Polymer supported nickel complex: Synthesis, structure and catalytic application

Article abstract of DOI:10.1007/s12039-014-0728-3

In the present investigation, a new synthetic route for a novel recyclable free [3-MOBdMBn-Ni] and polystyrene-anchored [P-3-MOBdMBn-Ni] nickel complexes is presented. The free and polymer-anchored metal complexes were synthesized by the reaction of nickel (II) with one molar equivalent of unsupported N N'-bis (2-Hydroxy-3-methoxybenzaldehyde) 4-Methylbenzene-1,2-diamine (3-MOBdMBn) or polymer-supported (P-3-MOBdMBn) Schiff-base ligand in methanol under nitrogen atmosphere. The advantages of these polymer-supported catalysts are the low cost of catalyst and recyclability up to six times, due to easy availability of materials and simple synthetic route. The higher efficiency of complexation of nickel on the polymer-anchored 3-MOBdMBn Schiff base than the unsupported analogue is another advantage of this catalyst system. The structural study reveals that nickel(II) complex of 3-MOBdMBn is square planar in geometry. The catalytic activity of nickel complex towards the oxidation of phenol was investigated in the presence of hydrogen peroxide. Experimental results indicate that the reactivity of P-3-MOBdMBn-Ni was dramatically affected by the polymer support compared to free 3-MOBdMBn-Ni. The rates of oxidation (R_p) for unsupported and supported catalysts are 1.37 × 10^-6 mole dm^-3 s^-1 and 2.33 × 10^-6 mole dm^-3 s^-1 respectively. [Figure not available: see fulltext.]

Full text of DOI:10.1007/s12039-014-0728-3

J. Chem. Sci. Vol. 126, No. 6, November 2014, pp. 1695–1705. ꢀc Indian Academy of Sciences.
Polymer supported nickel complex: Synthesis, structure and catalytic
application
a,∗
b
a,c
ALEKHA KUMAR SUTAR , TUNGABIDYA MAHARANA , YASOBANTA DAS and
PRASANTA RATH
a
c
Catalysis Research Lab, Department of Chemistry, Ravenshaw University, Cuttack - 3, Odisha, India
Department of Chemistry, National Institute of Technology, Raipur, India
School of Applied Sciences (Chemistry), KIIT University, Bhubaneswar, Odisha, India
b
c
e-mail: dralekhasutar@gmail.com
MS received 26 January 2014; revised 8 May 2014; accepted 29 May 2014
Abstract. In the present investigation, a new synthetic route for a novel recyclable free [3-MOBdMBn-Ni]
and polystyrene-anchored [P-3-MOBdMBn-Ni] nickel complexes is presented. The free and polymer-anchored
metal complexes were synthesized by the reaction of nickel (II) with one molar equivalent of unsupported
ꢁ
N N -bis (2-Hydroxy-3-methoxybenzaldehyde) 4-Methylbenzene-1,2-diamine (3-MOBdMBn) or polymer-
supported (P-3-MOBdMBn) Schiff-base ligand in methanol under nitrogen atmosphere. The advantages of
these polymer-supported catalysts are the low cost of catalyst and recyclability up to six times, due to easy
availability of materials and simple synthetic route. The higher efficiency of complexation of nickel on the
polymer-anchored 3-MOBdMBn Schiff base than the unsupported analogue is another advantage of this
catalyst system. The structural study reveals that nickel(II) complex of 3-MOBdMBn is square planar in geo-
metry. The catalytic activity of nickel complex towards the oxidation of phenol was investigated in the presence
of hydrogen peroxide. Experimental results indicate that the reactivity of P-3-MOBdMBn-Ni was dramatically
affected by the polymer support compared to free 3-MOBdMBn-Ni. The rates of oxidation (Rp) for unsupported
−6
−3 −1
−6
−3 −1
s respectively.
and supported catalysts are 1.37 × 10 mole dm
s
and 2.33 × 10 mole dm
Keywords. Polystyrene support; catalysis; organometallic catalyst, nickel complex; phenol oxidation.
in their catalytic activity9,10 but homogeneous catalysts
are not recovered easily.
1. Introduction
Schiff base catalysts have a great impact on both
the scholastic research and polymer industry. Schiff
base complexes have shown adaptable applications
in different organic transformation reactions such as
Further, oxidation of phenol is an industrially impor-
tant reaction as its products catechol and hydroquinone
are widely used as antioxidants, polymerization inhi-
bitors, photography chemicals, flavouring agents, drug
intermediates, etc. Since 1970, phenol oxidation,
has been widely investigated using various homoge-
1
,2
3,4
oxidation, epoxidation of olefins, and polymeriza-
5
,6
tion of ethylene. However, it was observed that the
homogeneous Schiff-base catalytic systems have two
key disadvantages: the lack of control of product which
causes the reactor fouling and the limitation of its
use in solution processes. Thus, binding these Schiff
base metal catalysts onto polymer supports can offer a
promising route to prevail over these drawbacks. In gen-
eral, the heterogeneous Schiff base catalytic system (or
supported system) apparently has a lower activity than
its corresponding homogeneous analogue. But polymer
supported transition metal complexes have shown high
11
neous and heterogeneous catalysts. Moreover, phenol
is an intermediate in the oxidation of many aromatic
compounds and a toxic molecule resistant to biotreat-
ment. For the treatment of wastewater containing
highly concentrated, toxic or hardly biodegradable
compounds, oxidation of such organic pollutants, into
non-toxic products in the presence of a catalyst is a
12,13
promising approach.
Oxidation of phenols using various chemical
reagents such as, hydrogen peroxide, permanganate,
7
,8
catalytic activity in comparison to homogeneous and
unsupported catalysts. Also, polymer-supported cata-
lysts are easily recovered without any substantial loss
14
molecular oxygen and ozone, are extensively used.
The use of hydrogen peroxide has the benefit of produ-
cing oxygen which can be used to supplement biological
15
degradation. Oxidation of phenol using unsupported
∗
16
For correspondence
Schiff base complexes of metal ions is reported, but
1
695

1
696
Alekha Kumar Sutar et al.
the oxidation of phenol using polymer-supported transi- carried out using Perkin-Elmer Pyris, Diamond Ther-
17
tion metal complexes was found to be high in the pres- mal Analyzer under nitrogen atmosphere at a heating
◦
ence of tert-butyl hydroperoxide (t-BHP). The activity rate of 5 C/min. The loading of metal ions on 3-
of polymer-supported Schiff base complexes of MOBdMBn Schiff base was determined by analyzing
iron(III), cobalt(II) and nickel(II) ions in the oxida- the solution with PerkinElmer 3100 Atomic Absorption
1
8,19
tion of phenol
rature, time, etc, which might be due to the change in MOBdMBn Schiff base anchored on polymer beads and
concentration of substrate or catalyst. its composition were estimated using Haraeus Carlo
also showed variation with tempe- Spectrometer at λmax of nickel ion. The amount of 3-
1
Although the oxidation of phenol in the presence Ebra 1108 Elemental Analyzer. The H-NMR spectra
of metal complexes of salen and hydrogen peroxide of 3-MOBdMBn Schiff base and its nickel complexes
as the oxidant is reported earlier the catalytic activity were recorded on an FT-NMR-Brucker 300 MHz Spec-
8
6
of metal complexes of N, N’-bis (2-hydroxy-3- trometer using DMSO-d as a solvent and tetramethyl-
methoxybenzaldehyde) 4-Methylbenzene-1,2-diamine silane (TMS) as an internal reference. The magnetic
Schiff base (3-MOBdMBn) is not reported in the lite- moment (μ) of nickel complexes were measured using
rature. Hence, in the present investigation an attempt has Vibrating Sample Magnetometer-155. The molecular
been made to prepare polymer-supported nickel com- weight of 3-MOBdMBn Schiff base and its nickel
plex of 3-MOBdMBn Schiff base and to characterize complexes were determined using a Vapor Pressure
them for their structures and catalytic activity towards Osmometer (Merk VAPRO 5600, Germany).
the oxidation of phenol in the presence of hydrogen per-
oxide as an oxidant. Here the substituent 4-methyl and
2
.3 Synthesis of 3-MOBdMBn Schiff base
3-methoxy play a significant role towards the stability
and its nickel complex
as well as in the catalytic activity of the Ni(II) complex.
The 3-MOBdMBn Schiff base was synthesized by
the modified procedure reported in the literature.7
The reaction mixture containing 2-hydroxy-3-
methoxybenzaldehyde (20 mmol, 3.043 g) and 4-
Methylbenzene-1,2-diamine (10 mmol, 1.22 g) in
2
. Experimental
2.1 Materials
◦
methanol was refluxed at 60 C for about 1 h. On cooling
Divinyl benzene cross-linked chloromethylated
polystyrene beads were obtained from Ion Exchange
India Ltd., Mumbai, India and were used to anchor syn-
thesized 3-MOBdMBn Schiff base complexes of metal
ions. The anhydrous chloro salts of nickel(II) ions were
purchased from Ranbaxy, Mumbai, India and used
without further purifications. The phenol, hydrogen per-
oxide (30.0 wt%), 2-hydroxy-3-methoxybenzaldehyde
at low temperature the reaction mixture produced light
orange red coloured crystals, which were filtered and
recrystallized with methanol. The metal complexes
of 3-MOBdMBn Schiff base were prepared by taking
100 mL methanolic solution of mixture of Schiff base
(
20 mmol, 7.81 g) and 20 mmol of Nickel metal ions
◦
in a round bottom flask and refluxing at 60 C. After
8
h, the solution was cooled and crystalline metal com-
(3-MOBd) and 4-Methylbenzene-1,2-diamine (MBn)
plexes were separated from the mother liquor. Finally,
metal complexes were recrytallized in methanol and
dried in a vacuum desiccator.
were procured form E. Merck, India. Other chemicals
and solvents were of analytical grade (>99.0 wt%) and
used after drying with standard methods.20
2.4 Synthesis of polymer-anchored 3-MOBdMBn
2.2 Characterization of 3-MOBdMBn Schiff base
Schiff base and its nickel complex
and its metal complexes
To prepare polymeranchored nickel complexes of
IR spectra of 3-MOBdMBn Schiff base and its 3-MOBdMBn Schiff base, the N, N’-bis (4-amino-2-
nickel complexes were recorded on KBr pellet using hydroxy-3-methoxybenzaldehyde) 4-Methylbenzene-
PerkinElmer 1600 FTIR Spectrophotometer. The elec- 1,2-diamine (A-3-MOBdMBn) Schiff base was
tronic spectra of 3-MOBdMBn Schiff base and its prepared by carrying out nitrosation and reduction
nickel complexes were recorded with Shimadzu 1601 reactions of 3-MOBdMBn Schiff base and then
PC UV–Vis Spectrophotometer by using sample mull reacting resulted A-3-MOBdMBn Schiff base with
in a cuvette. Thermogravimetric analysis (TGA) of 3- cross-linked chloromethylated polystyrene beads. The
MOBdMBn Schiff base and its nickel complexes were nitrosation of 3-MOBdMBn Schiff base was carried

Catalytic activity of polymer-supported nickel complex
1697
out using 20 mmol (7.81 g) of 3-MOBdMBn Schiff before starting the heating and stirring (1200 rpm) of
base with sodium nitrite (20 mmol) in 1.0 N hydroch- reaction mixture. Gas chromatography was used to
loric acid (100 mL) in an ice bath. The resultant N, follow these reactions at different time intervals. The
N’-bis (4-nitroso-2-hydroxy-3-methoxybenzaldehyde) retention time of standards was used to identify the
4-Methylbenzene-1,2-diamine
(NO-3-MOBdMBn) reaction products, and product selectivity was mea-
was filtered and washed with hot and cold water to sured using peak areas of reaction products in the chro-
remove reaction impurities. The reduction of NO- matograms. The reactions were also carried out taking
3
-MOBdMBn was carried out using 20 mmol of different molar ratios of substrates, hydrogen peroxide,
nitrosated Schiff base in 1.0 N hydrochloric acid and catalyst. The rate of oxidation for the oxidation of
50 mL) in the presence of metallic iron, which pro- phenol was determined by studying reactions at diffe-
(
duced A-3-MOBdMBn Schiff base. To immobilize the rent temperatures at constant molar ratios of substrate
prepared A-3-MOBdMBn Schiff base on cross-linked to H O and catalyst. The reactions were also studied
2
2
chloromethylated polystyrene, the methanol swollen without using catalysts to compare the effect of catalyst
polymer beads (5 g) were refluxed in 50 mL methanol towards the oxidation of phenol.
containing 20 mmol (8.41 g) A-3-MOBdMBn Schiff
base. After 8 h, the 3-MOBdMBn Schiff base anchored
3
. Results and Discussion
polymer beads were separated and dried in a vacuum
desiccator. The amount of 3-MOBdMBn Schiff base
loaded on polymer beads was estimated with elemental
analysis. Subsequently, the nickel ion was loaded by
keeping 3-MOBdMBn loaded polymer beads (5 g)
for 10 h in an aqueous solution (50 mL) of nickel ion
Investigations have revealed that polymer-supported
metal complexes are sometimes more catalytic and effi-
cient than free and unsupported analogues.
18,19
The
activity of catalysts also showed dependence on pro-
perties of polymer supports and amount of loaded cata-
lysts. Thermal stability of catalysts is required to be
high, if these catalysts have to be applied for high tem-
perature reactions. Therefore, thermal analysis of sup-
ported metal complexes was carried out to evaluate
their possible applications in high temperature reactions
and to provide a proof for complexation of metal ions
with polymer-anchored 3-MOBdMBn Schiff base. The
(
20 mmol). Finally, the polymer beads were separated
◦
and dried at 70 C in a vacuum oven after washing
with hot and cold water. The loading of nickel ion
on polymer bead was determined by analysing the
solution with an Atomic Absorption Spectrometer. The
loading of nickel ion on free and polymer-supported
3-MOBdMBn Schiff base was calculated as complex-
ation of nickel ion using the amount of 3-MOBdMBn
Schiff base taken initially and the amount of nickel ion
loaded on polymer beads.
TGA of polymer supported 3-MOBdMBn Schiff base
◦
(figure 1) showed a weight loss of 39.1 wt% at 500 C,
but its nickel(II) ion complex showed a weight loss of
35.1 wt% at the same temperature. This was a clear indi-
cation that nickel(II) ion complex was more stable than
its Schiff base ligand. Also, the nickel complex of P- 3-
MOBdMBn Schiff base was more stable than the poly-
mer supported nickel complex synthesized by Gupta
et al. (showed a weight loss of 45 wt% at 500 C),
may be due to the presence of 4-methyl and 3-methoxy
substituent in the Schiff base.
Further, polymer-supported 3-MOBdMBn Schiff
base and its nickel complex were also characterized by
IR and UV techniques to provide a proof for the com-
plexation of metal ions and to decide the structures
and geometry of the complex on the basis of elemental
analysis and magnetic properties.
Complexation of Nickel ion
Amount of Nickel loaded
=
× 100
21
Amount of Schiff base used for loading
2.5 Catalytic activity of metal complexes towards
19
◦
oxidation of phenol
To evaluate the catalytic activity of free and polymer-
supported nickel complexes of 3-MOBdMBn Schiff
base, the oxidation of phenol was carried out using
hydrogen peroxide as the oxidant at fixed ionic strength
(0.10 M) and hydrogen ions concentration (pH7.0) in
the reaction mixture. To carry out these reactions, a cal-
culated amount of metal-anchored polymer beads were
taken in a two-necked roundbottomed flask contain-
ing 0.05 M phenol (4.7 g). The oxidation of phenol was
carried out adding 5.67 g (0.05 M) hydrogen peroxide
3.1 Characterization of 3-MOBdMBn Schiff base
(30 wt%) in the reaction mixture and 2 mL chloroben- The yield of 3-MOBdMBn Schiff base was found
zene as an internal standard. The water condenser and to be 91.6 wt% (scheme 1). The IR spectrum of 3-
supply of nitrogen were attached with reaction flask MOBdMBn Schiff base showed absorption bands at

1
698
Alekha Kumar Sutar et al.
It is observed from the elemental analysis of NO-3-
MOBdMBn that (wt%): C = 62.01, N = 12.17, and
H = 4.95; Caltd (wt%): C = 61.60, N = 12.49 and
H = 4.50, which corresponds to C H N O formula of
23
20
4
6
nitrosated Schiff base. The molecular weight of NO-3-
1
1
MOBdMBn was 447.02 g mol (Caltd 448.43 g mol ).
The IR spectrum of NO-3-MOBdMBn showed absorp-
1
−1
−1
tion bands at 1604 cm (>C=N), 1260 cm (>C–O)
−1
phenolic, and 1533 cm and 131 cm for N–O
group. The NO-3-MOBdMBn Schiff base showed a
shift in NMR signals in comparison to NMR sig-
nals observed for 3-MOBdMBn Schiff base. The NO-
3
-MOBdMBn Schiff base showed proton signals at
δ/ppm = 2.82(3H), 3.75(6H), 5.15(2H), 7.15(2H),
.36(3H), 7.66(2H) and 8.64(2H), which corresponded
7
to the structure of nitrosated 3-MOBdMBn Schiff base
as shown in scheme 2. The protons ortho to nitroso
group in 3-MOBdMBn Schiff base were deshielded.
Hence, their signals appeared at 7.15 and 7.66 ppm
instead of 6.8 and 7.41 ppm respectively, as observed
for pure 3-MOBdMBn Schiff base. The proton sig-
nal at 7.25 ppm was missing due to the substitution
of nitroso group in the benzene ring. The reduction
of NO-3-MOBdMBn produced 81.3 wt% yield of A3-
MOBdMBn Schiff base as shown in scheme 2. The A3-
MOBdMBn Schiff base was characterized by different
techniques. The elemental analysis of A3-MOBdMBn
Schiff base showed (wt%): C = 63.92 N = 13.21 and
H = 7.02, Caltd (wt%): C = 65.7 N = 13.33, and H
Figure 1. Thermal stability of polymer-supported 3-
MOBdMBn Schiff base and its nickel complex.
1
1
1
609 cm (>C =N), 1263 cm (>C–O) phenolic (SI).
The elemental analysis of 3-MOBdMBn Schiff base
proved (wt%): C = 69.82, N = 7.03 and H = 6.31;
Caltd (%): C = 70.75, N = 7.17 and H = 5.68,
which corresponds to C H N O empirical formula of
23
22
,22
2
4
3
-MOBdMBn Schiff base.7
The molecular weight of Schiff base was
−1
−1
3
89.37 g mol (Caltd 390.43 g mol ). The electronic
spectra of 3-MOBdMBn Schiff base showed absorption
bands at 289 nm and 347 nm, which were assigned to
∗
∗
1
π → π and n → π transitions. The H-NMR spec-
trum of 3-MOBdMBn Schiff base (figure S3) showed
signals at δ/ppm = 2.79(3H), 3.81(6H), 5.13(2H),
=
5.75, which corresponded to C H N O empirical
23 24 4 4
formula of Schiff base. The molecular weight of A-3-
1
MOBdMBn Schiff base was found to be 418.23 g mol
(
1
6
.80(2H), 6.98(3H), 7.25(2H), 7.41(2H) and 8.61(2H)
Caltd 420.46 g mol ). The IR spectrum of A-3-
which corresponded to the structure of 3-MOBdMBn
Schiff base as shown in scheme 1.
MOBdMBn Schiff base showed absorption bands at
1
1
1
600 cm (>C= N), 1255 cm (>C–O) phenolic, and
1
a band between 1641 and 1619 cm for >C–N group.
The H-NMR spectrum of A-3-MHBdMBn Schiff base
1
3.2 Synthesis and characterization of A-3-MOBdMBn
showed proton signals at δ/ppm = 2.81(3H), 3.72(6H),
Schiff base and its anchoring on polymer beads
4.15 (4H), 5.15(2H), 6.16(2H), 6.46(2H), 7.36(3H) and
The nitrosation of 3-MOBdMBn Schiff base pro- 8.63(2H), which corresponded to the structure of A-3-
duced 87.5% yield of NO-3-MOBdMBn (scheme 2). MOBdMBn Schiff base as shown in scheme 2.
OCH3
OCH3
OCH3
OCH3
OH
HO
HO
OH
Reflux in MeOH
+
+
0
at 60 C for 1 hr
H N
NH2
O
N
O
2
N
2
-hydroxy-
-methoxybenzaldehyde
3-MOBd
2
-hydroxy-
-methoxybenzaldehyde
-MOBd
3
3
3
4
-Methylbenzene-1,2-diamine
N, N'-bis (2-hydroxy-3-methoxybenzaldehyde
4-Methylbenzene-1,2-diamine Schiff base
MBn
3-MOBdMBn
Scheme 1. Synthesis of 3-MOBdMBn Schiff base.

Catalytic activity of polymer-supported nickel complex
OCH
OCH
3
1699
OCH3
3
OCH3
OH
HO
OH
HO
HCl / NaNO2
Cold
N
N
NO
N
N
ON
[H]
NO- 3-MOBdMBn
Fe/HCl
3-MOBdMBn
OCH3
OH
OCH
3
HO
N
N
NH2
H N
2
A - 3-MOBdMBn
Scheme 2. Synthesis of A-3-MOBdMBn Schiff base.
The synthesized A-3-MOBdMBn Schiff base was in polymer- anchored 3-MOBdMBn Schiff base than
anchored on cross-linked chloromethylated polystyrene pure polymer beads was an evidence for anchoring of
beads by refluxing A-3-MOBdMBn Schiff base with 3-MOBdMBn Schiff base on polymer beads. The
◦
polymer beads in DMF for 8 h at 60 C. The amount appearance of new absorption bands and shift in cha-
of A-3-MOBdMBn Schiff base anchored on polymer racteristic absorption bands of 3-MOBdMBn Schiff
beads was 881 wt% (scheme 3). The anchoring of A- base were also used as evidence for anchoring of 3-
3-MOBdMBn Schiff base on polymer beads was con- MOBdMBn Schiff base on polymer beads.
firmed by comparing the IR spectrum of 3-MOBdMBn
Schiff base anchored polymer beads with IR spectrum
of pure polymer beads. The IR spectrum of polymer-
anchored Schiff base showed new absorption bands
3
.3 Characterization of free and polymer-anchored
metal complexes of 3-MOBdMBn Schiff base
−1
−1
at 1594 cm (>C=N), 1246 cm (>C–O) phenolic,
−1
and a broad band between 1644 cm (>C=N), which The loading of nickel ion on free and polymer-
were absent in the IR spectrum of pure polymer supported 3-MOBdMBn Schiff base was carried out
beads, but were present in free Schiff base. The IR by refluxing free Schiff base (scheme 4) and polymer-
◦
spectrum of pure polymer beads showed absorption anchored Schiff base in solution of metal salt at 60 C
−1
band at 1262 cm , which is attributed to C–Cl bond for 6 h (scheme 5). The metal complexes of free
7
of chloromethyl in cross-linked polymer beads. The Schiff base (3-MOBdMBn-Ni) and polymer-anchored
1
decrease in the intensity of absorption band at 1262 cm
Schiff base (P-3-MOBdMBn -Ni) after separation and
OCH3
OH
OCH3
OCH3
OCH3
HO
OH
HO
Polymer beads
CH Cl
N
N
NH
NH
N
NH2
H N
N
2
P
2
P
P
P - 3-MOBdMBn
A - 3-MOBdMBn
Scheme 3. Synthesis of polymer-anchored Schiff base (P-3-MOBdMBn).

1
700
Alekha Kumar Sutar et al.
OCH3
OCH3
OCH3
OCH3
OH
O
O
HO
Ni
Nickel salt solution
reflux, rt
N
N
N
N
Ni - 3-MOBdMBn
Scheme 4. Loading of nickel ion on unsupported 3-MOBdMBn Schiff base.
3-MOBdMBn
purification were analyzed for their structures and evidence for the complexation of metal ions with
loading of nickel ion. The complexation of nickel(II) 3-MOBdMBn Schiff base. The polymer-supported
ion on free 3-MOBdMBn Schiff base and polymer- Schiff base showed absorption bands at low frequency
anchored Schiff base was 86.34 and 89.88 wt% respec- in comparison to free Schiff base (SI) On complex-
tively (table 1). These results have clearly suggested ation with nickel(II) ion, the frequency of >C=N
that the loading of nickel ion on polymersupported absorption band of free Schiff base has decreased
−
1
3
-MOBdMBn Schiff base was higher than free 3- from 1609 to 1595 cm , whereas polymer- supported
−1
MOBdMBn Schiff base.
Schiff base showed variation from 1594 to 1582 cm
The complexation of nickel ion with 3-MOBdMBn (figure 2).7
Schiff base showed significant variations in IR bands
These variations in IR band corresponding to
for >C=N and >C–O groups and new absorption >C= N group of 3-MOBdMBn Schiff base were
bands appeared due to the formation of M–O and M–N used as evidence for interaction of metal ions with
bonds in metal complex. The disappearance of phenolic azomethine nitrogen (>C= N) of 3-MOBdMBn Schiff
broad band in the IR spectrum of 3-MOBdMBn Schiff base. The complexation of nickel(II) ions showed new
−1
base after the complexation of nickel ion was an absorption band at 424 cm with free Schiff base
OCH3
OH
OCH3
HO
N
N
NH CH2
P
P
CH2 NH
P- 3-MOBdMBn
reflux, rt Nickel salt solution
OCH3
OCH3
O
O
Ni
NH CH2
P
CH2 NH
N
N
P
P- 3-MOBdMBn - Ni
Scheme 5. Loading of nickel ion on polymer-supported Schiff base (P-3-MOBdMBn -Ni).

Catalytic activity of polymer-supported nickel complex
1701
Table 1. Efficiency of complexation (EC) of nickel ion Table 2. Electronic transitions in unsupported and poly-
(
wt%).
mer supported nickel complex of 3-MOBdMBn Schiff base.
Complexation (EC)
of nickel(II) ion (wt%)
Frequencies λmax / nm
π → π∗ n→ π∗ C→T d→d
Schiff base
Compounds
3
-MOBdMBn
86.34
89.88
3-MOBdMBn
P-3-MOBdMBn
289
282
347
344
–
–
–
–
P-3-MOBdMBn
[
3-MOBdMBn] = 20 mmol, [Nickel ion] = 20 mmol.
3
-MOBdMBn -Ni
280
273
299
291
315
308
403
398
P-3-MOBdMBn -Ni
OCH3
OCH3
O
O
Ni
N
N
Ni - 3-MOBdMBn (Square Planar)
Scheme 6. Geometry and structure of 3-MOBdMBn Schiff
base complex of nickel ion.
moment (μ) and electronic configurations have sug-
gested a square planar structure with dsp hybridization
for nickel complex (scheme 6).
Figure 2. FTIR spectrum of polymer supported
-MOBdDBn-Ni complex.
2
3
and at 417 cm−1 with polymer-anchored Schiff base,
which was due to the formation of M–N bond between
3.4 Oxidation of phenol
nickel(II) ion and Schiff base. The complexation of The catalytic activity of free and polymer-supported
nickel(II) ion showed another new band due to the for- nickel complex of 3-MOBdMBn Schiff base was eval-
mation of bond between nickel and phenolic oxygen uated by studying the oxidation of phenol in the pres-
(
–O–M) with polymer-supported 3-MOBdMBn Schiff ence of hydrogen peroxide. The gas chromatographic
−1
base at 518 cm (figure 2).
analysis was used to determine the product selectivity
The complexation of metal ions with Schiff base and to estimate the percent conversion of phenol. The
was further confirmed by comparing electronic spectra catechol (CTL) was a major reaction product in the oxi-
of nickel complex and pure 3-MOBdMBn Schiff base. dation of phenol (scheme 7). The formation of reaction
The complexation of nickel(II) ion with 3-MOBdMBn products was attributed to enzymatic behaviour of metal
∗
Schiff base showed variation in π → π transition complexes of 3-MOBdMBn Schiff base.
∗
from 289 nm to 280 nm and (table 2) the n → π tran-
The polymer support has facilitated the decompo-
sition of 3-MOBdMBn Schiff base was changed from sition of the intermediates. Hence, % conversion of
3
47 nm to 299 nm. The charge transfer (CT) and d → d phenol was more with polymer-supported nickel com-
transitions were also used as evidence for complexation plex (figure 3) in comparison to free complex of 3-
of nickel with Schiff base. These electronic transitions MOBdMBn Schiff base (SI). The conversion of phenol
corresponded to t e configurations for nickel(II) ion increased up to 240 min and after that the conversion
6
g
2
g
2
in this complex. The magnetic moment (μ) of nickel became almost constant due to substantial decrease in
Schiff base complex was found to be 0.83BM, which the concentration of oxidant and substrate in the reac-
indicated that nickel(II) ion complex was diamagnetic tion mixture for both supported and unsupported cata-
in nature with zero unpaired electron. The magnetic lysts (figure 3). The oxidation reaction also had shown

1
702
Alekha Kumar Sutar et al.
OH
OH
OH
OH
Catalyst
OH
+
Phenol (Ph)
Catechol (CTL) Hydroquinone (HQ)
Scheme 7. Oxidation of phenol.
for CTL remained unaffected on using recycled cata-
lysts, which was an indication for the structural stability
of polymer supported metal complexes as confirmed
by comparing IR spectra of recycled catalysts with IR
spectra of freshly prepared catalysts. The activity of
metal complexes in substrate conversion and product
selectivity was evaluated at different molar ratios of
substrate to hydrogen peroxide and catalyst.
70
60
50
40
30
20
10
0
P-3-MOBdDMBn-Ni
-MOBdDMBn-Ni
The rate of phenol conversion was higher (2.33 ×
3
−6
−3
−1
10
mole dm s ) in the presence of polymer-
supported 3-MOBdMBn Schiff base complexes of
nickel(II) ion than its unsupported Schiff base analogue
[Phenol] = [Catalyst] = [Oxidant] = 0.05M,
0
−6
−3 −1
Temp - 70 C
(1.37 × 10 mole dm s ) (table 4). The turnover
number (TON) for the oxidation of phenol was higher
0
50
100
150
200
1000 1500
−1
−1
(
15.82 g mol Ni hr ) in the presence of polymer sup-
Time(min)
ported Schiff base complex in comparison to unsup-
−1
−1
Figure 3. Effect of reaction time on oxidation of phenol ported Schiff base complex (9.33 g mol Ni hr ) at a
in presence of polymer-supported and unsupported nickel molar ratio of 1:1:1 of H O to phenol and catalyst
2
2
complexes. [Phenol]:[Catalyst]: [H2O2] = 1:1:1 (0.05 M),
(
table 4). The rate of substrate conversion and TON in
◦
Temp. = 70 C.
the oxidation of phenol varied significantly with molar
ratio of hydrogen peroxide but product selectivity in
both cases remained almost constant (table 4).
dependence on the type of catalyst. The high activity of
polymer supported metal complexes was due to more
facile interactions of the catalyst with substrate than
with unsupported catalysts. The low activity of catalyst
in solution was due to the formation of inactive dimers
or multimers of metal complexes in the solution. The
3
.5 Effect of the H O concentration/phenol
2 2
concentration/catalyst concentration on the oxidation
of phenol
amount of phenol oxidized with hydrogen peroxide was The oxidation of phenol was studied with the varia-
almost equal to the sum of amount of CTL and hydro- tion of the molar ratio of H O to phenol from 0.5
2
2
quinone (HQ) produced, which indicates the formation to 2.0 at a constant molarity of the substrate and cat-
of other reaction products such as polymeric phenols is alyst (0.05 M) in acetonitrile (2.0 mL). The reaction
◦
almost none.
temperature was maintained at 70 C. When the molar
The amount of CTL and HQ was equal to phenol con- ratio of H O was varied from 0.5 to 1.0, the oxida-
2
2
version, but reaction showed high selectivity for CTL as tion of phenol increased in the presence of supported
determined from the area under the GC chromatograms. 3-MOBdMBn Schiff base complex of nickel(II)
The supported catalysts were recycled and also further (figure 4). However, when the molar ratio of H O2
2
evaluated for their catalytic activity after their applica- was further increased (>1) in the reaction mixture,
tions in oxidation reactions. The efficiency of supported the oxidation of phenol showed a decreasing trend,
catalysts remained almost constant up to six recycles and a similar trend was observed with unsupported
and after that the efficiency decreased (table 3) which 3-MOBdMBn-Ni Schiff base complex analogue. The
may be due to either decomposition of the catalyst in decreasing trend in the conversion of phenol was due to
the reaction media or their extraction to the organic sol- the decrease in molar ratio of phenol and catalyst with
23
vent during product isolation. The product selectivity respect to molar ratio of H O .
2
2

Catalytic activity of polymer-supported nickel complex
1703
Table 3. Efficiency of recycled supported catalysts (P-3-MOBdMBn-M) towards
oxidation of phenol.
Recycle number
Supported Nickel complexes
Conversion
0
2
4
6
8
68.8
95.1
68.0
95.0
67.1
95.3
64.8
91.6
43.2
89.5
P-3-MOBdMBn -Ni
Selectivity
◦
[
Phenol]:[Catalyst]: [H2O2] = 1:1:1 (0.05M), Temp. = 70 C. CH3CN = 2.0 mL.
Table 4. Oxidation of phenol by [3-MOBdMBn-Ni]: Phenol conversion, selectivity
for catechol and kinetic parameters.
Complexes
Conversion (wt%)
Selectivity (wt%)
TON
Rp
3
-MOBdMBn -Ni
41.1
68.8
94.2
95.1
9.33
15.82
1.37
2.33
P-3-MOBdMBn -Ni
◦
[
2
1
H2O2]/[Phenol]/[Catalyst]= 0.05 M, Time = 1440min, Temp. = 70 C CH3CN =
−1
−1
.0 mL TON in terms of = g mol M hr , Rate of oxidation (Rp) in terms of =
−6
−3 −1
0
mole dm s
the Schiff base (figure 4), but when the molar ratio of
phenol increased further (>1), the conversion (%) of
phenol showed a decreasing trend, which was due to
7
6
5
4
3
0
0
0
0
0
the significant decrease in the molar ratio of H O2
2
to the catalyst in the reaction mixture in compari-
son with the molar ratio of phenol. The oxidation of
phenol was also evaluated at different molar ratios of
polymer-supported 3-MOBdMBn Schiff base complex
of nickel at a constant molar ratio (1: 1) of the sub-
strate and oxidant. The molar ratio of Schiff base com-
plex of nickel(II) ions was varied from 0.5 to 2.0 at
a constant molarity (0.05 M) of phenol and H O in
Effect of [H O ]
Effect of [phenol]
Effect of [P-3-MOBdDBn-Ni]
2
2
2
2
0
Temp. 70 C.
the reaction mixture. The conversion (%) of phenol
also showed the same trend as variation of substrate
concentration.
0
.000
0.025
0.050
0.075
0.100
0.125
-
3
[
Phenol] / [H O ] / [catalyst] mol dm
2 2
Figure 4. Effect of H2O2 concentration / phenol concen-
tration / concentration of polymer-supported nickel complex 3.6 Mechanism for oxidation of phenol
catalyst towards oxidation of phenol.
Considering the experimental findings for the oxida-
tion of phenol with free and supported 3-MOBdMBn
The catalytic efficiency of the [3-MOBdMBn-Ni] Schiff base complex of nickel ion, the following reac-
complexes towards the oxidation of phenol was evalu- tion steps are proposed (scheme 8). The free and
ated at different molar ratios of phenol in the reaction polymer-supported Schiff base complexes of nickel
mixture, whereas the molar ratio of H O to the cata- ion (3-MOBdMBn-Ni) have produced active species
2
2
-
lyst was kept constant. The molar ratio of phenol was (Ni3-MOBdMBn-HOO ) through fast interactions with
varied from 0.5 to 2.0 with respect to the molar ratio H O and 3-MOBdMBn Schiff base. The intermediate
of H O to the catalyst. During the molar ratio varia- (Ni3-MOBdMBn-Ph-HOO ) has facilitated the nucle-
2
2
-
2
2
-
tion of phenol, the concentration of H O and the cata- ophilic attack of OOH species on ortho and para posi-
2
2
lyst was kept constant (0.05 M). When the molar ratio tion of phenol to produce hydroxy-substituted phe-
of phenol was increased from 0.5 to 1.0 in the reac- nols (scheme 7). The reaction products, catalyst and
tion mixture, the conversion (%) of phenol increased hydroxyl ions were formed through decomposition
substantially with polymer supported nickel complex of of intermediates (scheme 8, step 4) and finally the

1
704
Alekha Kumar Sutar et al.
kd
H O
H +
+
OOH-
..1
2
2
Ni-3-MOBdMBn
+
OOH-
Ni-3-MOBdMBn-OOH-
Active species
...2
K
Ni-3-MOBdMBn-OOH-
O
+
Ph
Ni-3-MOBdMBn-Ph-OOH-
Intermedaite
..3
H
O
H
OCH3
O-
OH
OCH3
OH
OH
O
O
k
Ni
-
+
+ Ni-3-MOBdMBn + OH ...4
N
N
Catechol (CTL)
OH
Hydroquinone (HQ)
Ni - 3-MOBdMBn-Ph-OOH-
H +
+
HO-
H O
2
..5
Scheme 8. Reaction steps of oxidation of phenol.
hydroxyl ions reacted with hydrogen ions, which were 1H-NMR spectrum of 3-MOBdMBn Schiff base,
produced in step 1.
time variation data for conversion of phenol by
unsupported and polymer-supported nickel complex
(
Ni-3-MOBdMBn) are provided in supporting informa-
4. Conclusion
tion (SI) available at www.ias.ac.in/chemsci.
The unsupported and polymer-supported nickel com-
plexes of 3-MOBdMBn Schiff base were synthesized Acknowledgements
and characterized successfully for their structures and
catalytic activity towards the oxidation of phenol. The The authors are thankful to Department of Science and
polymer supported 3-MOBdMBn Schiff base nickel Technology (DST), Council of Scientific and Indus-
complex showed high catalytic activity than its free ana- trial Research (CSIR) and University Grants Com-
logue. The oxidation of phenol showed high selectivity mission (UGC), New Delhi, India for funding. The
for catechol. The supported catalysts showed higher authors are also grateful to Ravenshaw University, KIIT
rate of oxidation and TON than unsupported cata- University and National Institute of Technology, Raipur
lysts, which clearly suggested that polymer support has for providing research facilities.
played a significant role in increasing the rate for oxi-
dation of phenol in the presence of nickel complexes
of 3-MOBdMBn Schiff base. Concentration ration of
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