An immobilised Co(ii) and Ni(ii) Schiff base magnetic nanocatalyst via a click reaction: A greener approach for alcohol oxidation

Article abstract of DOI:10.1039/c5nj00218d

A Schiff base immobilised nanocatalyst was synthesized via copper catalysed alkyne azide cycloaddition (CuAAC) on a magnetic support. The nanocatalyst exhibited high accessible active sites with a surface area of 76 m² g^-1 for a cobalt complex and 57 m² g^-1 for a nickel complex. A strong interaction between the magnetic support and the Schiff base was achieved by avoiding leaching during the course of reaction. The nanocatalyst efficiently oxidised both primary and secondary alcohols to carbonyl with improved yield in a solventless system rendering a greener approach.

Full text of DOI:10.1039/c5nj00218d

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An immobilised Co(II) and Ni(II) Schiff base
magnetic nanocatalyst via a click reaction:
a greener approach for alcohol oxidation†
Cite this:DOI: 10.1039/c5nj00218d
Pooja B. Bhat and Badekai Ramachandra Bhat*
A Schiff base immobilised nanocatalyst was synthesized via copper catalysed alkyne azide cycloaddition
(CuAAC) on a magnetic support. The nanocatalyst exhibited high accessible active sites with a surface
area of 76 m2 gꢀ1 for a cobalt complex and 57 m2 gꢀ1 for a nickel complex. A strong interaction
between the magnetic support and the Schiff base was achieved by avoiding leaching during the course
of reaction. The nanocatalyst efficiently oxidised both primary and secondary alcohols to carbonyl with
improved yield in a solventless system rendering a greener approach.
Received (in Porto Alegre, Brazil)
26th January 2015,
Accepted 9th April 2015
DOI: 10.1039/c5nj00218d
www.rsc.org/njc
has proved this reaction to be one of the most powerful tool for
covalent attachment.6 Usually, the immobilization methods
Introduction
In recent years, researchers have explored different strategies often require harsh conditions with multiple-step syntheses
to improve yielding of immobilized complexes on a support. and are complicated by cross-reactivity of the functional groups
Covalent anchoring of metal complexes on a silane support present.7 Therefore, developing a new strategy with mild,
provides efficient immobilization.1 However, a slight amount of selective and water-compatible immobilization of ligands onto
leaching is observed. This may be due to the possible interaction magnetic nanomaterials is highly desirable. The 1,2,3-triazole
between the support and the metal complexes. A strong binding linkage provides a high tolerance functional group for covalent
between the support and nanoparticle must withstand the harsh attachment of the functionalized molecule on the support.8
conditions of the reaction.2 The immobilization strategy used for Immobilization of the 1,2,3-triazole linkage on silica supports
linking metal complexes to supports must be mild, preserving the offers several advantages in terms of thermal stability, high
chemical functional activity of the complex and giving quantitative surface areas and tailorable porosities at a lower cost. This
conversion.3 In this respect, click reaction is one of the powerful approach has been extensively used for functionalization of
tools for ligation, which prevents the leaching of the complexes surfaces by biomolecules and complexes.9–12 However, few studies
from the support during the reaction.
have been devoted for the immobilization of metal complexes
Immobilization methods that are often used for the attach- on magnetic nanoparticles (MNP) via a click reaction.13–15
ment of metal complexes to a surface either involve non-covalent In this respect, we tried to immobilize metal complexes on
interactions based on physical adsorption or direct covalent magnetic nanoparticles via a click reaction.
immobilization to chemically functionalized activated supports.
However, these methods suffer from certain drawbacks such as a
lower catalyst loading on the support, use of excess metal complex
and harsh reaction conditions. Hence, recently, researchers have
Results and discussion
indulged in determining different immobilization strategies to The electronic spectra of the ligands and complexes were
be carried out under mild reaction conditions with quantitative recorded by DMSO. The electronic spectra of all the complexes
conversions.
showed bands in the 300–380 nm region. The bands appearing
Copper catalysed alkyne azide cycloaddition reaction was in the 300–400 nm region have been assigned to intra ligand
first proposed by Sharpless’ group and Meldal’s group.4,5 The transitions. A band in the range of 380–400 nm corresponds to
simplicity in use, mild reaction conditions and high conversion the ligand to metal charge transfer transition (Fig. 1).
A coordinated Co(II) and Ni(II) complex shows d–d transitions
at 650 nm. As a result of coordination, the intra ligand transition
bands are red-shifted in the complexes. Magnetic susceptibility
Catalysis and Materials Laboratory, Department of Chemistry,
National Institute of Technology Karnataka, Surathkal, Srinivasanagar-575025,
measurements of all Co(II) complexes show magnetic moments
India. E-mail: ram@nitk.edu.in; Fax: +91 824-2474033
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5nj00218d in the range of 4.08–4.12 B.M., confirming the tetrahedral structure.
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Fig. 2 XRD of (a) Fe3O4, (b) FCoL1, (c) FCoL2, (d) FNiL1 and (e) FNiL2.
frequency of Fe–O vibration from 570 cmꢀ1 to 550 cmꢀ1
revealed grafting of a clicked metal complex on the nanoparticle.16
A Si–O peak at 1057 cmꢀ1 confirms the silane modification on
the nanoparticle. A peak at 2067 cmꢀ1 confirms the presence of
the azide functional group on the surface of the modified
ferrite.17 The reduced azide peak confirms click immobilization
of the metal complexes on the nanoparticle. The X-ray diffrac-
tion pattern of MNP-clicked metal complexes was same as raw
MNP, clearly indicating that crystallinity and morphology were
not altered after grafting (Fig. 2). A widened broad spectrum
with reduced intensity confirmed the immobilization of metal
complexes on the magnetic nanoparticle.18
FEG-SEM images of MNP-clicked metal complexes shows
further evidence for the attachment of the clicked metal
complex to MNP. The morphology of MNP-clicked metal com-
plexes showed a cluster of nanoparticles (Fig. 3). The molecular
crowding by a clicked ligand can cause slight agglomeration.
These ligands may block the active pores of the nanocatalyst
and reduce its surface area. TEM images also further confirmed
the size of a nanoparticle to be 20 nm. The TEM images exhibit
a layer of the ligand adhered on a nanoparticle (black dots).
Fig. 1 UV-visible spectra of the (a) cobalt complex, (b) ligand L1, (c) nickel
complex, and (d) ligand L2.
The Ni(II) complex was diamagnetic in nature. The HNMR of the
ligand exhibited a multiplet at around 6.9–7.9 ppm, which has
been assigned to the protons of phenyl groups present in the
Schiff base ligand. A peak observed at B8.5 ppm in complexes
has been assigned to azomethine proton (–CHQN–). A peak
around 5 ppm gives (O–CH2) and 3.5 for (HCRCH) confirms
propargylation of the ligand (ESI,† Fig. S1). The HNMR of the
Co complex could not be obtained due to its paramagnetic
nature (Scheme 1).
The covalent attachment and successful synthesis of silica-
immobilized Co(II) and Ni(II) Schiff base complexes via a click
reaction was confirmed by FTIR spectroscopy (ESI,† Fig. S2).
The FTIR spectral intensity of the MNP-grafted clicked complex is
weak due to the low concentration of the complex. The reduced
Scheme 1 Schematic representation of the synthesis of the nanocatalyst.
Fig. 3 FEG-SEM of (a) FCoL1, (b) FCoL2, (c) FNiL1 and (d) FNiL2.
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Fig. 6 (a) Nitrogen sorption isotherm and (b) pore size distribution of
FNiL1.
Fig. 4 FEG-SEM of (a) FCoL1, (b) FNiL1 and (c) SAED image of FCoL1.
The SAED image confirms the crystalline nature of the nano-
catalyst (Fig. 4c). Energy dispersive X-ray analysis (EDAX) further
confirmed the presence of ‘Si’, ‘Ni’, ‘Co’, ‘Fe’ in the anchored
clicked metal complexes on the MNP (ESI,† Fig. S3)
The nitrogen-sorption analysis showed a narrow pore-size
distribution with a mean pore-size of 5 nm. The nanocatalyst
had a surface area of 76 m2 gꢀ1 for the cobalt complex (Fig. 5)
and 57 m2 gꢀ1 for the nickel complex (Fig. 6), which is con-
siderably lower in comparison to that observed for bare ferrite
(330 m2 gꢀ1 surface area),19 indicating considerable loading of
the complex inside the active pores of the nanocatalyst. Thermal
behavior of the complexes was determined in a nitrogen atmosphere
(20 ml minꢀ1) with a heating rate of 10 1C minꢀ1 in the temperature
range of 30–800 1C. The thermograms of MNP-grafted complexes
showed a two-step weight loss at temperatures ranging from 30 to
800 1C. The ferrite showed only one step weight loss at 300 1C. The
first region at temperatures between 35 1C and 250 1C corresponds
to the removal of physically adsorbed water. The second region is
mainly related to the decomposition of Schiff base groups in the
temperature range from 450 1C to 800 1C. A higher rate of weight
loss of clicked metal complexes compared to ferrite further confirms
the immobilization of the clicked complexes on the ferrite (Fig. 7).
Fig. 7 Thermogravimetric analysis of (a) Fe3O4, (b) FCoL1 and (c) FNiL1.
The oxidation of primary and secondary alcohols catalyzed by
the magnetic core grafted clicked Schiff base complexes occurs
readily to form the corresponding aldehydes and ketones as
major products. The oxidation of benzyl alcohol was studied as a
model substrate in more detail to optimize the reaction variables
such as solvent, alcohol/oxidant, the alcohol/catalyst molar
ratio and length of the reaction time. In order to study the
effect of time on the activity, the product analysis was done at
regular intervals of time under similar reaction conditions (Fig. 8).
A total reaction time of 140 min gave a maximum conversion.
A significant change in conversion was not observed with
increase in time above 140 min. An increase in GC conversion
by 3% was achieved for acetonitrile solvent. This can be due to
the solvent effect observed due to acetonitrile (the chelating
ligand). However, the MNP-immobilised clicked metal complexes
via a click azide reaction exhibited comparable good efficiency
for the solvent-less system. Hence, further studies were carried
out in the solvent-less system. The water content present in
hydrogen peroxide (30%) can act as a solvent to the catalytic
Fig. 5 (a) Nitrogen sorption isotherm and (b) pore size distribution of FCoL1. system rendering a green synthesis. In order to study the effect
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Table 2 Oxidation of alcohols catalyzed by MNP-clicked metal complexes
Conversiona (%)
Substrate
Product
FCoL1 FCoL2 FNiL1 FNiL2
82.0
98.5
80.1
97.3
79.8
96.6
76.2
98.0
96.2
70.3
68.4
94.3
76.8
66.7
92.5
70.5
64.5
95.3
77.2
67.1
Fig. 8 Effect of time on nanocatalyst (a) FCoL1 and (b) FNiL1.
of the concentration of a catalyst with respect to the substrate,
the reaction was also studied at different substrate to catalyst
ratios. The reaction was also studied in the absence of a catalyst.
No conversion was observed in this case (Table 1, entries 1
and 6). This observation ruled out the possibility of the reaction
taking place due to the thermal decomposition of the oxidants.
0.02 g of catalyst was sufficient for the effective transformation
of benzyl alcohol to benzaldehyde. The reactions were also
studied at various substrate-to-oxidant ratios. From the Table 1,
it is observed that a minimum quantity of 5 mmol of the
oxidant was sufficient for the effective oxidation of 1 mmol
benzyl alcohol to benzaldehyde.
The oxidation was extended to a variety of alcohols, includ-
ing aromatic and aliphatic alcohols. All the synthesized MNP-
grafted clicked Schiff base complex were found to catalyse the
oxidation of alcohols to corresponding carbonyl compounds
in good yield (Table 2). Among the various alcohols studied,
those containing aromatic substituents were found to be more
reactive than aliphatic alcohols. It is clear from GC analysis that
no side reaction took place during the course of catalytic
oxidation. All the experiments were carried out in air at room
temperature because there is no significant change in conver-
sion observed when the experiments are carried out under a
higher temperature of 80 1C. This indicates that oxidation
64.6
58.7
72.3
74.2
62.9
59.3
70.5
70.6
62.4
57.6
69.2
66.9
60.4
58.9
73.1
71.5
33.6
24.2
30.3
27.3
35.7
26.4
36.2
21.9
(CH3)2CH2OH
(CH3)2CH2CHO
a
Reaction conditions: substrate (1 mmol); oxidant (H2O2, 30%, 5 mmol);
catalyst (0.02 g); time 140 min; solventless. Average of 3 GC trials.
involves peroxide adsorption on the surface of nanoparticles.
This also confirmed that the magnetic nanoparticle had a sole
role of a catalyst support. All the reactions occurred with
complete selectivity for aldehydes and no other products were
detected in the reaction mixture. A higher rate of oxidation was
observed for the clicked Co complex than for Ni complexes.
This can be due to feasibility of the Co2+ cation with a higher
number of unpaired electrons for orbital overlap. In the nano-
range, the ligands may passivate the active pores on the
nanocatalyst due to molecular crowding, thereby decreasing
the catalytic activity.
Catalyst reusability was checked by the extraction of the
nanocatalyst by a simple external magnet after the reaction.
The active surfaces of the nanocatalyst were activated by washing
with ethanol followed by sonication. The oxidation reaction was
carried out nearly 5 times under identical reaction conditions
to check the significant change in the catalytic activity observed.
Table 1 Optimisation of the catalyst amount and oxidant concentration
Conversiona (%)
Amount of
Entry
Catalyst (g)
oxidant (mmol)
FCoL1
FNiL1
1
2
3
4
5
6
7
0
5.00
5.00
5.00
5.00
5.00
0
2.6
47.0
82.0
53.2
39.2
1.3
2.23
43.5
79.8
48.7
32.4
1.8
0.01
0.02
0.04
0.06
0.02
0.02
2.00
36.2
32.8
a
Reaction conditions: 1 mmol substrate, time 140 min. Average of 3 GC
trials.
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the cobalt Schiff base to the silica support provided a simple
process with high catalyst loading without using excess reagents
under mild reaction conditions. Magnetic nanoparticles pro-
vided a strong binding interaction to the metal complexes on
the support, thereby avoiding low yields possible due to ligand/
complex dissociation.
Experimental
(a) Synthesis of ligands and complexes
Synthesis of N,N0-bis-(4 bromo-2-propynoxybenzylidine)-1,4-
phenylenediamine (L1) and N,N0-bis-(2-propynoxynapthalidine)-
1,4 phenylenediamine (L2). The precursor ligands were prepared
by the drop wise addition of a methanolic solution of p-phenylene-
diamine (0.1 mol) to methanolic solution of 4-bromo-2-hydroxy
benzaldehyde (0.2 mol) or 2-hydroxynapthaldehyde (0.2 mol). The
resulting solution was refluxed on water bath for 2 h. A yellow
coloured and bright red coloured solid mass was separated out,
which was filtered, washed and subsequently dried. The ligand
was found to be insoluble in non-polar solvents such as acetone
and benzene and soluble in polar solvents such as DMF and
DMSO (yield 90%). The ligand was treated with propargyl bromide
and potassium carbonate (base) at room temperature for 6 h. The
product was quenched with water and extracted with ethylacetate.
The propargyl armed ligand was soluble in polar solvent like
DMSO (yield 80%).
The ligand (0.1 mol) in the minimum quantity of methanol
solution was mixed with a methanolic solution of nickel(II)
acetate tetrahydrate or cobalt(II) acetate tetrahydrate (0.2 mol).
The resulting solution was refluxed with stirring on a magnetic
stirrer equipped with a heater at 80 1C for 2 h. The bright red (Ni)
or brown (Co) colour complex separated out, which was filtered,
washed and dried.
Fig. 9 Vibrating sample magnetometry analysis of (a) FNiL1, (b) FNiL2,
(c) FCoL1, (d) FCoL2 and (e) Fe3O4.
All the catalyst exhibited better stability (ESI,† Fig. S4). A decrease in
the catalytic activity of FNiL1 can be attributed to the overcrowding
of the oxidised products entrapped within the pores. The magnetic
curve obtained from vibrating sample magnetometry showed a
hysteresis curve without coercivity (Fig. 9). This indicated super-
paramagnetism of the ferrite nanoparticle.
The metal loading observed for clicked metal complexes was
4.6 wt% (Co complex) and 5.2 wt% (Ni Complex). The loading
of the azide group was confirmed by TGA as 0.36 mmol gꢀ1
.
An improved yielding was observed for the model substrate
with 80% conversion. Furthermore, leaching experiments were
undertaken to demonstrate that the catalysis with MNPs are
truly heterogeneous and that no catalytically active Ni or Co
species are dissolved in the solution. For these experiments, the
reactions were allowed to occur until around 50% of conversion
and the catalyst was extracted by an external magnet. The
filtrate was subjected to atomic absorption spectroscopy (AAS)
to determine the amount of the leached product (ESI,†
Table S1). Clearly, it was observed that a negligible amount of
leaching was observed during the oxidation process. This confirms
the catalyst to be heterogeneous.
Click immobilization on silane coated ferrite support exhibited
negligible amount of leaching with good yield. This confirms a
very strong interaction between the magnetic support and the
clicked metal complexes. 1,2,3-Triazole as a linker provides
stabilized magnetic nanoparticle. In addition, the heterocyclic
molecule assisted in enhancing the catalytic activity of the
nanocatalyst.
(b) Synthesis of iron oxide nanoparticle
Magnetic nanoparticles were prepared by chemical coprecipita-
tion technique. Aqueous solution of salts Fe2SO4ꢁ7H2O (1 mmol)
and FeCl3ꢁ6H2O (2 mmol) are mixed in D.I. water at 80 1C with a
slow addition of NH4OH (25%, 15 mL) under anaerobic condi-
tions to yield a black ppt.20 The precipitate was aged overnight
to obtain monodisperse nanoparticles. Furthermore, the black
precipitate was washed with ethanol (10 mL) and water (5 mL)
and dried at 80 1C.
(c) Azide functionalized nanoparticle
A solution of sodium azide (0.05 mmole) in DMF solution at
0 1C was treated with 3-chloropropyltrimethoxysilane (ClPTES;
1 mmole). The slurry was heated at 50 1C and stirred for 48 h to
yield 3-azidopropyltrimethoxysilane.3,21,22 The reaction mixture was
Conclusions
We have successfully synthesized MNP-immobilised clicked metal quenched in water and extracted with ethylacetate. The organic
complexes. The developed nanocatalyst showed a promising greener layer was dried with anhydrous sodium sulfate and concentrated
route with high catalytic activity for oxidation of primary and at reduced pressure. This yielded a viscous oil solution. FTIR:
secondary alcohols. A solventless greener approach with improved 2097 cmꢀ1. 1H NMR (CDCl3): 0.7 (2H), 1.7 (2H), 3.27 (2H), 3.5 (9H).
yield was achieved. The superiority of the copper catalysed
1 g of magnetic nanoparticles was treated with 1.1 g of
alkyne azide cycloaddition for the covalent immobilization of azidopropyltrimethoxysilane in the presence of toluene at 80 1C
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6 R. K. O’Reilly, M. J. Joralemon, K. L. Wooley and
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for 6 h in an argon atmosphere. The product was aged for 1 h,
centrifuged with toluene and ethanol and dried at 80 1C.
(d) Immobilisation via a click-azide reaction of metal
complexes on magnetic iron oxide
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One gram of azide functionalized ferrite nanoparticles was added
to a solution of ligand (100 mg) in 1 : 1 H2O2 :tert-butanol system.
CuSO4 (0.5 mmol) was added to initiate the reaction with sodium
ascorbate (1 mmol). The solution was reacted for 6 h. The brown
coloured nanoparticles were separated using a magnet and were
washed several times with dichloromethane and toluene.
Acknowledgements
13 P. C. Lin, S. H. Ueng, S. C. Yu, M. D. Jan, A. K. Adak, C. C. Yu
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The author Pooja would like to thank NITK for research fellow-
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