Development of an efficient magnetically separable nanocatalyst: Theoretical approach on the role of the ligand backbone on epoxidation capability

Article abstract of DOI:10.1039/c5ra17484h

Three chiral Schiff base ligands H₂L¹, H₂L², H₂L³ have been synthesized by treating (R)-1,2-diaminopropane separately with 3,5-dichlorosalicylaldehyde, 3,5-dibromosalicylaldehyde and 3,5-di

Full text of DOI:10.1039/c5ra17484h

View Article Online
View Journal
RSC Advances
PAPER
Development of an efficient magnetically separable
nanocatalyst: theoretical approach on the role of
the ligand backbone on epoxidation capability†
Cite this: RSC Adv., 2015, 5, 92634
Jaydeep Adhikary,a Arnab Datta,a Sanchari Dasgupta,a Aratrika Chakraborty,a
b
c
*
*
M. Isabel Menendez and Tanmay Chattopadhyay
´
Three chiral Schiff base ligands H2L1, H2L2, H2L3 have been synthesized by treating (R)-1,2-diaminopropane
separately with 3,5-dichlorosalicylaldehyde, 3,5-dibromosalicylaldehyde and 3,5-diiodosalicylaldehyde,
respectively. Three new asymmetric FeIII complexes, namely, FeL1Cl (1), FeL2Cl (2), FeL3Cl (3) have been
prepared from their corresponding ligands. The crystal structure of 2 reveals that the complexes are
mononuclear in nature. Circular dichroism (CD) studies suggest that the ligands and their corresponding
This article can be cited before page numbers have been issued, to do this please use: T. Chattopadhyay,
complexes contain an asymmetric center. The catalytic activity of these complexes toward the
J. Adhikary, A. Datta, S. Dasgupta, A. Chakraborty and M. I. Menendez-Rodriguez, RSC Adv., 2015, DOI:
epoxidation of alkenes has been investigated in the presence of iodosylbenzene (PhIO), in two solvents
10.1039/C5RA17484H.
CH3CN and CH2Cl2. The epoxide yield suggests that the order of their catalytic efficiency is 3 > 2 > 1.
This trend as well as the role of substitution on the ligand backbone on alkene epoxidation has also been
confirmed by density functional theory (DFT) calculations. For further adaptation, we attached our most
efficient homogeneous catalyst, 3, with surface modified magnetic nanoparticles (Fe3O4@dopa) and
thereby obtained the new magnetically separable nanocatalyst Fe3O4@dopa@FeL3Cl. This catalyst has
been characterized and its olefin epoxidation ability investigated in similar conditions to those used for
homogeneous catalysts. The enantiomeric excess of the epoxide yield reveals the retention of chirality
of the active site of Fe3O4@dopa@FeL3Cl. The catalyst can be easily recovered by magnetic separation
and recycled several times without significant loss of its catalytic activity.
Received 28th August 2015
Accepted 22nd October 2015
DOI: 10.1039/c5ra17484h
www.rsc.org/advances
can be stereospecically opened by nucleophiles to produce
various optically active 1,2-difunctional compounds.6,7 Besides,
Introduction
the chiral epoxides play an eminent role as drug intermediates
and chiral building blocks in the synthesis of optically active
complex molecules.8–11
Oxygen activation and transfer by cytochrome P-450 has
attracted the attention of organic chemists in particular, since it
catalyses the mono-oxygenation of various compounds, both
biotic and exobiotic, with high stereo- and regioselectivity under
mild conditions. Groves and Nemo proposed that the oxygen
transfer reaction proceeds through an oxohaem catalytic inter-
mediate.1,2 From these basic results, many optically active
complexes have been synthesized and found to be efficient
catalysts for the epoxidation of olens.3–5 The enantioselective
epoxidation of olens is one of the most important and chal-
lenging areas in organic synthesis because the resulting epoxide
In the literature, transition metal Schiff base complexes have
been extensively used as homogeneous catalysts for epoxida-
tion.12–15 They show high efficiency since the active site is easily
accessible, but their separation from the reaction mixture is
really a difficult task. Heterogeneous catalysts appear to avoid
this problems.16–18 In heterogeneous catalytic systems, the active
catalytic sites and the reactants are in the different phases, so
isolation and separation can be readily accomplished. Hetero-
genization is commonly achieved by entrapment or graing of
the active molecules on surfaces or inside the pores of a solid
support, such as silica, alumina, organic nanotube etc.19,20
However, the active sites in heterogeneous catalysts are not as
accessible as in a homogeneous system, and thus the activity of
such catalysts become lowered. So, a catalytic system showing
high activity and selectivity (like a homogeneous system) and
ease separation and recovery (like a heterogeneous system)
would be highly desirable. This goal can be achieved using
magnetic nanoparticles (MNPs), which are able to bridge the
aDepartment of Chemistry, University of Calcutta, 92 A. P. C. Road, Kolkata-700 009,
India
b
´
´
´
´
Departamento de Quımica Fısica y Analıtica, Universidad de Oviedo, C/ Julian
´
Claverıa 8, 33006 Oviedo, Spain. E-mail: isabel@uniovi.es
cDepartment of Chemistry, Panchakot Mahavidyalaya, Sarbari, Purulia, 723121,
India. E-mail: tanmayc2003@gmail.com
† Electronic supplementary information (ESI) available: FT-IR, UV-Vis spectra of
three complexes 1–3, TGA diagram of three complexes 1–3, DFT Cartesian
coordinates and absolute energies. CCDC 1025818 contains the supplementary
crystallographic data for complexes 1. For ESI and crystallographic data in CIF
or other electronic format see DOI: 10.1039/c5ra17484h
92634 | RSC Adv., 2015, 5, 92634–92647
This journal is © The Royal Society of Chemistry 2015

View Article Online
DOI: 10.1039/C5RA17484H
RSC Advances
Paper
Scheme 1 Schematic diagram for the formation of the three ligands (H2L1, H2L2, H2L3).
gap between homogeneous and heterogeneous catalysis, on the epoxide yield is yet to be explored. Thus we are prompted
preserving the desirable attributes of both systems. to look at this issue. For this purpose we have designed and
The development and growth of nanotechnology has caused synthesized FeIII complexes of H2L1, H2L2 and H2L3 (Scheme 1),
a continuous shi in every aspect of modern science. So far, it characterized them and investigated their catalytic efficiencies
has changed the perspective of the scientic community as epoxidation catalysts, in presence of terminal oxidant PhIO
towards catalysis and brought remarkable transformations in and in two solvents, CH3CN and CH2Cl2 under mild conditions.
the synthetic chemical processes. MNPs nowadays have attrac- The order of their experimental catalytic efficiency has also been
ted immense scientic and technological interest due to their ratied by theoretical calculations. Finally, we attached the
unique physical and chemical properties which make them an most efficient catalyst to surface modied nanoparticles (MNPs)
ultimate choice in the eld of catalysis.21–23 The preparation and and investigated its catalytic behavior. In this way we have
the use of MNPs in organic synthesis has become a subject of developed an efficient magnetically separable asymmetric
intense investigation as they offer advantages in clean and nanocatalyst for alkenes epoxidatation that preserve its activity
sustainable chemistry.24–27 The use of MNPs as catalysts in aer several catalytic cycles.
chemical synthesis has been extensively studied in recent years
as their magnetic properties allow ready separation from the
reaction mixture via the aid of an external magnet without
Results and discussion
Preparation and characterization of complexes (1–3)
cumbersome ltration and centrifugation techniques. The
magnetic eld-assisted separation can reduce energy
consumption, catalyst loss and save the time in achieving
catalyst recovery. Besides, these nanosized particles offer a large
number of potential active sites for the reactants as a result of
a large surface area to volume ratio which eventually results in
higher yields.
Recent reports have shown that the mechanism of epoxida-
tion of alkenes catalyzed by metal complexes, and the yield of
products depends on several factors: the nature of the alkenes,
oxidant, counter-ion of the catalyst, structure of the ligand
moiety of catalyst and the added Lewis base.28 Steric and elec-
tronic effects of catalyst and olenic substrates on epoxidation
property have been investigated by several research groups
also.29 However, the role of the halogen substituent present at
the ligand back bone in transition metal Schiff base complexes
Three purposely selected Schiff bases H2L1, H2L2 and H2L3 have
been prepared by treating 3,5-dichlorosalicylaldehyde, 3,5-
dibromosalicylaldehyde and 3,5-diiodosalicylaldehyde sepa-
rately with (R)-1,2-diaminopropane in ethanolic medium,
respectively. Further treatment of H2L1, H2L2 and H2L3 with
iron(III) chloride gives complexes 1, 2 and 3, respectively. We
failed to get X-ray diffractable single crystals of 1 and 3; only
a single crystal of 2 is obtained from DMF solvent. The
complexes are characterized by regular physiochemical
methods. All of them exhibit characteristic IR bands at the
range of ꢀ1609–1638 cmꢁ1 and ꢀ1498–1526 cmꢁ1 assigned to
C]N and skeletal vibrations (Fig. S1, ESI†), respectively. Elec-
tronic absorption spectra of the complexes in acetonitrile
medium show multiple intense bands in the UV and visible
regions (Fig. S2, ESI†). In these complexes, the absorption
This journal is © The Royal Society of Chemistry 2015
RSC Adv., 2015, 5, 92634–92647 | 92635

View Article Online
DOI: 10.1039/C5RA17484H
Paper
RSC Advances
maxima observed in the near-UV regions (below 300 nm) are
caused by p / p* transitions involving the phenolate units.
Intense, high energy bands are also observed in the region
between 300 and 350 nm, which are assigned to charge transfer
transitions from the out-of-plane pp orbital (HOMO) of the
2
2
2
phenolate oxygen to the half-lled dx ꢁy /dz orbital of high spin
iron(III). The lowest energy bands (around 500 nm) are proposed
to arise from charge-transfer transitions from the in-plane
pp orbital of the phenolate to the half-lled dp* orbital of
iron(III).30 Molar conductance of three complexes in acetonitrile
medium are 2.3, 4.5, 5.8 Uꢁ1 cm2 Mꢁ1, which implies that all are
nonelectrolyte in solution. Thermogram of the complexes
ꢂ
(Fig. S3, ESI†) reveal that 1, 2 are stable up to 80 C where 3 is
stable upto 255 ꢂC. Initial weight loss for both 1 and 2 are
ꢀ3.1% (theoretical wt. loss are 3.4% and 2.6% for 1 and 2
respectively) may be due to loss of one absorbed water molecule.
On further heating all three species generate Fe2O3 as the
thermally stable end product (for complex 1, expt. wt loss ¼
58.33%, theo. wt loss ¼ 56.51%; for complex 2 expt. wt loss ¼
69.74%, theo. wt loss ¼ 67.5% and for complex 3 expt. wt loss ¼
75.3%, theo. wt loss ¼ 73.82%). Circular dichroism (CD)
measurements for all the ligands and their corresponding
complexes have been performed. The CD spectra of ligand
(H2L3) and 3 (FeL3Cl) are presented in Fig. 1 (as representative
of all ligands and their corresponding complexes). Interestingly,
the nature of CD spectra for FeL3Cl is the reverse of H2L3,
suggesting the inversion of conguration aer complexation.
Fig. 2 Crystal structure of complex 2 with atom numbering scheme
having 50% ellipsoid probability.
and the two imines N atoms of Schiff-base constitute the basal
plane and one chlorine atom is occupying the axial position.
Two Fe–O (phenolic) have almost similar bond distance,
˚
1.888(6) and 1.890(6) A, respectively. Two Fe–N (imine)
˚
distances of 2.093(8) and 2.088(8) A, are also comparable
˚
whereas Fe–Cl bond distance is 2.222(3) A. The methyl group on
the 1,2-diaminopropane is disordered over two positions, C10a
and C10b with rened occupancies of 0.65/0.35 respectively.
This is intended to avoid steric clashes with the lattice dime-
thylformamide (DMF) molecule, which is also positionally
disordered and rened with restraints on its geometry. The
crystal is low diffracting (theta max ¼ 21.80ꢂ) for the disorder
detected. The atoms at lower occupancies (methyl C10b and
DMF molecule) have been isotropically rened.
Description of crystal structure of complex 2
The X-ray structure of complex 2 has been depicted in Fig. 2.
Selected bond lengths and bond angles are listed in Table S1
(ESI†). According to the X-ray structure the Schiff base ligand
coordinates to one iron atom in a tetradentate mode. The metal
center is ve-coordinated and the geometrical index (s5) is equal
to 0.18, slightly shied from zero, implying the geometry is
slightly distorted square-pyramid. The two phenolic O atoms
Optimization of the catalytic epoxidation conditions
In all cases, we have followed the same procedure to study the
epoxidation reaction catalyzed by the three homogeneous cata-
lysts (1–3). In a typical reaction, (E)-stilbene (30 mmol) (as
representative), 3 (0.1 mmol) and PhIO (30 mmol) are mixed in
25 mL dichloromethane/acetonitrile and stirred for 4 h at room
temperature (see Scheme 2). The progress of the reaction is
monitored by TLC. Aer usual work up and chromatographic
purication, the isolated yields of epoxide are found to be 74% in
acetonitrile and 62% when dichloromethane used as a solvent.
From this initial experiment we proceed to further optimization
of the catalysis conditions, viz. amount of catalyst and terminal
oxidant, along with the time required to obtain the maximum
Fig. 1 Circular dichroism (CD) spectra of the chiral ligand (H2L3) and Scheme 2 Model epoxidation reaction where (E)-stilbene is oxidized
the chiral complex 3 (FeL3Cl).
by iodosylbenzene, PhIO, in the presence of catalyst 3 (FeL3Cl).
92636 | RSC Adv., 2015, 5, 92634–92647
This journal is © The Royal Society of Chemistry 2015

View Article Online
DOI: 10.1039/C5RA17484H
RSC Advances
Paper
epoxide yield. To optimize the amount of catalyst, its concen-
tration is varied between 0.05 and 0.2 mmol per 30 mmol of (E)-
stilbene. Epoxide yield increases when the amount of catalyst is
increased from 0.05 mmol to 0.1 mmol but the yield remains the
same with further increment of catalyst amount up to 0.2 mmol.
Then, the reaction has been studied with varying amounts of
PhIO between 25 and 35 mmol per 30 mmol of (E)-stilbene. The
yield of the epoxide also is studied by varying the time period
between 2 and 6 h. We observe the epoxide yield attains the peak
aer 4 h of reaction and remain the same even aer 6 h. An
optimum of 87.84 mg (0.1 mmnol) of catalyst, PhIO (30 mmol)
and 4 h reaction time are ideal for achieving the best yield. Table
S2 (ESI†) represents the (E)-stilbene epoxide yields under
different conditions using 3 as catalyst. The optimum reaction
conditions thus determined have been followed in 1 and 2
catalyzed epoxidation reactions. The essential role played by the
catalyst is evident from the extremely low (<2%) yield of epoxide
found in a blank reaction carried out in absence of the catalyst.
Scheme 3 Schematic presentation of the generation of Fe(V) oxo
species during epoxidation of alkenes.
peaks at 306 and 487 nm. But, aer addition of PhIO both
disappear and a shoulder around 377 nm emerges (Fig. S4,
ESI†). This observation clearly suggests the generation of higher
valent FeV]O species as previously reported (see Scheme 3).15c,d
The generation of FeV]O species is very common that were
conrmed previously both experimentally an theoretically.31–36
Catalytic epoxidation with PhIO
Here we investigate the epoxidation of three alkenes [(E)-stil-
bene, (Z)-stilbene, or styrene] catalyzed by complexes 1–3 in
CH3CN or CH2Cl2, with PhIO as oxidant (Table 1). From Table 1
three main features may be drawn. First, in all cases CH3CN
Theoretical investigation
Theoretical investigation on the effect of halogen substit-
solvent renders higher conversions and yields than CH2Cl2. uent on the oxidation of iron complex catalysts
Second, using the same catalyst for the epoxidation of the three
Geometry and charge distribution of the non-oxidized Fe(III)
olens, all of them produce similar epoxide yields, with a very complexes. All the initial complexes are electrically neutral and
slight preference for (E)-stilbene. Finally, complex 3 emerges as only differ in the halogen at the ortho and para positions of the
the best catalyst for the epoxidation reaction under study, two benzene rings of the Schiff base bonded to the Fe(III) center
rendering yields close to 70% in CH3CN. However, complex 2 (Fig. 3). The non-oxidized initial complexes could present low
and complex 1 show average yields of 65% and 54% in CH3CN, (2S + 1 ¼ 2) or high (2S + 1 ¼ 6) spin depending of the distri-
respectively. The epoxidation process has also been monitored bution of the 5 most external iron electrons over the non-
by UV-Vis spectroscopy. The isolated Fe(III) complex has two degenerate d orbitals of the metal in the complexes. We have
Table 1 Epoxidation of alkenes catalyzed by the complexes 1–3 in CH3CN and CH2Cl2 with PhIO
Catalysta
Substrate
(E)-Stilbene
(Z)-Stilbene
Styrene
Solvent
Conversion (%)
TONb
Yieldc (%)
eed,f (%)
1
CH3CN
CH2Cl2
CH3CN
CH2Cl2
CH3CN
CH2Cl2
CH3CN
CH2Cl2
CH3CN
CH2Cl2
CH3CN
CH2Cl2
CH3CN
CH2Cl2
CH3CN
CH2Cl2
CH3CN
CH2Cl2
58
55
61
52
60
47
64
61
61
57
57
59
79
66
78
74
72
67
43.5
41.4
45.9
39
45
35.4
48
45.9
45.7
42.9
50.4
44.4
59.4
49.5
58.5
55.5
54
55
50
Tracee
Tracee
18 (trans)
15 (trans)
20
55 (cis : trans ¼ 70 : 30)
48 (cis : trans ¼ 80 : 20)
53
44
68
57
Tracee
16
2
3
(E)-Stilbene
(Z)-Stilbene
Styrene
Tracee
25 (trans)
21 (trans)
Tracee
Tracee
21
67 (cis : trans ¼ 77 : 23)
54 (cis : trans ¼ 60 : 40)
61
53
74
62
(E)-Stilbene
(Z)-Stilbene
Styrene
14
72 (cis : trans ¼ 90 : 10)
27 (trans)
22 (trans)
11
67 (cis : trans ¼ 75 : 25)
69
65
50.4
Tracee
a
Catalyst (0.1 mmol), alkenes (30 mmol), PhIO (30 mmol), and CH3CN or CH2Cl2 (25 mL), are stirred at room temperature for 4 h in air. b TON ¼
moles of substrate converted per mole of catalyst. c Isolated epoxide yield. d Determined by 1H NMR (300 Hz) in the presence of Eu(hfc). e Trace ¼
concentration < 5%. f Conguration not determined.
This journal is © The Royal Society of Chemistry 2015
RSC Adv., 2015, 5, 92634–92647 | 92637

View Article Online
DOI: 10.1039/C5RA17484H
Paper
RSC Advances
Fig. 4 Main DFT geometrical parameters of the final oxidized Fe(V)
complexes. (UB3LYP/6-311+G(d,p) with LANL2DZ pseudopotential for
Fe, Cl, Br, and I.) All distances in angstroms.
Fig. 3 Main DFT geometrical parameters of the initial halogen Fe(III)
complexes. (UB3LYP/6-311+G(d,p) with LANL2DZ pseudopotential for
Fe, Cl, Br, and I.) All distances in angstroms.
alkenes, it is interesting to characterize the oxidized metal
complexes through DFT calculations. The three oxidized
complexes, complex 1-oxo, complex 2-oxo, and complex 3-oxo,
have been optimized including the released chloride anion in
such a way that the whole system is electrically neutral (Fig. 4).
It is interesting to note that the presence of the oxygen atom at
the apical position of the complex yields less distorted square
pyramid geometries for the Fe(V) than that observed for Fe(III).
computationally checked that high spin initial complexes are
about 15 kcal molꢁ1 more stable in Gibbs energy than the cor-
responding low spin ones (Table S3, ESI†).
DFT geometrical parameters essentially match those
provided by X-ray except for a slight lengthening of bond
distances. Metal ion is bonded through stronger bonds to both O
˚
N–Fe bonds (1.930 A in average) are now stronger and closer in
˚
length to O–Fe bonds (1.917 A in average), whereas O–Fe–O
˚
atoms (bond distances of 1.91 A) than to both N atoms (average
and N–Fe–N angels are both of about 85ꢂ. Concerning NBO
charges, the same trend described for the initial complexes is
found for the oxidized ones: heavier halogens more easily
donate electron charge that goes by through O and N atoms
bonded to Fe(V) and from the metal to the apical oxygen
(oxygen charge evolves from ꢁ0.350 to ꢁ0.372 and to ꢁ0.385
when going from chlorine to bromine and iodine oxidized
complexes).
˚
bond distances of 2.12 A), forming average O–Fe–O and N–Fe–N
bonds of 100.3ꢂ and 77.4ꢂ, respectively. The metal is also bonded
˚
to a chloride anion at a distance of 2.37 A and placed slightly
over a distorted plane of a square pyramid. Such similar geom-
etries for the three initial complexes indicate that geometry is
not a factor determining a different oxidation capability.
As oxidation is mainly and electronic issue, we also checked
the NBO charge of some relevant atoms at each initial complex.
The charges of the halogens in the aromatic rings become
increasingly positive when moving from chlorine to bromine
and iodine systems (ꢁ0.02 to +0.08 to +0.20, respectively for
para halogens), as expected due to their diminishing electro-
negativity. The electron density that halogens donate makes the
negative charge on atoms bonded to Fe(III) slightly higher as the
atomic number of the halogen increases. Thus, as an average, O
atoms charges go from ꢁ0.686 to ꢁ0.688 to ꢁ0.693, whereas
those of N atoms vary from ꢁ0.521 to ꢁ0.522 to ꢁ0.525 for
chlorine, bromine, and iodine systems, respectively. However,
the slightly more negative charges of atoms bonded to Fe(III) do
not make the metal more negatively charged since it lets the
charge ow to chloride ligand (Fe charges goes from 1.219 to
1.220 and to 1.222 whereas that of chloride goes from ꢁ0.502 to
ꢁ0.505 and to ꢁ0.511).
Reaction energy for the intermediate oxidation of the catalyst
by PhIO
As already said, initial Fe(III) complexes are assumed to be
oxidized by PhIO ((a) in Fig. 5) to render the oxidized complexes
just described and PhI ((b) at Fig. 5), being this oxidation an
Geometry and charge distribution of the oxidized Fe(V)
complexes. Assuming that the metal complexes become
oxidized by iodosylbenzene, PhIO, during the epoxidation of
Fig. 5 Main DFT geometrical parameter of the oxidized ((a), PhIO) and
reduced ((b), PhI) iodobenzine molecule. (UB3LYP/6-311+G(d,p) with
LANL2DZ pseudopotential for I.) Distance in angstroms.
92638 | RSC Adv., 2015, 5, 92634–92647
This journal is © The Royal Society of Chemistry 2015

View Article Online
DOI: 10.1039/C5RA17484H
RSC Advances
Paper
Table 2 Reaction electronic and Gibbs energy in acetonitrile solution
calculated at B3LYP/6-31+G(d,p) with LANL2DZ psudopotential for Fe,
Cl, Br, and I. Left values at each column for the reaction between the
most stable spin-states of reactants and products; right values for the
reaction in the doublet state (see text)
Table 2 collects the electronic and Gibbs energy of the
catalyst oxidation step (calculated as the summation of the
energy of the corresponding products menus summation of the
energy of the corresponding reactants in eqn (1)) for both
hypotheses. For the rst one, considering the electronic energy,
only the products of oxidized complex 3 are more stable than
the corresponding initial reactants. In terms of Gibbs energy in
acetonitrile solution, all the oxidized products are less stable
than the initial ones. However, both electronic and Gibbs
energies agree to show that the yield of oxidized complexes is
expected to increase from chlorine to bromine and iodine
complexes. The second hypotheses, renders the same trend,
now predicting exothermic processes in all the cases. As
a consequence, what is most important for this study is that
theoretical calculations based on any of the possible assump-
tions agree to point the iodine catalyst as the most effective one.
Complex
DEe (kcal molꢁ1
)
DGsol (kcal molꢁ1
)
1
2
3
6.6
3.7
ꢁ0.6
9.1
6.2
2.1
intermediate step in the epoxidation of alkenes. The oxidation
of the catalyst is schematically represented by eqn (1).
Complex + PhIO 5 complex-oxo + PhI
(1)
Preparation and characterization of Fe3O4@dopa@FeL3Cl
Theoretical results show that the high-spin initial complexes
are about 15 kcal molꢁ1 more stable in Gibbs energy than low-
spin ones, whereas oxidized complexes present similar energies
for both spin states (Table S3, ESI†). With these data at hand
and to fulll the required spin-conservation along the reaction
displayed in eqn (1), two situations are possible. On the one
hand, an ancillary reaction involving a spin transition from
a quartet to a sextet could happen associated to the oxidation of
the catalyst to allow its spin crossover, or, on the other hand, the
oxidation of the catalyst could take place in the doublet state.
Few examples of spin ip during catalysis are already re-
ported.37–39 The second assumption involves an energy supply
from the environment to excite Fe(III) complexes from the most
stable sextet to the doublet one.
A schematic diagram of Fe3O4@dopa@FeL3Cl formation is
presented in Scheme 4. Firstly, the preparation of magnetic
nanoparticles Fe3O4 and then their surface modication has
been done following the procedure earlier reported by R. S.
Varma et al. in 2009.40 For the preparation of Fe3O4@
dopa@FeL3Cl, 1 g of FeL3Cl is added in the dispersed acetoni-
trile solution of amine-functionalized nano-Fe3O4 (500 mg). The
mixture is stirred for 12 h at room temperature. The product is
allowed to settle, washed several times with acetonitrile, and
ꢂ
dried under vacuum at 60 C for 2 h. Newly prepared Fe3O4@
dopa@FeL3Cl has been characterized by the following process.
Fourier-transform infrared spectra (FTIR) of Fe3O4,
Fe3O4@dopa and Fe3O4@dopa@FeL3Cl are represented in
Fig. 6. The peaks at around 583 cmꢁ1 and 636 cmꢁ1 are the
Scheme 4 Schematic representation of the preparation of Fe3O4@dopa@FeL3Cl particles.
This journal is © The Royal Society of Chemistry 2015
RSC Adv., 2015, 5, 92634–92647 | 92639

View Article Online
DOI: 10.1039/C5RA17484H
Paper
RSC Advances
Fig. 6 Fourier-transform infrared spectra (FTIR) spectra of Fe3O4 NPs,
Fe3O4@dopa and Fe3O4@dopa@FeL3Cl.
Fig.
8
Solid state UV spectra of Fe3O4 NPs, Fe3O4@dopa and
Fe3O4@dopa@FeL3Cl.
characteristic absorptions of the Fe–O bond, which conrmed
the presence of iron oxide, as previously reported.41 For Fe3-
O4@dopa a peak around 1485 cmꢁ1 arises, which may be due to
the vibration of the benzene ring present at dopamine moiety.
Several new peaks are generated for Fe3O4@dopa@FeL3Cl,
along with the characteristic peak of Fe3O4. The peak at 1526
cmꢁ1 may be assigned as the skeleton vibration of the complex,
the sharp peak at 1624 cmꢁ1 may be due to the C]N vibration
of incorporated complex moiety. The FTIR spectra allow to
conclude that the desired surface medication of MNPs has been
successfully done.
The degree of crystallinity of magnetic Fe3O4, Fe3O4@dopa
and Fe3O4@dopa@FeL3Cl are obtained from PXRD measure-
ments (Fig. 7). The PXRD data of the synthesized magnetic
nanoparticles show diffraction peaks at 2q ¼ 29.69ꢂ, 35.19ꢂ,
42.61ꢂ, 56.5ꢂ, 59.5ꢂ and 62.36ꢂ which can be assigned to the
(220), (311), (400), (422), (511) and (440) planes of Fe3O4,
respectively, indicating that the Fe3O4 particles in the nano-
particles are pure Fe3O4 with a cubic spinel structure. These
match well with the standard Fe3O4 sample. The same peaks are
observed in both of the Fe3O4@dopa and Fe3O4@dopa@FeL3Cl
in PXRD patterns, indicating that the resultant nanoparticles
contain pure Fe3O4 with a spinel structure and that the graing
Fig. 7 Power X-ray diffraction (PXRD) spectra of Fe3O4 NPs, Fe3- Fig. 9 Thermogravimetric analysis (TGA) diagrams of Fe3O4 NPs,
O4@dopa and Fe3O4@dopa@FeL3Cl.
Fe3O4@dopa and Fe3O4@dopa@FeL3Cl.
92640 | RSC Adv., 2015, 5, 92634–92647
This journal is © The Royal Society of Chemistry 2015

View Article Online
DOI: 10.1039/C5RA17484H
RSC Advances
Paper
Fig. 10 Scanning electron microscopy (SEM) images of (a) Fe3O4-NPs, (b) Fe3O4@dopa and (c) Fe3O4@dopa@FeL3Cl.
process does not induce any phase change of Fe3O4 as previ- morphologies of Fe3O4@dopa (Fig. 10(b)) and Fe3O4@
ously mentioned by other group previously.42 dopa@FeL3Cl (Fig. 10(c)) are quite different from that Fe3O4-
Solid state UV spectrum of Fe3O4 is very similar to a previ- NPs. Both Fe3O4@dopa and Fe3O4@dopa@FeL3Cl contained
ously reported one.43 The existence of Fe3O4 in the Fe3O4@dopa agglomerated particle having larger sized particle in the case of
and Fe3O4@dopa@FeL3Cl species has been proved by Fe3O4@dopa@FeL3Cl. Alteration in morphology from Fig. 10(a)
comparing the solid state UV (Fig. 8). FeL3Cl has a broad and (b) conrmed the surface modication process.
shoulder at ꢀ495 nm (Fig. S2, ESI†). Increment of the absor-
TEM images of Fe3O4-NPs, Fe3O4@dopa and Fe3O4@
bance at ꢀ500 nm for Fe3O4@dopa@FeL3Cl clearly suggests the dopa@FeL3Cl are shown in Fig. 11(a)–(c), respectively. A close
conjugation of FeL3Cl onto Fe3O4@dopa.
examination of the TEM image in Fig. 11(a) reveals that the
For conrmation of the successful surface modication of magnetic nanoparticles are quasi-spherical with the average
magnetic Fe3O4 and further FeL3Cl graing on the surface of diameter of 10–15 nm. The measurement of the hydrodynamic
Fe3O4@dopa, thermogravimetric analysis (TGA) is carried out. size of Fe3O4-NPs by dynamic light scattering (DLS) shows stable
As shown in Fig. 9, the trace amount of weight loss within the non-aggregated particles with a mean diameter of 30 nm
range of 100–200 ꢂC is caused by the trace amount of water (Fig. S6, ESI†). The NPs show good stability in water. The
vapor adsorbed by magnetic Fe3O4. 7% weight loss occurred in observed sizes of the NPs from TEM images are approximately
ꢂ
the range of 30–800 C for Fe3O4-NPs. Weight loss within the smaller than the hydrodynamic diameter obtained from the
same range for Fe3O4@dopa and Fe3O4@dopa@FeL3Cl are 19% DLS experiment. Transmission electron microscopy measure
and 27.6%, respectively. TGA results clearly suggest that the the size in the dried state of the sample, whereas DLS measure
enhancing weight loss is due to the increasing amount of the size in the hydrated state of the sample, so the size
attached organic moiety from Fe3O4@dopa to Fe3O4@ measured by DLS is a hydrodynamic diameter and is larger. The
dopa@FeL3Cl. Similar results have been described by other nanoparticles, depicted in Fig. 11(b) aer dopamine encapsu-
groups.44 Raman spectrum of FeL3Cl and Fe3O4@dopa@FeL3Cl lation step, have a discrete core–shell structure, and Fe3O4 is
are depicted in Fig. S5 (ESI†). FeL3Cl has peaks at 500, 594, 776, surrounded by 3–5 nm thick dopamine shell as previously re-
876, 967, 1312, 1442, 1516, 1590 cmꢁ1. FT Raman spectra for ported.41 Fig. 11(c) illustrates the graing of FeL3Cl on to the
Fe3O4@dopa@FeL3Cl resemble the unbound FeL3Cl demon- surface of Fe3O4@dopa. Energy-dispersive X-ray spectroscopy
strating that the catalyst moiety remains intact during the (EDX) spectrum of Fe3O4@dopa@FeL3Cl is depicted in Fig. 12.
process of encapsulation as earlier reported.20c,45
Fe and O signals come from the Fe3O4 nanoparticles and carbon
SEM images of Fe3O4, Fe3O4@dopa and Fe3O4@ (C) from dopamine. Signals of I and Cl are responsible for the
dopa@FeL3Cl are presented in Fig. 10. Fig. 10(a) suggests Fe3O4 presence of those elements in FeL3Cl. The gold (Au) signals
NPs have square shaped morphology with 20–30 nm size. The come from the coating material of the instrument.
Fig. 11 Transmission electron microscopy (TEM) images of (a) Fe3O4-NPs, (b) Fe3O4@dopa and (c) Fe3O4@dopa@FeL3Cl.
This journal is © The Royal Society of Chemistry 2015
RSC Adv., 2015, 5, 92634–92647 | 92641

View Article Online
DOI: 10.1039/C5RA17484H
Paper
RSC Advances
Fig.
12 Energy-dispersive
X-ray
(EDX)
spectrum
of
Fe3O4@dopa@FeL3Cl.
Epoxidation property of Fe3O4@dopa@FeL3Cl
Fig. 13 Magnetic curves of Fe3O4 NPs, Fe3O4@dopa and Fe3O4@
dopa@FeL3Cl; and inset: image of the efficiency of the magnetic
separation of the catalyst.
We also evaluated the optimum reaction conditions to achieve
the maximum epoxide yield with the heterogeneous catalyst as
already done for the homogeneous system (Table S4, ESI†). For
this purpose the weight of Fe3O4@dopa@FeL3Cl is varied
between 50 and 200 mg per 3 mmol of (E)-stilbene. Enhance-
ment in the yield of epoxide is observed when the amount of
catalyst is increased from 50 to 100 mg but the yield remain
same with further increment of catalyst amount up to 200 mg.
The reaction has also been studied with varying the amounts of
terminal oxidant, PhIO and the time. The results show that
using 3 mmol PhIO and 6 h stirring the best epoxidation yield is
obtained. At the end of the reaction, the catalyst has been
magnetically separated out and reuse for further epoxidation.
Here it is noteworthy that we did not nd any leaching of
catalysts during the epoxidation reaction.
diamagnetic organic materials from Fe3O4 to Fe3O4@
dopa@FeL3Cl. Furthermore, in comparison with the bulk
magnetite nanomaterials that typically show a saturation
magnetization value of 92 emu gꢁ1, the Ms value of the Fe3O4
nanoparticles is found to be much lower. Since the magneti-
zation of a particle in an external eld is a function of its size, it
is normal that small nanoparticles show in the Ms value.46
However, the net magnetism exhibited by the nal nanocatalyst
is sufficiently good for an effective separation from the solution
medium through the application of an external magnetic force.
Again, here we have studied Fe3O4@dopa@FeL3Cl catalyzed
epoxidation of three different alkenes [(E)-stilbene, (Z)-stilbene,
or styrene] in CH3CN or CH2Cl2 with PhIO and the results are
presented in Table 3.
Characterization and reusability Fe3O4@dopa@FeL3Cl
We have studied the recycling efficiency of the magnetically
separable heterogeneous catalyst Fe3O4@dopa@FeL3Cl, i.e.,
whether the catalyst can be reused further for several cycles. (E)-
Stilbene has been chosen as a representative case for recycling
experiments. Aer each reaction cycle the catalysts are recov-
ered by magnetic separation, washed thoroughly with acetoni-
trile and dried at 100 ꢂC for 2 h. The used catalyst has been
further characterized by FTIR, UV-Vis in solid state and scan-
ning electron microscopy (Fig. 14). Comparing all the experi-
mental results of the used catalyst with virgin one it is easy to
Magnetization behavior of the (a) Fe3O4-NPs, (b) Fe3O4@
dopa and (c) Fe3O4@dopa@FeL3Cl nanoparticles under the
applied magnetic eld is depicted in Fig. 13. The curves exhibit
an extremely interesting phenomenon showing a decrease in
the values of the saturation magnetization (Ms) from the Fe3O4
nanoparticles (58.06 emu gꢁ1) to Fe3O4@dopa (39.24 emu gꢁ1
)
)
to the nal Fe3O4@dopa@FeL3Cl nanocatalyst (26.39 emu gꢁ1
which can be attributed to the gradual increment of
Table 3 Epoxidation of alkenes catalyzed by Fe3O4@dopa@FeL3Cl in CH3CN and CH2Cl2 with PhIO
Catalysta
Substrate
(E)-Stilbene
(Z)-Stilbene
Styrene
Solvent
Conversion (%)
TONb
Yieldc (%)
eed,f (%)
Fe3O4@dopa @FeL3Cl
CH3CN
CH2Cl2
CH3CN
CH2Cl2
CH3CN
CH2Cl2
79
66
78
74
72
67
40.5
33.8
40.0
37.9
36.9
34.3
62
57
Tracee
18
67 (cis : trans ¼ 80 : 20)
29 (trans)
21 (trans)
17
62 (cis : trans ¼ 75 : 25)
63
55
Tracee
a
Catalyst (100 mg), alkenes (3 mmol), PhIO (3 mmol), and CH3CN or CH2Cl2 (25 mL), were stirred at room temperature for 6 h in air. b TON ¼ moles
of substrate converted per mole of catalyst per hour. c Isolated epoxide yield. d Determined by 1H NMR (300 Hz) in the presence of Eu(hfc). e Trace ¼
concentration < 5%. f Conguration not determined.
92642 | RSC Adv., 2015, 5, 92634–92647
This journal is © The Royal Society of Chemistry 2015

View Article Online
DOI: 10.1039/C5RA17484H
RSC Advances
Paper
Fig. 14 (A) Fourier-transform infrared spectra (FTIR), (B) solid state UV and (C) scanning electron microscopy (SEM) images of used Fe3O4@
dopa@FeL3Cl. (D) Recyclability and reusability test of Fe3O4@dopa@FeL3Cl for the epoxidation of (E)-stilbene.
conclude that no signicant change in the catalyst aer epoxi- Thermal analyses (TG-DTA) were carried out on a Mettler Toledo
dation reaction. That implies no structural deformation has (TGA/SDTA851) thermal analyzer in owing dinitrogen (ow
taken place in the active site of catalyst which is probably the rate: 30 cm3 minꢁ1). Field Emission Scanning Electron Micro-
key factor for its reusability. The catalytic reactions have been scope (FE-SEM) measurement was carried out with JEOL JSM-
carried out following the same experimental procedure as that 6700F eld-emission microscope. X-ray powder diffraction
with the original catalysts, and for all cases, the yields are not (PXRD) was performed on a XPERT-PRO Diffractometer mono-
signicantly different through rst to h use (Fig. 14).
chromated Cu-Ka radiation (40.0 kV, 30.0 mA) at room
temperature. Raman spectra (Horiba Jobin Yvon, T64000
model) were recorded in solution via 90ꢂ scattering by exciting
the sample with an Ar+ ion laser source of 48 mW power at the
sample. The spectra of the samples were recorded at room
temperature in solid phase. The light irradiation dependent
study was done aer different irradiation times with a white
light source compiled with a <420 nm cut off lter. Conductance
of the methanolic solution of complexes was measured using
a SYSTRONICS 306 conductivity meter. A vibrating sample
magnetometer (EV-9, Microsense, ADE) was utilized for
obtaining the magnetization curves. The CD spectra were
measured with a Jobin Ivon CD 6 spectrophotometer. All the
spectra were recorded in acetonitrile.
Experimental
Materials and methods
All chemicals were obtained from commercial sources and used
as received. Solvents were dried according to standard proce-
dure and distilled prior to use. 3,5-Dichloro salicylaldehyde; 3,5-
dibromo salicylaldehyde; 3,5-diiodo salicylaldehyde; (R)-1,2-
diaminopropane, styrene, (E)-stilbene and (Z)-stilbene were
purchased from Aldrich and used in epoxidation experiments
without further purication. Iron(III) chloride was purchased
from Merck.
Elemental analyses (carbon, hydrogen and nitrogen) were
performed using a Perkin-Elmer 240C elemental analyzer.
Infrared spectra (4000–500 cmꢁ1) were recorded at 27 ꢂC using
Syntheses of catalysts
a Perkin-Elmer RXI FT-IR spectrophotometer with KBr pellets.
Synthesis of [FeL1Cl] (1). 10 mL of ethanolic solution of (R)-
ꢂ
Electronic spectra (800–200 nm) were obtained at 27 C using 1,2-diaminopropane (0.74 g, 1 mmol) was added slowly to the 20
a Shimadzu UV-3101PC with methanol as solvent and reference. mL ethanolic solution of 3,5-dichlorosalicylaldehyde (0.382 g, 2
This journal is © The Royal Society of Chemistry 2015
RSC Adv., 2015, 5, 92634–92647 | 92643

View Article Online
DOI: 10.1039/C5RA17484H
Paper
RSC Advances
mmol). Immediate precipitation of H2L1 took place. 10 mL anisotropically. All the calculations were performed using the
ethanolic solution of ferric chloride hexahydrate (0.135 g, 0.5 WinGX System, Ver 1.80.05,50 PLATON99,51 ORTEP3 (ref. 52)
mmol) was added to suspension of H2L1. Yellow precipitate of programs. Selected crystallographic data and renement details
H2L1 was disappearing readily and the solution was turned deep are displayed in Table 4.
reddish brown. The solution was allowed to stir for 1 h. Brown
colored micro crystals of 1 were formed. Anal. calcd for 1: C,
Preparation of iodosylbenzene
40.11; H, 2.35; N, 5.50. Found: C, 40.08; H, 2.30; N, 5.41. IR
(KBr): n(C]N) 1638 cmꢁ1; n(skeletated vibration) 1526 cmꢁ1. UV
It was prepared by hydrolysis of the corresponding diacetate
with aqueous sodium hydroxide according to literature
method.53 In every epoxidation experiment freshly prepared
lmax (ACN)/nm 267, 308, 482sh (3/dm3 molꢁ1 cmꢁ1 24 806,
15 253, 5816).
Synthesis of [FeL2Cl] (2). Complex 2 was synthesized
PhIO was used.
following the same procedure as mentioned for 1. Here 3,5-
dibromosalicylaldehyde (0.56 g, 2 mmol) was used in place of
3,5-dichlorosalicylaldehyde. The solution was allowed to stir for
1 h. Brown colored micro crystals of 2 were formed. Micro-
crystals were dissolved in DMF and kept the solution in dark. X-
ray suitable single crystals of 2 were obtained from the solution.
Anal. calcd for 2: C, 29.71; H, 1.74; N, 4.07. Found: C, 29.68; H,
1.70; N, 4.01. IR (KBr): n(C]N) 1631 cmꢁ1; n(skeletated vibra-
tion) 1511 cmꢁ1. UV lmax (ACN)/nm 265, 309, 484sh (3/dm3
molꢁ1 cmꢁ1 27 214, 15 712, 5816).
Epoxidation study of catalysts
Epoxidation of alkenes catalyzed by complexes (1–3). To
a solution of alkene (30 mmol) in acetonitrile or dichloro-
methane (25 mL), 0.1 mmol of complex was added and then
PhIO (30 mmol) was added portion wise to that solution and
then the resultant mixture was stirred at room temperature for 4
h in air. The reaction progress was monitored by TLC. Aer
removal of solvent, the crude product was puried by ash
chromatography. Identication of the epoxide was performed
by 1H NMR spectroscopy.
Synthesis of [FeL3Cl] (3). Complex 3 was prepared by
adopting the same procedure as that for 1 using 3,5-diiodosa-
licylaldehyde (0.748 g, 2 mmol) in place of 3,5-dichlor-
osalicylaldehyde. Anal. calcd for 3: C, 23.32; H, 1.37; N, 3.20.
Epoxidation of alkenes catalyzed by Fe3O4@dopa@FeL3Cl.
To a solution of alkene (3 mmol) in acetonitrile or dichloro-
methane (25 mL), 100 mg of Fe3O4@dopa@FeL3Cl was sus-
pended and then PhIO (3 mmol) was added and the resultant
mixture were stirred at room temperature for 6 h in air. Here
also we added iodosylbenzene portion wise to the solution. The
reaction progress was monitored by TLC. Aer completion of
the reaction, the catalyst was separated with magnet and the
solvent was removed by rotary evaporator. The crude product
was thus obtained was puried by ash chromatography.
Identication of the epoxide was performed by 1H NMR
spectroscopy.
Found: C, 23.28; H, 1.32; N, 3.18. IR (KBr): n(C]N) 1609 cmꢁ1
;
n(skeletated vibration) 1498 cmꢁ1. UV lmax (ACN)/nm 266, 317,
495sh (3/dm3 molꢁ1 cmꢁ1 21 414, 12 026, 4423).
Synthesis of Fe3O4 NPs. Preparation of magnetic nano-
particles Fe3O4 have been done following the same procedure as
reported earlier by R. S. Varma et al. at 2009.40
Synthesis of Fe3O4@dopa. Fe3O4@dopa has been prepared
by following the similar method as mentioned previously.40
Synthesis of Fe3O4@dopa@FeL3Cl. Fe3O4@dopa@FeL3Cl
has been synthesized by following similar procedure20c,45 but
with some modication where 1 g of FeL3Cl is added in the
dispersed acetonitrilic solution of Fe3O4@dopa (500 mg). The
mixture is stirred for 12 h in room temperature. The product is
allowed to settle, washed several times with acetonitrile, and
dried under vacuum at 60 ꢂC for 2 h. IR (KBr): n(C]N) 1624
cmꢁ1; n(skeletated vibration) 1526, 1485 cmꢁ1; n(Fe3O4) 583
cmꢁ1. UV lmax (solid)/nm ꢀ502sh, ꢀ279.
Table 4 Crystal data and details of the structure determination for
complex 2
Empirical formula
Formula mass
Crystal system
Space group
C17H12Br4ClFeN2O2, C3H7NO
760.28
Monoclinic
P21/n (no. 14)
13.314(6)
14.720(6)
13.228(6)
90
101.192(6)
90
2543.2(19)
4
˚
a (A)
˚
b (A)
˚
c (A)
X-ray data collection and structure determination
a (ꢂ)
b (ꢂ)
g (ꢂ)
Diffraction data for complex 2 was collected at room tempera-
ture (293 K) on a Bruker Smart CCD diffractometer equipped
3
˚
V (A )
˚
with graphite-monochromated MoKa radiation (l ¼ 0.71073 A).
Z
Cell renement, indexing and scaling of the data set were
carried out using Bruker SMART APEX and Bruker SAINT
package.47 The structure was solved by direct methods and
subsequent Fourier analyses48 and rened by the full-matrix
least-squares method based on F2 with all observed reections
using SIR-92 and SHELX-97,49 soware. For the complex, all
non-hydrogen atoms were rened with anisotropic thermal
parameters and the hydrogen atoms were xed at their respec-
tive positions riding on their carrier atoms and rened
T (K)
293
7.009
1.986
1468
m(MoKa) (mmꢁ1
)
Dcalc (Mg mꢁ3
F(000)
qmax (degree)
Tot., uniq. data, Rint
Observed (I > 2s(I))
Nref, Npar
)
21.8
8389, 2956, 0.071
1883
2956, 292
0.0484, 0.1247, 1.02
ꢁ0.53, 0.45
R, wR2, S
Residual extrema (e A
ꢁ3
˚
)
92644 | RSC Adv., 2015, 5, 92634–92647
This journal is © The Royal Society of Chemistry 2015

View Article Online
DOI: 10.1039/C5RA17484H
RSC Advances
Paper
University of Calcutta, through DST-FIST program. Authors are
thankful to Professor Debasis Das, Department of Chemistry,
University of Calcutta, Kolkata, India for his generous help.
Authors also wish to thank Dr Partha Mahata, Inspire Faculty,
SNBNCBS, Kolkata, India for his help to collect the magnetic
data.
Computational method
All calculations were carried out with the Gaussian 09 series of
programs.54 The implicit Polarizable Continuum Model (PCM)
of Tomasi and coworkers55 was used to take into account the
effect of acetonitrile as solvent (dielectric constant, 3 ¼ 35.688),
using atomic radii derived from the standard Universal Force
Field (UFF). Full geometry optimization of all reactants and
products has been carried out using Density Functional Theory
with the UB3LYP56 functional and the 6-311+G(d,p) basis set for
C, N, O and H atoms as well as with LANL2DZ pseudopotential
for Fe, Cl, Br, and I atoms. Geometry optimizations were done
with the Schlegel's algorithm.57 The located stationary points
were checked to be true minima by the analytical computation
of the harmonic vibrational frequencies at the same theory
level. Due to the open shell nature of the iron complexes under
study, the possible low and high spin states have been calcu-
lated for non-oxidized and oxidized complexes. High spin
structures were found to be more stable than low spin ones in
all cases, and they showed practically no spin contamination at
the theory level here used. For interpretation purposes,
a natural bond orbital (NBO) analysis was performed on the
non-oxidized and oxidized complexes in solution.57–59 Thermo-
dynamic magnitude DG was also calculated within the ideal gas,
rigid rotor, and harmonic oscillator approximations at a pres-
sure of 1 atm and a temperature of 298.15 K.60
References
1 (a) J. T. Groves, T. E. Nemo and R. S. Myers, J. Am. Chem. Soc.,
1979, 101, 1032; (b) J. T. Groves and T. E. Nemo, J. Am. Chem.
Soc., 1983, 105, 5786; (c) J. T. Groves, R. C. Haushalter,
M. Nakamura, T. E. Nemo and B. J. Evans, J. Am. Chem.
Soc., 1981, 103, 2884; (d) J. T. Groves and Y. Watanabe, J.
Am. Chem. Soc., 1988, 110, 8443.
2 H.-L. Shyu, H.-H. Wei, G.-H. Lee and Y. Wang, J. Chem. Soc.,
Dalton Trans., 2000, 915–918.
3 S. O'Malley and T. Kodadek, J. Am. Chem. Soc., 1989, 111,
9116.
4 Y. Naruta, F. Tani, N. Ishihara and K. Maruyama, J. Am.
Chem. Soc., 1991, 113, 6865.
5 S. J. Yang and W. Nam, Inorg. Chem., 1998, 37, 606.
6 For recent advances, see: R. A. Sheldon, J. Mol. Catal. A:
Chem., 1997, 117, 1–478; C. T. Dalton, K. M. Ryan,
V. M. Wall, C. Bousquet and D. G. Gilheany, Top. Catal.,
1998, 5, 75.
7 R. E. White and M. J. Coon, Annu. Rev. Biochem., 1980, 49,
315.
8 T. Hamada, K. Daikai, R. Irie and T. Katsuki, Tetrahedron:
Asymmetry, 1995, 6, 2441.
9 B. B. De, B. B. Lohray, S. Sivaraman and P. K. Dhal,
Tetrahedron: Asymmetry, 1995, 6, 2105.
10 K. Bemardo, S. Leppard, A. Robert, G. Commenges, F. Dahan
and B. Meunier, Inorg. Chem., 1996, 35, 387.
11 H. Sasaki, R. Irie, T. Hamada, K. Susuki and T. Katsuki,
Tetrahedron, 1994, 50, 11827.
12 H. Yoon, T. R. Wagler, K. J. O'Connor and C. J. Burrows, J.
Am. Chem. Soc., 1990, 112, 4568.
13 X.-H. Lu, Q.-H. Xia, H.-J. Zhan, H.-X. Yuan, C.-P. Ye, K.-X. Su
and G. Xu, J. Mol. Catal. A: Chem., 2006, 250, 62.
14 (a) L. Gomez, I. Garcia-Bosch, A. Company, X. Sala, X. Ribas
and M. Costas, J. Chem. Soc., Dalton Trans., 2007, 47, 5539;
(b) C. Coperet, M. Chabanas, R. P. Saint-Arroman and
J. M. Basset, Angew. Chem., Int. Ed. Engl., 2003, 42, 156.
15 (a) D. Das and C. P. Cheng, J. Chem. Soc., Dalton Trans., 2000,
1081; (b) T. Chattopadhyay, S. Islam, M. Nethaji, A. Majee
and D. Das, J. Mol. Catal. A: Chem., 2007, 267, 255–264; (c)
T. Chattopadhyay and D. Das, J. Coord. Chem., 2009, 62,
845–853; (d) B. Biswas, M. Mitra, J. Adhikary,
G. R. Krishna, P. P. Bag, C. Malla Reddy, N. Aliaga-Alcalde,
T. Chattopadhyay, D. Das and R. Ghosh, Polyhedron, 2013,
53, 264–268.
Conclusion
In conclusion, here we have demonstrated that among three
asymmetric mononuclear Fe(III) complexes of chloro, bromo
and iodo substituted Schiff base ligands, the most efficient
catalyst towards alkene epoxidation is iodo substituted Schiff
base Fe(III) complex. This efficiency is related to the stability of
FeV]O intermediate which forms during the catalytic process
in presence of PhIO. This factor has been well rationalized by
DFT calculations. Then, we have adapted an economically
viable and energy efficient catalytic process using the iodo
substituted Schiff base Fe(III) complex over magnetically sepa-
rable nanoparticules for the selective epoxidation of alkenes at
room temperature. The simple operation, stability and rigidity
of catalyst, the use of inexpensive and gentle magnetic nano-
particles as support, the easy recoverability and reusability of
the catalyst, along with the high epoxide yield made the
proposed protocol a potential candidate for addressing the
challenges of sustainability. We consider that this novel
magnetic nano composite system would nd applications in
several other industrially signicant catalytic processes as well
as the general synthetic organic transformations.
Acknowledgements
Financial support by the Science and Engineering Research 16 N. Mizuno and M. Misono, Chem. Rev., 1998, 98, 199.
Board (a statutory body under DST, New Delhi, India (F. No. SB/ 17 F. Lefebvre and J. M. Basset, J. Mol. Catal. A: Chem., 1999,
FT/CS-185/2013 dt. 30-06-2014) is gratefully acknowledged by
146, 3.
TC. We also thank DST, New Delhi, for providing single crystal 18 (a) J. M. Thomas, Angew. Chem., Int. Ed., 1999, 38, 3588; (b)
diffractometer facility at the Department of Chemistry,
S. Koner, K. Chaudhari, T. K. Das and S. Sivasanker, J. Mol.
RSC Adv., 2015, 5, 92634–92647 | 92645
This journal is © The Royal Society of Chemistry 2015

View Article Online
DOI: 10.1039/C5RA17484H
Paper
RSC Advances
Catal. A: Chem., 1999, 150, 295; (c) S. Seelan, A. K. Sinha, 34 B. Karamzadeh, D. Singh, W. Nam, D. Kumar and S. P. de
D. Srinivas and S. Sivasanker, J. Mol. Catal. A: Chem., 2000, Visser, Phys. Chem. Chem. Phys., 2014, 16, 22611–22622.
157, 163; (d) E. F. Murphy, D. Ferri, A. Baiker, 35 W.-P. To, T. W.-S. Chow, C.-W. Tse, X. Guan, J.-S. Huang and
S. V. Doorslaer and A. Schweiger, Inorg. Chem., 2003, 42, C.-M. Che, Chem. Sci., 2015, 6, 5891.
2559; (e) F. Bedioui, Coord. Chem. Rev., 1995, 144, 39; (f) 36 K. Chen, M. Costas and L. Que Jr, J. Chem. Soc., Dalton Trans.,
P. K. Saha, S. Saha and S. Koner, J. Mol. Catal. A: Chem.,
2003, 203, 173.
2002, 672–679.
37 B. Minaev, Chem. Chem. Technol., 2010, 4, 1–16.
¨
19 (a) M. Pillinger, I. S. Goncualves, A. D. Lopes, J. Madureira, 38 S. Shaik, D. Kumar, V. S. de and A. Altun, Chem. Rev., 2005,
P. Ferreira, A. A. Valente, T. M. Santos, J. Rocha, 105, 2279.
J. F. S. Menezes and L. D. Carlos, Dalton Trans., 2001, 39 V. A. Minaeva and V. A. Minaeva, Ukr. Bioorg. Acta, 2008, 2,
1628; (b) G.-J. Kim and S.-H. Kim, Catal. Lett., 1999, 57, 139. 56.
20 (a) D. Brunel, N. Bellocq, P. Sutra, A. Cauvel, M. Lasperas, 40 V. Polshettiwar, B. Baruwati and R. S. Varma, Green Chem.,
P. Moreau, F. D. Renzo, A. Galarneau and F. Fajula, Coord. 2009, 11, 127–131.
Chem. Rev., 1998, 178–180, 1085; (b) A. Choplin and 41 R. K. Sharma, Y. Monga and A. Puri, J. Mol. Catal. A: Chem.,
F. Quignard, Coord. Chem. Rev., 1998, 178–180, 1679; (c) 2014, 393, 84–95.
X.-G. Zhou, X.-Q. Yu, J.-S. Huang, S.-G. Li, L.-S. Li and 42 H. Wan, Z. Wu, W. Chen, G. Guan, Y. Cai, C. Chen, Z. Li and
C.-M. Che, Chem. Commun., 1999, 18, 1789; (d) X. Liu, J. Mol. Catal. A: Chem., 2015, 398, 127–132.
T. Chattopadhyay, M. Kogiso, M. Aoyagi, H. Yui, 43 H. Firouzabadi, N. Iranpoor, M. Gholinejad, S. Akbaria and
M. Asakawa and T. Shimizu, Green Chem., 2011, 13, 1138; N. Jeddi, RSC Adv., 2014, 4, 17060.
(e) J. Adhikary, A. Guha, T. Chattopadhyay and D. Das, 44 (a) W. Li, Y. Tian, B. Zhang, L. Tian, X. Li, H. Zhang, N. Ali
´
Inorg. Chim. Acta, 2013, 406, 1.
21 D. Zhang, C. Zhou, Z. Sun, L. Z. Wu, C. H. Tung and
T. Zhang, Nanoscale, 2012, 4, 6244–6255.
and Q. Zhang, New J. Chem., 2015, 39, 2767–2777; (b)
N. Azgomi and M. Mokhtary, J. Mol. Catal. A: Chem., 2015,
398, 58–64.
22 M. Mazur, A. Barras, V. Kuncser, A. Galatanu, V. Zaitzev, 45 C.-J. Liu, W.-Y. Yu, S.-G. Li and C.-M. Che, J. Org. Chem.,
K. V. Turcheniuk, P. Woisel, J. Lyskawa, W. Laure, 1998, 63, 7364–7369.
A. Siriwardena, R. Boukherrouba and S. Szunerits, 46 V. S. Zaitsev, D. S. Filimonov, I. A. Presnyakov, R. J. Gambino
Nanoscale, 2013, 5, 2692–2702. and B. Chu, J. Colloid Interface Sci., 1999, 212, 49–57.
23 S. Singamaneni, V. N. Bliznyuk, C. Binek and E. Y. Tsymbal, 47 Bruker, SMART, SAINT. Soware Reference Manual, Bruker,
J. Mater. Chem., 2011, 21, 16819–16845.
AXS Inc., Madison, Wisconsin, USA, 2000.
48 G. M. Sheldrick, Acta Crystallogr., Sect. A: Found. Crystallogr.,
2008, 64, 112–122.
49 G. M. Sheldrick, SHELXL 97, Program for the Renement of
Crystal Structures, University of Gottingen, Germany,
1997.
24 R. S. Varma, Sustainable Chem. Processes, 2014, 2, 11.
25 M. B. Gawande, A. K. Rathi, P. S. Branco and R. S. Varma,
Appl. Sci., 2013, 3, 656.
26 M. B. Gawande, P. S. Branco and R. S. Varma, Chem. Soc.
Rev., 2013, 42, 3371.
27 R. B. N. Baig and R. S. Varma, Green Chem., 2013, 15, 398.
50 L. J. Farrugia, J. Appl. Crystallogr., 1999, 32, 837–838.
28 E. N. Jacobsen and M. H. Wu, in Comprehensive Asymmetric 51 A. L. Spek, PLATON, Molecular Geometry Program, University
Catalysis, ed. A. Pfaltz, E. N. Jacobsen and H. Yamamoto,
of Utrecht, The Netherlands, 1999.
Springer-Verlag, Berlin, 1999, vol. 2, p. 649.
52 L. J. Farrugia, J. Appl. Crystallogr., 1997, 30, 565.
29 (a) E. P. Talsi and K. P. Bryliakov, Coord. Chem. Rev., 2012, 53 H. Saltzman and J. G. Sharein, Org. Synth., 1973, 5, 658–
256, 1418; (b) A. Murphy, G. Dubois and T. D. P. Stack, J. 660.
Am. Chem. Soc., 2003, 125, 5250; (c) J. Rich, M. Rodrıquez, 54 M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria,
´
I. Romero, L. Vaquer, X. Sala, A. Llobet, M. Corbella,
M. N. Collomb and X. Fontrodona, Dalton Trans., 2009,
8117; (d) R. V. Ottenbacher, K. P. Bryliakov and E. P. Talsi,
Adv. Synth. Catal., 2011, 353, 885; (e) N. Saravanan,
M. Sankaralingam and M. Palaniandavar, RSC Adv., 2014,
4, 12000–12011.
M. A. Robb, J. R. Cheeseman, G. Scalmani, V. Barone,
B. Mennucci, G. A. Petersson, H. Nakatsuji, M. Caricato,
X. Li, H. P. Hratchian, A. F. Izmaylov, J. Bloino, G. Zheng,
J. L. Sonnenberg, M. Hada, M. Ehara, K. Toyota,
R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda,
O. Kitao, H. Nakai, T. Vreven, J. A. Montgomery Jr,
J. E. Peralta, F. Ogliaro, M. Bearpark, J. J. Heyd,
E. Brothers, K. N. Kudin, V. N. Staroverov, R. Kobayashi,
J. Normand, K. Raghavachari, A. Rendell, J. C. Burant,
S. S. Iyengar, J. Tomasi, M. Cossi, N. Rega, J. M. Millam,
M. Klene, J. E. Knox, J. B. Cross, V. Bakken, C. Adamo,
J. Jaramillo, R. Gomperts, R. E. Stratmann, O. Yazyev,
A. J. Austin, R. Cammi, C. Pomelli, J. W. Ochterski,
R. L. Martin, K. Morokuma, V. G. Zakrzewski, G. A. Voth,
P. Salvador, J. J. Dannenberg, S. Dapprich, A. D. Daniels,
30 (a) B. N. Figgis and M. A. Hitchman, Ligand Field Theory and
its Applications, Wiley-VCH, USA, 2000; (b) R. K. Dean,
C. I. Fowler, K. Hasan, K. Kerman, P. Kwong, S. Trudel,
D. B. Leznoff, H.-B. Kraatz, L. N. Dawe and C. M. Kozak,
Dalton Trans., 2012, 41, 4806–4816.
31 I. Wasbotten and A. Ghosh, Inorg. Chem., 2006, 45, 4910.
32 E. Derat, D. Kumar, R. Neumann and S. Shaik, Inorg. Chem.,
2006, 45, 8655.
33 E. Kwon, K.-B. Cho, S. Hong and W. Nam, Chem. Commun.,
2014, 50, 5572.
¨
O. Farkas, J. B. Foresman, J. V. Ortiz, J. Cioslowski and
92646 | RSC Adv., 2015, 5, 92634–92647
This journal is © The Royal Society of Chemistry 2015

View Article Online
DOI: 10.1039/C5RA17484H
RSC Advances
Paper
D. J. Fox, in Gaussian 09, Gaussian, Inc., Wallingford CT, 57 (a) H. B. Schlegel, J. Comput. Chem., 1982, 3, 214–218; (b)
2009.
H. B. Schlegel, Theor. Chem. Acc., 1984, 66, 333–340; (c)
X. Li and M. J. Frisch, J. Chem. Theory Comput., 2006, 2,
835–839.
55 (a) J. Tomasi and R. Cammi, J. Comput. Chem., 1996, 16,
1449–1458; (b) J. Tomasi and M. Persico, Chem. Rev., 1994,
94, 2027–2094.
58 E. Reed, L. A. Curtiss and F. Weinhold, Chem. Rev., 1988, 88,
899–926.
56 (a) A. B. Becke, Phys. Rev. A, 1988, 38, 3098–3100; (b)
A. B. Becke, J. Chem. Phys., 1993, 98, 5648–5662; (c) C. Lee, 59 F. Weinhold, Natural Bond Orbital Methods, John Wiley &
W. Yang and R. G. Parr, Phys. Rev. B: Condens. Matter
Mater. Phys., 1988, 37, 785–789.
Sons, Chichester, UK, 1998, vol. 3.
60 D. A. McQuarrie, Statistical Mechanics, Harper and Row, New
York, 1976.
This journal is © The Royal Society of Chemistry 2015
RSC Adv., 2015, 5, 92634–92647 | 92647