Oxalate capped iron nanomaterial: From methylene blue degradation to bis(indolyl)methane synthesis

Article abstract of DOI:10.1039/c4ra04214j

An efficient, sustainable and green procedure for the synthesis of selective orthorhombic iron(oxalate) capped Fe(0) [Fe(ox)-Fe(0)] nanomaterial is developed using sodium borohydride (NaBH₄) reduction of iron(ii) salt in the presence of oxalic acid at room temperature in water. The reported method is a cost-effective chemical route for producing Fe(ox)-Fe(0) nanomaterial on the gram scale without high-temperature calcination. The oxidation of Fe(0) to Fe₃O₄ at room temperature in open air leads to Fe-oxalate capped Fe₃O₄ [Fe(ox)-Fe ₃O₄] nanomaterial on the gram scale. The Fe(ox)-Fe ₃O₄ nanomaterial is found to be useful as a magnetically recoverable catalyst for the selective synthesis of bis(indolyl)methanes from the condensation between aldehydes and indoles in water. The as-prepared Fe(ox)-Fe₃O₄ nanomaterials also show an excellent ability as a reusable catalyst for the degradation of methylene blue (MB) under UV irradiation and are expected to be useful in many other applications. Aqueous reaction medium, easy synthesis, effortless separation of the catalyst using an external magnet, and efficient recycling of the catalyst make the protocol economical and sustainable. the Partner Organisations 2014.

Full text of DOI:10.1039/c4ra04214j

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Oxalate capped iron nanomaterial: from methylene
blue degradation to bis(indolyl)methane synthesis†
Cite this: RSC Adv., 2014, 4, 33446
*
Rupa Pegu, Krishna Joyti Majumdar, Dhruba Joyti Talukdar and Sanjay Pratihar
An efficient, sustainable and green procedure for the synthesis of selective orthorhombic iron(oxalate)
capped Fe(0) [Fe(ox)–Fe(0)] nanomaterial is developed using sodium borohydride (NaBH4) reduction of
iron(II) salt in the presence of oxalic acid at room temperature in water. The reported method is a cost-
effective chemical route for producing Fe(ox)–Fe(0) nanomaterial on the gram scale without high-
temperature calcination. The oxidation of Fe(0) to Fe3O4 at room temperature in open air leads to Fe-
oxalate capped Fe3O4 [Fe(ox)–Fe3O4] nanomaterial on the gram scale. The Fe(ox)–Fe3O4 nanomaterial is
found to be useful as a magnetically recoverable catalyst for the selective synthesis of bis(indolyl)methanes
from the condensation between aldehydes and indoles in water. The as-prepared Fe(ox)–Fe3O4
nanomaterials also show an excellent ability as a reusable catalyst for the degradation of methylene blue
(MB) under UV irradiation and are expected to be useful in many other applications. Aqueous reaction
medium, easy synthesis, effortless separation of the catalyst using an external magnet, and efficient
recycling of the catalyst make the protocol economical and sustainable.
Received 7th May 2014
Accepted 23rd July 2014
DOI: 10.1039/c4ra04214j
www.rsc.org/advances
anisotropic dipolar attraction in magnetic iron oxide nano-
structures and thus lose their specic properties associated
Introduction
with single-domain. To prevent such type of aggregation,
various capping agents with relatively high concentrations are
oen required.4 At the same time, the presence of large
amounts of capping agents in these systems may hamper its
catalytic activity, due to the binding of the capping agents to the
active surface of nanoparticle.5 In addition, the reactivity of iron
oxide particles has been shown to greatly increase as their
dimensions are reduced.6 However, it is still a big challenge to
develop simple and reliable synthetic methods for relatively
smaller iron oxide nanoparticle with designed chemical
components and controlled morphologies, which strongly
affect the properties of iron oxide nanomaterials. Particularly,
phase selective synthesis of iron oxide nanoparticle of smaller
size is very important, as iron oxide have many phases. Till date,
few methods are available in the literature for phase selective
iron oxide synthesis. Wan et al. shown the controlled synthesis
of three of the most common iron oxides a-Fe2O3, g-Fe2O3, or
Fe3O4, from the reaction between FeCl3$6H2O, urea, and tetra-
butylammonium bromide (TBAB) in ethylene glycol by altering
the calcinations conditions.7 Woo et al. also prepared maghe-
mite (a-Fe2O3) and magnetite (Fe3O4) nanoparticles from
thermal decomposition of Fe(CO)5 in the presence of residual
oxygen of the system and by consecutive aeration.8 Bronstein
et al. shown the inuence of the Fe–oleate complex structure on
its thermal properties and decomposition products, i.e., iron
oxide nanoparticles, their size, size distribution, and structure.9
Recently, Jia et al. shown a novel approach for synthesizing
Amongst various nanoparticles (NPs), metallic magnetic nano-
particles (NPs) having different shapes and sizes have received
considerable attention in the past decade because of their
various applications.1 In particular, iron oxide nanoparticles
have been of great interest, not only in fundamental properties
caused by their multivalent oxidation states, abundant poly-
morphism, and the mutual polymorphous changes in nano-
phase, but also in technological applications such as high
density magnetic recording media, sensors, catalysts, and
clinical usage. The enormous popularity of iron oxides as
catalysts derives from their broad application potential due to
easy handling, reasonably low cost, nontoxicity, and environ-
mentally friendly character.2 Currently, iron oxide catalysts are
utilized on large scale in laboratory, industrial, and environ-
mental processes to accelerate various reactions including
oxidation of carbon monoxide, decomposition of soot and NOx
in diesel exhausts, Fischer-Tropsch synthesis of hydrocarbons,
water–gas shi reactions, catalytic oxidations of other various
organic compounds, and catalytic decomposition of industrial
dyes.3 Most of these applications require the iron oxide nano-
particles to be chemically stable, uniform in size, and well-
dispersed in liquid media. The pristine nanoparticles of iron
oxides lean to aggregate into large clusters because of
Department of Chemical Sciences, Tezpur University, Napaam, Asaam-784028, India.
E-mail: spratihar@tezu.ernet.in; spratihar29@gmail.com
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c4ra04214j
single crystal a-Fe2O3 nanorings, employing
a double
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anion-assisted hydrothermal method.10 Along with this, several rather than to treat or clean up waste aer it has been created.17
other types of surfactants, such as cationic, anionic, or Maintaining greenness in synthetic methodology by prevention
nonionic, have been used for the synthesis of various type of of waste, avoiding the use of expensive catalyst or auxiliary
iron oxide.11 On the other hand, templates such as aluminum- substances (e.g. solvents, additional reagents) is always
oxide, silica, carbon nanotubes, polymer bers, and egg-shell needed.18 Towards this, the magnetic [Fe(ox)–Fe3O4] material
membranes have also been employed. Along with these was found to be useful as a magnetically recoverable catalyst for
template promoted iron oxide synthesis, metal oxalates the synthesis of bis(indolyl)methane derivatives in water. The
[M(C2O4)x/2] (x ¼ valency of the metal) have been also used as reusability of [Fe(ox)–Fe3O4] nanocatalyst was successfully
precursors for the production of oxidic and metallic powders.12 examined eight times with only a very slight loss of catalytic
The frequent use of metal oxalates can mainly be attributed to activity. The as-prepared Fe(ox)–Fe3O4 nanomaterials also
the stability of oxalate salts and the thermal decomposition of shown an excellent ability as a recyclable catalyst for the
the oxalate anion, which results in the formation of metal degradation of methylene blue (MB) under UV irradiation in
oxides (or metal) and carbon monoxide (or carbon dioxide).13 water.
Depending upon the decomposition parameters and chemical
composition of the starting material, various iron oxide phases,
such as maghemite (g-Fe2O3) and hematite (a-Fe2O3), can be
selectively obtained. Hermanek et al. have reported the forma-
Results and discussions
Characterization of iron nanomaterial
tion of nanocrystalline magnetite (Fe3O4) particles by heating
Previous literature showed that Fe(C2O4)$2H2O crystallizes in
two allotropic forms, a monoclinic C2/c and b orthorhombic
Cccm. As an important contribution, Tirado et al. showed the
synthesis of mesoporous Fe(C2O4) by dehydration (calcination)
of bulk monoclinic and micellar orthorhombic Fe(C2O4)$2H2O
precursors at 200 ꢀC.19 They observed that the nanoribbon
shaped particle can be preserved even aer dehydration. They
further applied the material as high-capacity lithium storage
materials with improved rate performance. Herein, we report
the synthesis of iron(oxalate) capped Fe(0) [Fe(ox)–Fe(0)] nano-
material from the aqueous phase room temperature reduction
of Mohr's salt, (NH4)2SO4$FeSO4 with sodium borohydride
(NaBH4) in presence of oxalic acid. Aer the completion of
reaction, [Fe(ox)–Fe(0)] nanomaterial was separated from the
reaction mixture by using an external magnet and washed with
ꢀ
iron oxalate to (more than) 360 C under exclusion of air and
moisture.14 Whereas, at higher temperatures (T > 400 C), the
ꢀ
coexistence of metallic (a-Fe) and thermodynamically unstable
¨
wustite (FeO) was observed. The decomposition temperature
and atmosphere can substantially inuence the oxalate-to-oxide
transformation chemistry, because the formation of the nal
product depends on the selective or simultaneous cleavage of
M–O and C–O units in metal oxalates.
Despite a large number of reports available on the applica-
tion of Fe(C2O4)$2H2O as precursors, for example, in sol–gel
method, hydrothermal techniques, a systematic study on the
control of reaction chemistry by the judicious choice of chem-
ical additives (oxidizing or reducing agent) is still on search,
which motivated us to undertake this work. Herein, we report
an efficient green synthesis of orthorhombic iron(oxalate) cap-
ped Fe(0) [Fe(ox)–Fe(0)] nonmaterial using sodium borohydride
(NaBH4) reduction of iron(II) salt in presence of oxalic acid at
room temperature in water. The reported method is a cost-
effective chemical route to produced Fe(C2O4) capped iron
nano particles [hereaer Fe(ox)–Fe(0)] on gram level without
high-temperature calcinations. The morphology has been
characterized by different physical methods such as FTIR, XRD,
FESEM, and TEM. The transformation of Fe(0) to Fe3O4 aer
keeping the Fe(ox)–Fe(0) at room temperature in water for a
prolonged time (1–2 days) also produced Fe(C2O4) capped Fe3O4
magnetic nanomaterial [hereaer Fe(ox)–Fe3O4] in gram scale.
On the other hand, the catalyst separation and reuse have
been increasingly important goals in the chemical community
from economic, safety, and environmental points of view. The
strategy of magnetic separation of nanoparticles is more effec-
tive than ltration or centrifugation as it prevents loss of the
catalyst. Towards this goal, various strategies have been
successfully demonstrated the applications of magnetite Fe3O4
nanoparticle-immobilized or -supported catalysts.15 In this
respect, Li et al. successfully demonstrated the synthesis of
various propargylamines from a robust and magnetically
recoverable Fe3O4 nanoparticle catalyzed three-component
coupling of aldehyde, alkyne, and amine.16 In view of the
ꢀ
water, and dried under vacuum at 50–60 C. The nanomaterial
characterization by X-ray diffraction (XRD) conrm the forma-
tion of single-phase orthorhombic Fe(C2O4)$2H2O, as the peaks
in XRD are in good agreement with reported XRD pattern of
Fe(C2O4)$2H2O (Fig. 1). From the XRD, we believe the presence
of Fe(0) in the material as both the peak for Fe(ox) and Fe(0)
overlapped each other. Some of the peaks in XRD are well
inuence of green chemistry, it is always better to prevent waste Fig. 1 XRD pattern of Fe(ox)–Fe(0) nano particle.
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matched with Fe3O4, which further suggests the oxidation of
Fe(0) to Fe3O4. At this stage, we thought that aer sodium
borohydride reduction, reduced iron(0) has been trapped in 3-D
network of orthorhombic iron oxalate and formed the iron-
(oxalate) capped Fe(0) [Fe(ox)–Fe(0)] nanomaterial (Fig. 4). FTIR
spectra of oxalic acid, Fe(C2O4)$2H2O, and the synthesized
Fe(ox)–Fe(0) nano particle were recorded to know the binding
behaviour of oxalic acid in Fe(ox)–Fe(0) nanoparticle. The peak
at 1710 cmꢁ1 for oxalic acid is characteristic for carbonyl group.
Other two peaks for oxalic acid appear at 1260 and 1123, which
is due to C–O and C–C bond of oxalic acid (Fig. 2). The char-
acteristic IR bands for meta carboxylates are in the range of
1650–1510 cmꢁ1 for the asymmetrical vibrations and 1400–
1280 cmꢁ1 for the symmetrical vibrations. FTIR analysis of
Fe(C2O4)$2H2O shows characteristic peaks at 1640, 1362, 1315,
819, and 493 cmꢁ1. The position and separation of n(COOꢁ)
bands, D, in the 1300–1700 cmꢁ1 region can be used to deduce
the carboxylate coordination mode. For D > 200 cmꢁ1, a
unidentate ligand is expected, whereas for D < 110 cmꢁ1, it is a
bidentate ligand. For a bridging ligand, D remain in between.
The peaks at 1640 and 1362 cmꢁ1 were typical for metal
carboxylate, in which oxalic acid is acting as a bidentate ligand
in Fe(C2O4)$2H2O. The other peak at 1315, 819 cmꢁ1 occur due
to C–O and C–C stretching vibration. The IR band of synthesized
Fe(ox)–Fe(0) nanomaterial well matched with Fe(C2O4)$2H2O,
which suggest the presence of iron oxalate in the synthesized
material. TEM micrographs of Fe(ox)–Fe(0) nanomaterial
showed brous morphology with layered structure (Fig. 3).
Further, the presence of Fe(0) in Fe(ox)–Fe(0) nanomaterial has
been checked from the reducing property of the material. For
this, reduction reaction of methylene blue was chosen as a
model reaction as it can be monitored visually as well as
spectrophotometrically.
Fig. 3 TEM/HR-TEM image of Fe(ox)–Fe(0).
Fig. 4 Schematic diagram of iron oxalate capped iron(0) nanoparticle.
visually dramatic reversible color change, but also provide an
engaging with the multifarious illustration of redox phenomena
of a species, reaction kinetics, and the principles of chemical
titration.20 There are some reports of reversible MB to LMB
redox reactions with the use of various reagents like; sulte–
iodate, iodate–arsenous acid, iodine–bisulfate.21 Snehalatha
et al. demonstrates the clock reaction involving methylene blue
and L-ascorbic acid.22 As an important contribution Pande et al.
showed Cu2O promoted oscillation between blue MB and col-
ourless LMB solution on periodic shaking.23 Through various
studies they showed that the oscillation cycle between MB and
LMB can be reproduced up to endless number due to the
formation of redox Cu(II)/Cu(I) system. Using the reversible
redox reaction between MB and LMB, we wanted to check the
existence of iron(0) nanoparticle in Fe(ox)–Fe(0) nanomaterial.
In aqueous medium the initial blue colour of the dye, MB,
faded away upon the addition of Fe(ox)–Fe(0) nanomaterial,
producing colorless leuco methylene blue (LMB) (Fig. 5). The
steady decrease of the two absorbance maxima at two specied
band (662 and 290 nm) and appearance of new band at 255 nm
due to the formation of leuco methylene blue (LMB) indicates the
progress of the reaction (Fig. 5). In the absence of Fe(ox)–Fe(0), no
noticeable decrease in absorbance of the dye was observed. The
sole use of sodium borohydride (NaBH4) and Fe(C2O4)$2H2O
was also found to be inactive towards the reduction of
Determination of iron(0) in oxalate bound iron nanomaterial
with reduction of methylene blue
The reversible redox reactions of methylene blue (MB) to leuco
methylene blue (LMB) not only provide a delightful reaction,
Fig. 2 Comparative FT-IR spectrum of Fe(ox)–Fe(0) with oxalic acid
and Fe(C2O4)$2H2O.
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During the course of iron nano particle promoted reduction
reaction of MB, black color of iron nanoparticle slowly turned
into light brown which may be due to the oxidation of Fe(0) to
its oxide. Detailed characterization of the oxide material was
done and elaborated in next section.
Iron(0) to iron oxide formation
Preliminary attempts were made to determine the oxidized
product of Fe(ox)–Fe(0) nanomaterial by employing UV-vis spec-
troscopy. The brown material was washed with distilled water
and homogeneously dispersed into water to record the absorp-
tion behaviour. Interestingly, two absorption peak (Lmax ¼ 307
and 370 nm) was observed in UV-vis window, which corresponds
to iron oxide (please see Fig. S1 in ESI†). The FTIR spectrum of
brown material showed two peaks at 1415 and 1660 cmꢁ1 due to
the oxalate coordination to metal oxide (Fig. 7).
The peak at 413 cmꢁ1 corresponds to Fe–O symmetric
bending vibration and at 563 cmꢁ1 corresponds to the Fe–O–Fe.
The FT-IR study of the brown material also veries the presence
of oxalate in the oxidized material. Further attempts were made
to determine the crystalline phase and degree of crystallinity by
employing X-ray diffraction analysis of brown material. The
XRD pattern of the sample conrms the structure of Fe3O4
nanocrystal because the position and the relative peak intensity
of the main peaks are well matched with the JCPDS card (85-
1436) for magnetite Fe3O4.8 On the other hand, some of the XRD
peak position and intensity also matched with orthorhombic
Fe(C2O4)$2H2O. From XRD analysis, we viewed the presence of
both Fe(ox) and Fe3O4 in the oxidized material [Fe(ox)–Fe3O4].
Interestingly, even aer keeping the Fe(ox)–Fe(0) sample in
open air for 1–2 days also produced brown material at room
temperature, which also contained Fe(ox) and Fe3O4. So, both
the MB oxidation and aerial oxidation of Fe(0) in Fe(ox)–Fe(0)
produced brown material, Fe(ox)–Fe3O4. A typical TEM image
of Fe(ox)–Fe3O4 was shown in Fig. 9. The initial brous
type morphology of Fe(ox)–Fe(0) has been retained in the
Fe(ox)–Fe3O4, which consists of Fe(ox) template bound Fe3O4
nanoparticle.24 So, aerial oxidation of Fe(0) bound with Fe(ox)
Fig. 5 Fe(ox)–Fe(0) promoted reversible methylene blue to leuco
methylene blue redox reaction.
MB to LMB, which further conrms the presence of Fe(0) in
Fe(ox)–Fe(0). Interestingly, aer the reduction of MB, the
observed colourless solution turned into blue, aer shaking the
colourless reaction mixture or purging air through it at room
temperature. While on standing, the blue colour of the solution
is diminished again by the addition of excess Fe(ox)–Fe(0) in
aqueous solution. The kinetics of Fe(ox)–Fe(0) nanomaterial
promoted MB to LMB reduction reaction was done by moni-
toring the steady decrease of absorbance at 662 nm band at an
interval of 5 second. The absorbance versus time plot shows a
prole of exponential nature, which indicated the pseudo-rst-
order nature of the reaction (Fig. 6). Furthermore, a plot of
ln(A) vs. time leads to a straight line, further conrms the pseudo-
rst order reaction kinetics (Fig. 6). The rate constant of iron
promoted reduction reaction of MB is 13.3 ꢂ 10ꢁ3 sꢁ1. Interest-
ingly, freshly prepared Fe(ox)–Fe(0) sample is more active.
The activity of the Fe(ox)–Fe(0) material towards the reduction
of MB to LMB is decreased upon keeping the sample in open air
for prolonged time. This may be due to the lesser no of active
Fe(0) particle in Fe(ox)–Fe(0), due to surface oxidation of Fe(0) to
Fe3O4 in air.
Fig. 6 Rate kinetics of Fe(ox)–Fe(0) catalyzed reduction of methylene
blue.
Fig. 7 Comparative FT-IR spectrum of Fe(ox)–Fe(0) and Fe(ox)–Fe3O4.
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Fig. 10 HR-SEM image of Fe(ox)–Fe3O4.
precursor upon heating the Fe(ox)–Fe3O4 nanomaterial at
150 ꢀC (Fig. 11). The size of these nanoparticles ranged between
4–15 nm. The selected area electron diffraction (SAED) analysis
exhibited the high polycrystalline nature of Fe3O4 nanoparticle
(Fig. S2 in ESI†). The SAED pattern also suggested the formation
of Fe3O4. We have also analyzed the lattice fringes of the
nanocrystal (Fig. 11), the lattice spacing between two planes is
observed to be ꢃ0.29 nm, corresponding to the distance of two
(220) planes of Fe3O4. The SEM image also suggested the
agglomerated structure of Fe3O4 because of the magnetic dipole
interaction between magnetic Fe3O4 nanoparticles. Further-
more, EDAX analysis authenticates the elemental composition
of the reddish-brown material, conrming the Fe3O4 formation
(Fig. 12).
The optical property of Fe(ox)–Fe3O4 NPs was investigated by
DRS experiment. The band gap of the synthesized material has
been calculated from the plot of (aEp)2 versus Ep based on the
direct transition and the extrapolated value of Ep at a ¼ 0. The
band gap value of Fe(ox)–Fe3O4 material was found to be 2.0 eV.
On the other hand, the band gap value of Fe3O4 is 1.7 eV (Fig. S2
and S3 in ESI†). The slightly higher band gap of the Fe(ox)–Fe3O4
compared to Fe3O4 may be attributed to small particle size
and some crystal lattice defects.25 The catalytic application of
Fig. 8 XRD pattern of Fe(ox)–Fe3O4 and its decomposed product
Fe3O4.
may transform Fe(0) to Fe3O4 keeping Fe(ox) intact. The calcu-
lated lattice spacing between two planes of Fe(ox)–Fe3O4 from
HR-TEM was ꢃ0.24 nm (Fig. 9). Further, scanning electron
microscope (SEM) of Fe(ox)–Fe3O4 material indicates porous
layered type structure (Fig. 10). Interestingly, pores are
produced between layers as a result of the aggregated assembly
between magnetic nanoparticles.
Furthermore, upon heating the Fe(ox)–Fe3O4 at 150 ꢀC in
oven for 36 h, brown material transformed into reddish-brown.
The XRD pattern of reddish-brown material (Fig. 8) agreed well
with the standard XRD pattern of magnetite Fe3O4 (magnetite,
JCPDS 85-1436). The conversion of Fe(ox)–Fe3O4 to Fe3O4 may
be due to the decomposition of Fe-bound oxalate upon heating.
TEM images of decomposed Fe3O4 material at different
magnication revealed that initial brous morphology of
Fe(ox)–Fe3O4 had been transformed into interconnected nano-
particles, because of the removal of organic species in the
Fig. 9 TEM/HR-TEM image of Fe(ox)–Fe3O4.
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Fig. 11 TEM/HR-TEM image of Fe3O4.
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indole has been chosen as a model reaction for the study.
Among the solvents tested, water was the most effective reaction
medium for this condensation reaction. Other organic solvent
like methanol, ethanol, dimethyl sulfoxide (DMSO) are also
effective for the reaction. But, slightly lower yields were
obtained with the solvent like dichloroethane, toluene, tetra-
hydrofuran (THF). When compared to other known catalytic
system, we found superiority of Fe(ox)–Fe3O4 material towards
the condensation between 4-bromo benzaldehyde and indole in
water at 100 ꢀC in air (Table 1). The Fe(ox)–Fe3O4 nanomaterial
catalyzed model reaction failed at room temperature. Although
quantitative yield of bis(indolyl)methanes product 1a was ach-
ieved with Fe(ox)–Fe3O4 as a catalyst in water however, in most
of the cases, solid lump of product in water was observed. So,
aer the completion of the reaction, water was removed from
the reaction mixture by decantation and acetone was added to
the reaction vessel to solubilize the product. Aer that the
insoluble catalyst Fe(ox)–Fe3O4 in acetone has been recovered
from the reaction mixture with the help of a tiny magnet
(Fig. 13). No signicant difference was observed when slightly
increasing the catalyst loading. But upon decreasing the catalyst
mol% from 5 to 2, signicant fall down in the product yield has
been observed. From screening of solvent, temperature, and
catalyst loading the optimized reaction conditions were
following: 1.0 equiv. of aldehyde, 2.1 equiv. of indole, and
5 mol% of Fe(ox)–Fe3O4 nanoparticles as a catalyst in water at
Fig. 12 HR-SEM image of Fe3O4.
Fe(ox)–Fe3O4 magnetic material towards organic transformation
and photo degradation reaction in water has been done and
elaborated in next section.
Fe(ox)–Fe3O4 promoted synthesis of bis(indolyl) methanes
The development of new methodologies for the synthesis of
bis(indolyl)methanes has received considerable amount of
interest to synthetic organic/medicinal chemists since bis(in-
dolyl)methanes possess a wide range of biological activity.26
Generally bis(indolyl)methanes are prepared by the condensa-
tion of indoles with various aldehydes or ketones in the pres-
ence of catalyst such as; protic or Lewis acids inorganic or
organic salt, ionic liquid or ionic liquid in conjugation with
some Lewis acids, zeolite, silica supported catalyst, etc.27
However, some of the reported methods have one or more of the
following limitations: for example, use of expensive catalyst and
volatile organic solvents, tedious work-up, complicated reaction
set-up, longer reaction times, low-yields of products, etc. In
terms of environmental compatibility, reusability, operational
simplicity, and ease of isolation, we also wanted to check the
activity of Fe(ox)–Fe3O4 towards the synthesis of bis(indolyl)
methanes. The reaction between 4-bromo benzaldehyde and
ꢀ
100 C in air. Furthermore, the blank test with model reaction
under optimized reaction condition provided corresponding
bis(indolyl)methane with low yield (Table 1).28 The involvement
of Fe(ox)–Fe3O4 towards the formation of bis(indolyl)methanes
from the condensation between aldehyde and indoles was done
from the FTIR experiment.29,30
For practical applications of such heterogeneous magneti-
cally recoverable systems, the lifetime of the catalyst and its
reusability are very important factors. For this, a set of experi-
ments was done for the condensation between 4-bromo
benzaldehyde and indole using Fe(ox)–Fe3O4 nanoparticles as a
catalyst. Aer the completion of the rst reaction, the catalyst
was recovered magnetically, washed with acetone for 3–4 times.
A new reaction was then performed with fresh reactants under
the optimized reaction conditions.
Table 1 Effect of catalyst for the synthesis of bis(indolyl)methanes in
water
#
Catalyst
Mol (%)
Time (h)
Yield (%)
1
2
3
4
5
6
7
8
9
Fe(ox)–Fe3O4
Fe(ox)–Fe3O4
Fe(ox)–Fe3O4
Fe3O4
FeCl3$6H2O
FeCl2$6H2O
Oxalic acid
Iodine
10
5
2
5
5
5
5
5
—
2
2
6
6
6
6
6
6
6
89
90
68
41
17
14
22
12
21
Fig. 13 Photo of (a): Fe(ox)–Fe3O4 nanoparticle in water; (b): Fe(ox)–Fe3O4
nanoparticle adsorbed on the magnetic stirring bar; (c): Fe(ox)–Fe3O4
nanoparticle in the reaction mixture; (d): after the completion of
reaction in acetone–water; (e): a magnet attract the Fe(ox)–Fe3O4
nanoparticle and magnetic stirring bar.
—
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Table 2 Substrate scope for Fe(ox)–Fe3O4 catalyzed condensation
reaction between aldehyde and indoles in water
In terms of TOF, Fe(ox)–Fe3O4 catalyst could be reused at
least 5 times without any change in activity (Fig. 14). However,
slight drop down in the product yield and TOF was observed in
6–8th cycle. The XRD analysis of the used Fe(ox)–Fe3O4 nano-
material aer 8th cycle was done. No appreciable change was
found in the XRD spectrum, which indicates the robustness of
the synthesized Fe(ox)–Fe3O4 material (Fig. 15). The generality
of the reaction has been tested with various aldehydes and
indoles under optimized reaction condition (Table 2). The
reaction with the electron-withdrawing aldehydes went
smoothly. However, the electron-donating group containing
aldehyde gave slightly lower yield of the corresponding bis(in-
dolyl)methanes. We have also tried the reaction with electron
rich aldehyde such as; 4-(n,n-dimethyl) benzaldehyde and pen-
tanal. In both the cases, reaction do not proceed to lead any
bis(indolyl)methane derivatives. On the other hand, both
electron-donating and withdrawing group containing indoles
#
R
R0
Product
Time (h)
Yield (%)
1
2
3
4
5
6
7
8
9
10
H
Me
Br
NO2
Br
NO2
NO2
Me
Me
H
H
H
H
H
OMe
NO2
Br
OMe
NO2
OMe
1a
1b
1c
1d
1e
1f
1g
1h
1i
1
2
1
1
1
3
3
4
4
3
90
82
90
92
78
76
85
68
72
81
1j
underwent the condensation reaction smoothly to provide the
desired bis(indolyl)methanes product in moderate to good
yields.
Application of Fe(ox)–Fe3O4 towards the degradation of
methylene blue
The robust metal oxide material catalyzed photo degradation is
very important to clean the industrial effluents especially
colored dye from textile, paper, pulp, and various industries.31
The photo degradation of any dye is related to the surface of
the oxide material and also to the light source (UV light or
visible light). To know the photo catalytic activity of the porous
Fig. 14 Catalytic cycle vs. turn over frequency (TOF) and yield plot of
Fe(ox)–Fe3O4 catalyzed condensation reaction between 4-bromo Fe(ox)–Fe3O4 material, methylene blue solution was used as a
benzaldehyde and indole.
representative dye. The Fe(ox)–Fe3O4 photocatalyst (10 mg) was
suspended in 5 mL of aqueous solution of MB (2 ꢂ 10ꢁ5 M).
Then, the solution was exposed to UV irradiation (365 nm) at
room temperature and time-dependent photo degradation of
the dye was studied using UV-vis absorption spectrophotom-
eter. Gratifyingly, magnetically recoverable Fe(ox)–Fe3O4 nano-
material catalyzed degradation of MB was observed in the
presence of UV light. The degradation of MB was found to be
very slow process in visible light (Fig. 16). However, in absence
of Fe(ox)–Fe3O4, the photo degradation of MB was stopped. To
know the effect of catalyst amount on the photo degradation of
MB, the rate kinetics was studied with varying amount of
Fe(ox)–Fe3O4. There is an enhancement of photo degradation
rate of MB with the increase of catalyst concentration up to
certain limit. Aer that, the photo degradation rate is inde-
pendent with catalyst amount (Fig. 17). The magnetically
recoverable Fe(ox)–Fe3O4 catalyst could be reused at least 10
times with no appreciable loss in its activity. The adsorption
plays very crucial role in the degradation of dye.32 For this the
Fig. 15 XRD pattern of Fe(ox)–Fe3O4 before and after the completion
of 8 cycle.
Fe(ox)–Fe3O4 promoted reaction of methylene blue was carried
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addition, the color of the solution slowly turned to yellow. Aer
few minutes, black Fe nano particle began to appear in a solu-
tion, which was separated from the solution with one tiny
magnet and washed with water, and dried under vacuum at 50
ꢀ
to 60 C, and collected for further study.
Reversible redox reaction between MB and LMB
At rst a 50 mL of aqueous homogeneous solution of methylene
blue (10ꢁ3 M) was prepared for the study. Then 200 mL of the
stock solution has been mixed with 20 mg of Fe(ox)–Fe(0)
nanomaterial in a UV-cuvette to record the progress of the reac-
tion. The progress of the reaction has been accounted from a
steady decrease of the two absorbance maxima at specied band
(662 and 290 nm) positions. At the same time one steady increase
of absorbance maxima at specied band of 255 nm was also
observed due to the formation of reduced leuco methylene blue
(LMB). The blue color of MB disappeared in 2 min. The solution
regained its original blue color just aer shaking. The visual
dramatic reversible color change goes on for about 5 cycles by
shaking and resting the reaction mixture with Fe(ox)–Fe(0).
Fig. 16 Fe(ox)–Fe3O4 promoted photo degradation of methylene
blue under different environment.
Degradation of methylene blue (MB) with Fe(ox)–Fe3O4
At rst a 50 mL of aqueous homogeneous solution of methylene
blue (10ꢁ3 M) has been prepared for the study. Then 100 mL of
the stock solution has been mixed with 20 mg of Fe(ox)–Fe3O4
nanoparticle in a 5 mL conical ask, and the volume of the
solution made up to 3 mL and kept under UV-light. The prog-
ress of the reaction has been accounted from a steady decrease
of the absorbance maxima at specied band (662 nm) positions
(Fig. 15).
Typical procedure for Fe(ox)–Fe3O4 catalyzed condensation
reaction between aldehydes and indoles
In a 10 mL Schlenk ask equipped with a magnetic bar, was
charged with Fe(ox)–Fe3O4 (0.05 mmol), in water (3 mL) in open
air and stirred vigorously for 5 min. Aer that the appropriate
aldehyde (1 mmol) and indole (2.2 mmol) was added to it and
placed into a constant temperature bath at 110 ꢀC and allowed
to continue. Aer completion, acetone was added to the reac-
tion mixture to solubilize the product. The catalyst was sepa-
rated with one tiny magnet and the product extracted with
ethylacetate (20 mL ꢂ 3) and washed with water (10 mL ꢂ 3),
brine (10 mL) and dried over anhydrous Na2SO4. Aer removing
the solvent the residue was subjected to silica gel column
chromatography (60–120 mesh, ethyl acetate–petroleum ether,
and gradient elution) to afford pure cross coupling product.
Fig. 17 Fe(ox)–Fe3O4 promoted photo degradation of methylene blue
under UV-light and the effect of catalyst on photodegradation rate.
out in dark. Interestingly, no gradual hypochromic shi was
observed in the dark, but slight decrease in absorbance was
found, which suggest slight adsorption of the MB in dark aer
prolonged time (Fig. S12 and S13†).33 We have also tried the
degradation reaction for other dye like rhodamine B or rose
Bengal. Although, the slow degradation rate was found for
rhodamine B, but no degradation was observed for rose Bengal.
Experimental section
Synthesis of iron nanoparticle
Instruments
In a typical procedure, 284 mg of Mohr's salt, (NH4)2SO4$FeSO4 Absorption spectra were recorded in a Dynamica Halo DB-30
was taken in 150 mL of distilled water in a 500 mL conical ask. double beam digital spectrophotometer (Switzerland) attached
To this 252 mg of oxalic acid, H2C2O4$2H2O was added and with
a Lab Companion RW-0525G chiller, and also in
stirred vigorously. In another 500 mL conical ask, a solution of SHIMADZU UV 2550 spectrophotometer with quartz cuvette. All
1.2 g of sodium borohydride, NaBH4 was prepared in 100 mL the samples for FTIR study were properly washed with distilled
distilled water. Now, NaBH4 solution was added slowly in an water at least ve times, and then dried under vacuum. Finally,
earlier prepared solution with vigorous stirring. During the samples for the FTIR spectra were recorded using IMPACT 410
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Thermo-Nicolet instrument from a thin transparent KBr pellet.
The XRD patterns of the sample was recorded on a Philips PW-
1710 X-ray diffractometer (40 kV, 20 mA) using Cu Ka radiation
1j. dH (400 MHz; acetone-d6): 3.62 (6H, s, –OMe), 5.82 (1H, s,
–CH), 6.68–6.74 (2H, m), 6.81–6.83 (4H, m), 7.25–7.29 (4H, m),
7.40 (2H, d, J ¼ 8.4 Hz), 9.84 (2H, s, –NH). dC (100 MHz, acetone-
d6): 40.4, 54.9, 101.6, 111.1, 111.8, 118.6, 124.4, 125.8, 127.6,
127.9, 128.7, 132.4, 145.1, 153.5.
ꢀ
˚
(k ¼ 1.5418 A) in the 2q range of 10–90 at a scanning rate of
0.5ꢀ minꢁ1. The XRD data were analyzed using JCPDS soware.
Surface morphology of all the samples were recorded by using a
scanning electron microscope (JEOL JSM5800) with an acceler-
ated voltage 5–20 kV. TEM images were acquired using JEOL
JEM-2010 microscopes with an operating voltage of 200 kV.
Conclusions
In summary, iron(oxalate) capped Fe(0) nanoparticle was
synthesized using a facile approach. The aerial oxidation of
Fe(0) leads to iron(oxalate) capped Fe3O4 nano material, which
is found to be useful as a magnetically recoverable catalyst for
the selective synthesis of bis(indolyl)methanes from the
condensation between aldehydes and indoles in water. In terms
of environmental compatibility, reusability, operational
simplicity, the magnetic iron(oxalate) capped Fe3O4 nano-
particles were very simple, effective and economical. The as-
prepared Fe(ox)–Fe3O4 nanomaterials also shows an excellent
ability as a recyclable catalyst for the degradation of methylene
blue (MB) under UV irradiation and are expected to be useful in
many other applications.
NMR data of bis(indolyl) methanes
1a. dH (400 MHz; acetone-d6) 5.89 (1H, s, –CH), 6.78 (2H, s),
6.86 (2H, t, J ¼ 8.0 Hz), 7.03 (2H, t, J ¼ 8.0 Hz), 7.15 (1H, t, J ¼ 7.6
Hz), 7.24 (2H, t, J ¼ 7.6 Hz), 7.31–7.38 (6H, m), 9.97 (2H, s, –NH).
dC (100 MHz, Acetone-d6) 40.3, 111.3, 118.5, 119.0, 119.4, 121.2,
123.7, 125.8, 127.2, 128.0, 128.7, 137.3, 145.1.
1b. dH (400 MHz; acetone-d6): 2.25 (3H, s, –CH3) 5.83 (1H, s,
–CH), 6.76 (2H, s), 6.85 (2H, t, J ¼ 7.6 Hz), 7.00–7.05 (4H, m),
7.24 (2H, d, J ¼ 8.2 Hz), 7.30–7.36 (4H, m), 9.97 (2H, s, –NH). dC
(100 MHz, acetone-d6) 28.4, 39.9, 111.3, 118.4, 119.2, 119.5,
121.2, 123.7, 127.3, 128.6, 128.6, 135.0, 137.3, 142.1.
1c. dH (400 MHz; DMSO-d6): 5.79 (1H, s, –CH), 6.79–6.83 (4H,
m), 7.00 (2H, t, J ¼ 8.0 Hz), 7.22–7.32 (6H, m), 7.40 (2H, d, J ¼ 8.0
Hz), 10.4 (2H, s, –NH). dC (100 MHz, DMSO-d6) 40.0, 112.0,
118.0, 118.8, 119.3, 119.5, 121.5, 124.1, 127.0, 131.0, 131.4,
137.1, 144.9.
1d. dH (400 MHz; acetone-d6): 6.07 (1H, s, –CH), 6.86–6.91
(4H, m), 7.06 (2H, t, J ¼ 7.2 Hz), 7.32 (2H, d, J ¼ 7.6 Hz), 7.38 (2H,
d, J ¼ 7.6 Hz), 7.62 (2H, s), 8.12 (2H, d, J ¼ 8.4 Hz), 10.1 (2H, s,
–NH). dC (100 MHz, acetone-d6) 40.1, 111.5, 117.6, 118.8, 119.2,
121.5, 123.3, 124.0, 126.9, 129.7, 137.3, 146.5, 153.1.
1e. dH (400 MHz; acetone-d6): 3.60 (6H, s, –OMe), 5.80 (1H, s,
–CH), 6.72 (2H, s), 6.79 (4H, s), 7.24–7.32 (4H, m), 7.41 (2H, d, J
¼ 7.0 Hz), 9.86 (2H, s, –NH). dC (100 MHz, acetone-d6) 39.7, 54.9,
101.5, 111.3, 112.0, 118.0, 119.1, 124.5, 127.5, 130.8, 131.0,
132.4, 144.6, 153.6.
1f. dH (400 MHz; acetone-d6): 6.48 (1H, s, –CH), 7.20 (2H, s),
7.59 (2H, d, J ¼ 9.2 Hz), 7.72 (2H, d, J ¼ 8.8 Hz), 8.0 (2H, d, J ¼
9.2 Hz), 8.20 (2H, d, J ¼ 8.8 Hz), 8.36 (2H, s), 10.93 (2H, s, –NH).
dC (100 MHz, acetone-d6): 39.1, 112.0, 116.2, 117.1, 119.9, 123.7,
126.1, 127.9, 129.7, 140.3, 141.4, 146.9, 151.5.
1g. dH (400 MHz; acetone-d6): 6.12 (1H, s, –CH), 6.93 (2H, s),
7.19 (2H, d, J ¼ 8.8 Hz), 7.37 (2H, d, J ¼ 8.4 Hz), 7.50 (2H, s), 7.64
(2H, d, J ¼ 8.4 Hz), 8.17 (2H, d, J ¼ 8.4 Hz), 10.33 (2H, s, –NH). dC
(100 MHz, acetone-d6): 39.5, 111.8, 113.5, 117.1, 121.5, 123.5,
124.3, 125.7, 128.7, 129.6, 135.9, 146.7, 152.2.
1h. dH (400 MHz; acetone-d6): 2.25 (3H, s, –CH3), 3.59 (6H, s,
–OMe), 5.76 (1H, s, –CH), 6.66–6.72 (2H, m), 6.77–6.81 (4H, m),
7.08 (2H, d, J ¼ 7.6 Hz), 7.23–7.25 (4H, m), 9.81 (2H, s, –NH). dC
(100 MHz, acetone-d6): 20.0, 39.9, 54.9, 101.8, 111.0, 111.8,
118.8, 124.4, 124.4, 127.7, 128.6, 132.4, 134.9, 142.1, 153.3.
1i. dH (400 MHz; acetone-d6): 2.26 (3H, s, –CH3), 6.16 (1H, s,
–CH), 7.09–7.13 (5H, m), 7.30 (2H, d, J ¼ 8.4 Hz), 7.98 (2H, d, J ¼
8.8 Hz), 8.33 (2H, s), 10.81 (2H, s, –NH). dC (100 MHz, acetone-
d6): 20.1, 39.1, 111.8, 116.4, 116.8, 121.3, 126.3, 127.5, 128.4,
129.1, 135.9, 140.3, 140.6, 141.1.
Acknowledgements
Financial support of this work by DST-New Delhi (to SP for
INSPIRE research grant) is gratefully acknowledged. SP fondly
dedicate this work to Prof. M. K. Chaudhuri on the occasion of
his 66th birthday. SP is also thankful to Anu for her constant
help and support.
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involvement of the catalyst in the reaction, 1 : 1 mixture
of Fe(ox)–Fe3O4 and 2a was taken in a motor pestle and
grinded for 0.5 h. The FTIR analysis of the mixture
showed shiing of peaks for both the C]O and C–H
stretching vibration to lower wave numbers as compared
to free aldehyde. The possible shi to lower wave
number is evident for the intimate interaction between
Fe(ox)–Fe3O4 catalyst and carbonyl group (for details
please see ESI†).
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