bound metal tetrahydroborate complexes in non-
polar solvents.27 However, the polar solvents are not suitable for
the generation of tetrahydroborate complexes. Thereby in polar or-
ganic solvents, alkylation of soluble [Co(I)Pc] with atrazine gives
atrazine-Co(III)Pc as an intermediate by leaving chloride ion. This
addition is in accordance with the alkylation of cobalt complexes
with halogenated compounds28–31 which was further supported
by UV–vis spectroscopic titrations. The UV–vis titration of Co(II)Pc
with sodium benzophenone ketyl as a reducing agent generates
Co(I) species which on subsequent addition of methyl iodide reacts
to give the bands corresponding to the formation of methyl-Co(III)
Pc species20b,30a (Fig. 6 refer Supplementary data). The homolytic
cleavage of labile Co–C bond29,32 of alkyl-Co(III)Pc gives alkyl rad-
ical and Co(II) Pc.28 Accordingly, Co–C bond of atrazine-Co(III) Pc
moiety was homolytically cleaved to form atrazine radical and
regenerates Co(II) Pc that can be reused several times without
any loss in its catalytic activity. Deuterated studies indicate that
the hydrogen replacing the –Cl group of atrazine comes from
methanol, based on these results it has been proposed that the
atrazine radical takes an electron from sodium borohydride and
subsequently abstract a proton from methanol to give 2.33,34
In summary, the reductive dechlorination of atrazine with so-
dium borohydride catalysed by metallophthalocyanines (3) gave
the reduced product in excellent yields in shorter reaction time.
The cobalt(II) phthalocyanines are catalytically more efficient than
other metal phthalocyanines. The cobalt(II) phthalocyanine bear-
ing electron withdrawing group at the peripheral position gave
the reduced product in a 98% yield. Further, the present method of-
fers various advantages such as the use of cheap and commercially
available reducing agent, milder reaction conditions, easy work-up
and the reusability of catalyst.
3a
10
11
12
CoCl2
NiCl2
FeCl3
33
a
Reaction conditions: Reactions were performed at room temperature in
methanol; substrate: catalyst: NaBH4 = 1:0.1:4.
b
Based on substrate (1).
Isolated yields.
Catalyst was reused.
c
d
high catalytic activity of cobalt(II) phthalocyanine bearing t-butyl
group (3h) is primarily due to the nonaggregating property of 3h
in the solution phase25 (Table 2, entry 8). After the reaction, the
catalysts MPc (3) were quantitatively recovered and used again
for reductions without any loss in catalytic activity (Table 2, entry
9), even though partial decomposition of 3g was observed during
the reduction and could not be reused. The catalyst 3h could not
be separated by direct filtration because of its good solubility in
methanol, limiting its applicability in reductions. However, 3h
could be recovered quantitatively by column chromatography.
The use of metal salts such as CoCl2, NiCl2 and FeCl3 was sluggish
giving the dechlorinated product 2 in 54%, 42% and 26% yields,
respectively (Table 2, entries 10–12) even for a longer reaction
time. Moreover, these catalysts decomposed after the reductions
and could not be reused. According to the results obtained the
reductions using MPc 3 as catalysts in the place of metal salts
are more advantageous and apparently proceed through different
pathway. The present study suggested the outstanding role of met-
allophthalocyanines as a catalyst and provides an opportunity to
examine the mechanism for the reductive dechlorination reactions
of cobalt(II) phthalocyanine.
In the reductive dechlorination of atrazine, the reaction of atra-
zine (1) using sodium borodeuteride (NaBD4) in methanol (CH3OH)
gave 2 whereas, the reaction of atrazine (1) using sodium borohy-
dride (NaBH4) in deuterated methanol (CD3OD) gave 4 (Scheme 2).
The formation of 2 and 4 in the reactions justifies that the hydro-
gen which replaces the –Cl group present at the aromatic ring in
dechlorinated product 2 came from methanol in the reduction
H
Acknowledgment
CoPc, NaBD4
N
N
MeOH
We gratefully acknowledge the financial assistance from the
Council of Scientific & Industrial Research (CSIR), New Delhi.
R2HN
N
NHR1
a
b
Cl
2
N
N
Supplementary data
R2HN
N
NHR1
D
1
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.tetlet.2011.10.080.
CoPc, NaBH4
MeOD
N
N
R1= CH2CH3
R2= CH(CH3)2
R2HN
N
NHR1
References and notes
4
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Scheme 2. Dechlorination of atrazine, (a) with NaBD4 in MeOH, (b) with NaBH4 in
MeOD.

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Poonam et al. / Tetrahedron Letters 52 (2011) 7083–7086
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borohydride catalysed by metal phthalocyanines (3) in methanol: In a two-necked
50 ml round bottom flask, fitted with a nitrogen inlet tube and a condenser.
Atrazine (1) (2.32 mmol) and cobalt(II) phthalocyanine (3a) (0.232 mmol)
were added in methanol under nitrogen atmosphere. When the atrazine was
completely dissolved, sodium borohydride (13.95 mmol) was added to the
reaction mixture slowly within a period of 10 min. The reaction mixture was
stirred at room temperature for 30–60 min. The progress of the reaction was
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2
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phthalocyanines and metal salts in reductive dechlorination of atrazine. Data
for N2-ethyl-N4-isopropyl-1,3,5-triazin-2,4-diamine (2): mp: 181–182 °C (lit.22
Poonam,
; Kumari, P.; Nagpal, R.; Chauhan, S. M. S. New J. Chem. 2011.
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mp 182 °C), IR (KBr, m
/cmÀ1): 3488, 3342, 3178, 3116, 2925, 2851, 1684, 1638,
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1588, 1527, 1300, 1180, 1085, 994, 916, 796, 638, 561, 455; 1H NMR (400 MHz,
CDCl3, d): 7.93 (1H, s, Ar), 6.09 (1H (NH), m, (br), 5.09 (1H (NH), d (br)), 4.12
(1H, s, (br), CH), 3.38 (2H, s, (br), CH2), 1.17 (9H, s, CH3); 13C NMR (75 MHz,
CDCl3, d) 14.68 (CH3), 22.58 (CH3), 22.89 (CH3), 35.35 (CH2), 42.18 (CH), 164.38,
164.89, 165.54 (Ar–C); ESI-MS: m/z calcd for C8H15N5 181.1327, observed:
181.1654 [M+]. Data for (4): mp: 178–180 °C 1H NMR (400 MHz, CDCl3, d): 5.72
(1H (NH), m (br), 5.01 (1H (NH), d (br)), 4.14 (1H, s, (br), CH), 3.41 (2H, s, (br),
CH2), 1.18 (9H, s, CH3). LC–MS: m/z calcd for C8H14DN5 182.2, observed: 182.2
[M+].
34. Spectral data for reduced compounds and UV–vis spectral studies are provided
in Supplementary data.
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