A route to magnetically separable nanocatalysts: Combined experimental and theoretical investigation of alkyl substituent role in ligand backbone towards epoxidation ability

Article abstract of DOI:10.1002/aoc.3663

We have prepared two chiral Schiff base ligands, H₂L¹ and H₂L², and one achiral Schiff base ligand, H₂L³, by treating 2,6-diformyl-4-methylphenol separately with (R)-1,2-diaminopropane, (R)

Full text of DOI:10.1002/aoc.3663

Received: 22 July 2016
DOI 10.1002/aoc.3663
Revised: 15 September 2016
Accepted: 20 September 2016
F U L L PA P E R
A route to magnetically separable nanocatalysts: Combined
experimental and theoretical investigation of alkyl substituent role
in ligand backbone towards epoxidation ability
Tanmay Chattopadhyay1 | Aratrika Chakraborty2 | Sanchari Dasgupta2 | Arnab Dutta2 |
M. Isabel Menéndez3 | Ennio Zangrando4
1 Department of Chemistry, Panchakot
WehavepreparedtwochiralSchiffbaseligands, H2L1 andH2L2, andoneachiralSchiff
Mahavidyalaya, Sarbari, Purulia 723 121, India
2 Department of Chemistry, University of Calcutta,
base ligand, H2L3, by treating 2,6‐diformyl‐4‐methylphenol separately with (R)‐1,2‐
92 A. P. C. Road, Kolkata 700 009, India
3 Departamento de Química Física y Analítica,
diaminopropane, (R)‐1,2‐diaminocyclohexane and 1,1′‐dimethylethylenediamine, in
ethanolic medium, respectively. The complexes MnL1ClO4 (1), MnL2ClO4 (2),
MnL3ClO4 (3), FeL1ClO4 (4), FeL2ClO4 (5) and FeL3ClO4 (6) have been obtained
Universidad de Oviedo, C/ Julián Clavería 8,
33006 Oviedo, Spain
4 Dipartimento di Scienze Chimiche, University of
by reacting the ligands H2L1, H2L2 and H2L3 with manganese(III) perchlorate or iron
(III) perchlorate in methanol. Circular dichroism studies suggest that ligands H2L1
Trieste, Via L. Giorgieri 1, 34127 Trieste, Italy
and H2L2 and their corresponding complexes have asymmetric character. Complexes
Correspondence
Tanmay Chattopadhyay, Department of Chemistry,
Panchakot Mahavidyalaya, Sarbari, Purulia 723
121, India.
1–6 have been used as homogeneous catalysts for epoxidation of alkenes. Manganese
systems have been found to be much better than iron counterparts for alkene epoxida-
tion, with 3 as the best catalyst among manganese systems and 6 as the best among iron
systems. The order of their experimental catalytic efficiency has also been rationalized
bytheoretical calculations. We have observed higher enantiomeric excess product with
catalysts 1 and 4, so they were attached to surface‐modified magnetic nanoparticles to
obtain two new magnetically separable nanocatalysts, Fe3O4@dopa@MnL1 and
Fe3O4@dopa@FeL4. They have been characterized and their alkene epoxidation
ability has been investigated. These catalysts can be easily recovered by magnetic
separation and recycled several times without significant loss of catalytic activity.
Hence our study focuses on the synthesis of a magnetically recoverable asymmetric
nanocatalyst that finds applications in epoxidation of alkenes and at the same time
can be recycled and reused.
Email: tanmayc2003@gmail.com
M. Isabel Menéndez, Departamento de Química
Física y Analítica, Universidad de Oviedo,
C/ Julián Clavería 8, 33006 Oviedo, Spain.
Email: isabel@uniovi.es
Ennio Zangrando, Dipartimento di Scienze
Chimiche, University of Trieste, Via L. Giorgieri
1, 34127 Trieste, Italy.
Email: ezangrando@units.it
Funding information
Science and Engineering Research Board, Grant/
Award Number: SB/FT/CS‐185/2013 dt. 30‐06‐
2014.
KEYWORDS
asymmetric complex, catalysis, epoxidation, magnetic nanoparticles, recyclable
1
|
INTRODUCTION
complexes in the presence of terminal oxidants, e.g.
iodosylbenzene (PhIO), NaOCl and H2O2.[3] The extensive
Catalysis is a broad field which has already been deeply
studied in specific areas but still leaves much scope for the
development of catalytic systems at different levels. Thus
research in this field remains significant in the area of
chemical sciences and allied branches. Epoxides are versatile
intermediates in organic chemistry that can constitute conve-
nient building blocks for the synthesis of many commodity
and fine chemicals.[1,2] Numerous studies have focused on
the preparation of epoxides using transition metal salen
use of metal salen complexes in homogeneous catalysis can
be attributed to their high activity, selectivity and
enantioselectivity (when chiral complexes are used).[4–7]
The easy availability of active sites of homogeneous catalysts
enhances their efficiency, but their liability lies in the
difficulty in their separation from reaction mixtures. Alterna-
tively, for heterogeneous catalysts simple separation
techniques are available, although lower catalytic activity
may be seen due to the difficulty of accessing the active sites.
Appl Organometal Chem 2016; 1–11
wileyonlinelibrary.com/journal/aoc
Copyright © 2016 John Wiley & Sons, Ltd.
1

2
CHATTOPADHYAY ET AL.
It is obvious that a catalyst system having the advantageous
attributes of both homogeneous and heterogeneous systems
would be highly desirable. In this line of thinking, magneti-
cally separable nanocatalysts emerge as an adequate solution,
as has been recently reported.[8–10] Magnetic separation is sim-
ple, ecological and allows easy recycling of the catalyst.[9–14]
Besides, magnetically separable nanocatalysts also offer
excellent surface‐to‐volume ratio, which enhances the contact
between reactants and catalyst, which increases the overall
activity of the catalyst for the desired transformations.
procedures and distilled prior to use. Iodosylbenzene was
prepared according to a literature method.[16]
2.2 | Syntheses
2,6‐Diformyl‐4‐methylphenol was prepared according to a
literature method.[16] Schiff bases H2L1, H2L2 and H2L3 were
prepared by reacting 2,6‐diformyl‐4‐methylphenol separately
with (R)‐1,2‐diaminopropane, (R)‐1,2‐diaminocyclohexane
and 1,1′‐dimethylethylenediamine, in ethanolic medium in
2:1 ratio. All the complexes were synthesized and character-
ized according to the same procedure as previously
reported.[16,17] Very similar analytical results were also
observed for 5 as earlier described.[17]
Several research groups have recently investigated steric
and electronic effects of manganese and non‐heme iron
complexes on alkene epoxidations,[15] and our own research
team has given a deeper insight into the effect of halogen
substituents at the ligand backbone of Schiff base Fe(III)
complexes on epoxide yield.[7] The relevance of the topic
encourages us to delve further, by exploring the effect of alkyl
groups at the non‐aromatic chain of the ligands. For this pur-
pose we have selected three Mn(III) and three Fe(III) Schiff
base complexes, as shown in Scheme 1, among which 2
and 5 had been previously synthesized and tested as catalysts
for the epoxidation of olefins in the presence of PhIO in
CH3CN under mild conditions.[16,17] They are included in
the present work for the sake of completeness due to their
close relationship with the four new catalysts here analysed.
Further, theoretical calculations helped us to rationalize the
experimental catalytic efficiency of the whole set of catalysts.
Also, in the work presented here we devised a technique to
attach some selected manganese and iron catalyst complexes
to surface‐modified nanoparticles in order to build efficient,
stable and easy‐to‐recover magnetically separable
nanocatalysts. To summarize our work, we have designed
an efficient magnetically separable asymmetric nanocatalyst
for alkene epoxidation that maintains its activity even after
several catalytic cycles, hence solving the problems of
tedious separation procedure of heterogeneous catalysts.
2.2.1
|
[MnL1ClO4] (1)
Yield: 88% (with respect to L1). Anal. calcd (%): C, 48.62;
H, 3.89; N, 5.40. Found (%): C, 48.58; H, 3.84; N, 5.45.
IR (KBr, ν, cm−1): ν(C═O) 1655, ν(C═N) 1626, ν(skeletal
vibration) 1545, ν(ClO4) 1109. UV (λmax(MeOH), nm):
422 (sh,
ε
=
7300 dm3 mol−1 cm−1), 360
(ε = 10000 dm3 mol−1 cm−1). MS (ESI positive mode):
m/z (%) = 418.92 (100) [M]+.
2.2.2
|
[MnL2ClO4] (2)
Yield: 85% (with respect to L2). Anal. calcd (%): C, 51.58; H,
4.33; N, 5.01. Found (%): C, 51.38; H, 4.30; N, 5.11. IR (KBr,
ν, cm−1): ν(C═O) 1660, ν(C═N) 1624, ν(skeletal vibration)
1547, ν(ClO4) 1112. UV (λmax(MeOH), nm): 412 (sh,
ε = 7500 dm3 mol−1 cm−1), 360 (ε = 15 000 dm3 mol−1
cm−1). MS (ESI positive mode): m/z (%) = 459.12 (100) [M]+.
2.2.3
|
[MnL3ClO4] (3)
Yield: 84% (with respect to L3). Anal. calcd (%): C, 49.59;
H, 4.16; N, 5.26. Found (%): C, 49.53; H, 4.14; N, 5.30.
IR (KBr, ν, cm−1): ν(C═O) 1657, ν(C═N) 1624, ν(skeletal
vibration) 1548, ν(ClO4) 1103. UV (λmax(MeOH), nm):
422 (sh,
ε
=
7600 dm3 mol−1 cm−1), 360
(ε = 14300 dm3 mol−1 cm−1). MS (ESI positive mode):
m/z (%) = 433.12 (100) [M]+.
2
| EXPERIMENTAL
2.1 | Materials and methods
2.2.4
|
[FeL1ClO4] (4)
Yield: 84% (with respect to L1). Anal. calcd (%): C, 48.55; H,
3.85; N, 5.39. Found (%): C, 49.01; H, 4.02; N, 5.30. IR
(KBr, ν, cm−1): ν(C═O) 1660, ν(C═N) 1632, ν(skeletal
vibration) 1548, ν(ClO4) 1104. UV (λmax(MeOH), nm): 526
(sh, ε = 5600 dm3 mol−1 cm−1), 330 (ε = 12 300 dm3 mol
All chemicals were purchased from commercial sources and
used as received. Solvents were dried according to standard
−1
cm−1). MS (ESI positive mode): m/z (%) = 685.48 (100)
[M + 4CH3OH + 6Na]+.
2.2.5
|
[FeL3ClO4] (6)
Yield: 86% (with respect to L3). Anal. calcd (%): C, 49.51; H,
4.15; N, 5.25. Found (%): C, 49.53; H, 4.14; N, 5.30. IR (KBr,
ν, cm−1): ν(C═O) 1655, ν(C═N) 1628, ν(skeletal
vibration) 1545, ν(ClO4) 1100. UV (λmax(MeOH), nm): 522
SCHEME 1 Schematic representation of the preparation of metal–Schiff
base complexes

CHATTOPADHYAY ET AL.
3
(sh,ε = 5500 dm3 mol−1 cm−1), 350 (ε = 8000 dm3 mol−1
cm−1). MS (ESI positive mode): m/z (%) = 434.22 (100) [M]+.
package,[18] and programs Denzo and Scalepack.[19] The
structure was solved by direct methods and Fourier analyses
and refined by full‐matrix least‐squares based on F2 with
all observed reflections.[20] All non‐hydrogen atoms were
refined with anisotropic displacement coefficients. A disor-
dered lattice ethanol molecule was fixed with half occupancy.
Crystallographic data and details of refinement are provided
in Table 1.
2.2.6
| Preparations of Fe3O4‐NPs, Fe3O4@dopa,
Fe3O4@dopa@MnL1 and Fe3O4@dopa@FeL1
Fe3O4‐NPs and Fe3O4@dopa were prepared following
the same procedure as we reported earlier.[7]
Fe3O4@dopa@MnL1 and Fe3O4@dopa@FeL1 were synthe-
sized with a slightly modified procedure than before: 1 g of
FeL1ClO4 or MnL1ClO4 was added to a dispersed acetonitrile
solution of Fe3O4@dopa (500 mg). The mixture was stirred
for 10 h at room temperature. The product was allowed to set-
tle, washed several times with acetonitrile and dried under
vacuum at 60°C for 2 h. For Fe3O4@dopa@MnL1:
ν1(C═N) 1626 cm−1; ν2(C═N) 1655 cm−1; ν(skeletal
2.5 | Epoxidation study of catalysts 1–6
To a solution of alkene (0.30 mmol) in acetonitrile (15 ml),
0.01 mmol of the complex was added followed by addition
of PhIO (0.30 mmol) portion‐wise and then the resultant
mixture was stirred at room temperature for 2 h in air. The
reaction progress was monitored by TLC. After removal of
solvent, the crude product was purified by flash chromatogra-
vibration) 1531 cm−1
Fe3O4@dopa@FeL1: ν1(C═N) 1621 cm−1
1645 cm−1; ν(skeletal vibration) 1537 cm−1; ν(Fe3O4)
589 cm−1
;
ν(Fe3O4) 577 cm−1
.
For
;
ν2(C═N)
1
phy. Identification of the epoxide was performed using H
NMR spectroscopy.
.
2.6 | Epoxidation study of Fe3O4@dopa@MnL1 and
2.3 | 1.3 Characterization
Fe3O4@dopa@FeL1
Elemental analyses (carbon, hydrogen and nitrogen) were
performed using a PerkinElmer 240C elemental analyser.
To a solution of alkene (2 mmol) in acetonitrile (15 ml),
100 mg of nanocatalyst was suspended followed by addition
of PhIO (2 mmol) and the resultant mixture were stirred at
room temperature for 2 h in air. PhIO was added portion‐wise
to the solution here as well. The reaction progress was mon-
itored by TLC. After completion of the reaction, the catalyst
was magnetically separated and the solvent was removed
using a rotary evaporator. The crude product thus obtained
was purified by flash chromatography. Identification of the
Fourier transform infrared (FT‐IR) spectra (500–4000 cm−1
)
were recorded at 27°C using a PerkinElmer RXI FT‐IR
spectrophotometer with KBr pellets. Electronic spectra
(200–800 nm) were obtained at 27°C using a Shimadzu UV‐
3101PC with methanol as solvent and reference. Thermal
analyses were carried out with a Mettler Toledo (TGA/
SDTA851) thermal analyser in flowing dinitrogen (flow rate:
30 cm3 min−1). Electrospray mass spectra were recorded with
a MICROMASS Q‐TOF mass spectrometer. Field emission
scanning electron microscopy (SEM) observation was carried
out with a JEOL JSM‐6700F field‐emission microscope. The
particle size and microstructure were studied using transmis-
sion electron microscopy (TEM) with a JEOL (Japan) JEM
2100 high‐resolution transmission electron microscope
operating at 200 kV. Powder X‐ray diffraction (PXRD) spectra
were recorded at room temperature using an XPERT‐PRO dif-
fractometer with monochromated Cu Kα radiation (40.0 kV,
30.0 mA). A vibrating sample magnetometer (EV‐9,
Microsense, ADE) was utilized for obtaining the magnetiza-
tion curves. Circular dichroism (CD) spectra were measured
with a Jobin Ivon CD 6 spectrophotometer in acetonitrile.
1
epoxide was performed using H NMR spectroscopy.
TABLE 1 Crystal data and details of refinement for complex 5
Empirical formula
C51H65Cl2Fe2N4O21.50
1260.67
Formula weight
Crystal system, space group
Monoclinic, C2
20.1210(12)
10.2560(8)
a (Å)
b (Å)
c (Å)
28.6220(15)
106.60(2)
β (°)
Volume (Å3)
5660.3(8)
Z, calculated density (Mg m−3
)
4, 1.479
μ (mm−1
F(000)
)
0.610
2628
2.4 | X‐ray crystallography
θ range data collection (°)
Reflections collected / unique
R(int)
1.46–29.98
48 408 / 16 945
0.0232
Single‐crystal data for 5 were collected at the X‐ray diffrac-
tion beamline XRD1 of the Elettra Synchrotron, Trieste
(Italy), using the rotating crystal method with a monochro-
matic wavelength of 0.7000 Å, using a Dectris Pilatus 2 M
detector. Measurements were performed at 100(2) K using a
nitrogen stream cryo‐cooler. Cell refinement, indexing and
scaling of the data set were performed using the CCP4
Data / parameters
16945 / 767
1.054
Goodness‐of‐fit on F2
Final R indices [I > 2σ(I)]
R indices (all data)
R1 = 0.0404, wR2 = 0.1106
R1 = 0.0413, wR2 = 0.1117
1.124, −0.756
Residuals (e. Å−3
)

4
CHATTOPADHYAY ET AL.
2.7 | Computational method
All calculations were carried out with the Gaussian 09 series
of programs.[21] The implicit polarizable continuum model of
Tomasi and co‐workers[22,23] was used to take into account
the effect of acetonitrile as solvent (dielectric constant
ε = 35.688), using atomic radii derived from the standard
universal force field. Full geometry optimization of all reac-
tants and products was carried out using density functional
theory with the UB3LYP[22,23] functional and the
6–311 + G(d,p) basis set for C, N, O and H atoms as well
as with LANL2DZ pseudopotential for Mn and Fe atoms.
This theory level has rendered accurate results in previous
studies of related systems.[24,25] Geometry optimizations
were done with the Schlegel algorithm.[26–28] 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 Mn and Fe complexes under study, the possible
low‐ and high‐spin states were calculated 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
used here.
FIGURE 1 CD spectra of the chiral complexes MnL1 (1), MnL2 (2), FeL1
(4) and FeL2 (5)
The X‐ray structure of FeL2 complex is depicted in
Figure 2. This structural analysis evidenced two crystallo-
graphically independent FeL2 complexes (A, B) besides two
perchlorate anions, and two ethanol molecules (one of which
disordered at half occupancy) (Figure S3, supporting infor-
mation). The two complexes slightly differ in their conforma-
tional geometry, although both metals exhibit an octahedral
coordination environment comprising the tetradentate Schiff
base and two aqua ligands at axial positions. The Fe─O
(phenoxo) and Fe─N bond distances fall in the range 1.898
(2)–1.9030(18) and 2.084(2)–2.096(2) Å, respectively. On
the other hand, a larger variation is observed in the Fe─OH2
bond lengths that vary from 2.075(3) to 2.115(2) Å, likely
modulated by the packing and the hydrogen bonds fashioned
by the ligands. These data are in agreement with those found
for similar salen‐type [Fe(L)(H2O)2] complexes previously
reported.[29,30] The different conformation assumed by the
chelating ligand in the two independent complexes A and B
is noteworthy. For Fe1 in the A complex the equatorial
coordination set N2O2 presents a slight tetrahedral distortion
(displacement of 0.12 Å), and thus the two chelating
moieties (almost coplanar) are tilted forming a dihedral angle
of 12.82°; for Fe2 in the B complex the ligand assumes a step
3
| RESULTS AND DISCUSSION
3.1 | Synthesis and characterization
Six purposely selected complexes have been prepared
following a procedure slightly modified from our earlier
reports.[16,17] Here Schiff bases H2L1, H2L2 and H2L3 have
been prepared in ethanolic medium by treating
2,6‐diformyl‐4‐methylphenol separately with (R)‐1,2‐
diaminopropane, (R)‐1,2‐diaminocyclohexane and 1,1′‐
dimethylethylenediamine. Further treatment of H2L1, H2L2
and H2L3 with manganese(III) perchlorate gives complexes
MnL1 (1), MnL2 (2) and MnL3 (3), and on reaction with
iron(III) perchlorate complexes FeL1 (4), FeL2 (5) and FeL3
(6). Only complex 5 rendered single crystals suitable for X‐
ray diffraction. All the complexes have been characterized
by standard physicochemical methods. We observed similar
characteristic FT‐IR bands in the ranges ca 1660–1670, ca
1610–1630 and ca 1500–1548 cm−1 assigned to C═O,
C═N and skeletal vibrations for all the complexes, as we
reported earlier for 2 and 5.[16,17] We also found characteristic
FT‐IR bands at the range ca 1090–1110 cm−1 for perchlorate
anion present in all the prepared complexes. Electronic
absorption spectra of all the complexes in methanolic
medium show various intense bands in the UV and visible
regions as we reported earlier for 2 and 5 (Figure S1 and
S2, supporting information).[16,17] CD measurements for all
the complexes were also performed and their CD spectra
are presented in Figure 1. As expected, 3 and 6 are CD inac-
tive due to absence of chirality.
FIGURE 2 ORTEP drawing (50% probability ellipsoids) of the complex
cation A of 5

CHATTOPADHYAY ET AL.
5
arrangement with the N2O2 coordination mean plane forming
dihedral angles of 18.22° and 20.19° with the phenyl ring of
each side, as displayed in Figure S4 (supporting information).
The compound crystallizes in non‐centrosymmetric space
group C2 and the chelate ring formed by the 1,2‐
diaminocyclohexane fragment in both complexes presents δ
chirality, with R configuration of both hexyl carbon atoms
connected to the nitrogen donors. The complexes are
arranged in pairs connected through hydrogen bonds occur-
ring between the axial aqua ligands of one complex and the
formyl oxygen atoms of the other, crystallographically related
by a two‐fold axis (Figure S5 and S6, supporting informa-
tion). In addition each water molecule interacts through a
hydrogen bond with ethanol oxygen or with a perchlorate
anion. Selected coordination bond distances and angles for
5 are listed in Table S1 (supporting information). Hydrogen
bond parameters are listed in Table S2 (supporting informa-
tion). This packing leads the metals to be separated by
6.465 and 6.371 Å in the two A and B complex dimers. A
similar structure with the formation of dimers in the solid
state was detected in the manganese derivative with chloride:
[MnL3(H2O)2]Cl.[16]
was used to monitor the progress of the reaction. After usual
work‐up and chromatographic purification, the isolated epox-
ide yield was found to be 90%, which is very similar to that
reported in our previous study.[16] To find the best catalytic
conditions we kept 0.30 mmol of (Z)‐stilbene whereas the
catalyst amount was varied between 0.005 and 0.01 mmol,
the amount of terminal oxidant, PhIO, between 0.25 and
0.35 mmol, and the reaction time between 1 and 3 h. We
found that 0.01 mmol of catalyst 2, 0.30 mmol of PhIO and
2 h reaction time provided the best yield. These optimum
reaction conditions were applied to all homogeneously
catalysed 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 the
absence of the catalyst.
3.3 | Catalytic epoxidation of various alkenes with
PhIO
Here we examined the epoxidation of three alkenes,
(E)‐stilbene, (Z)‐stilbene or styrene, catalysed by complexes
1–6 in CH3CN, with PhIO as terminal oxidant. Based on our
previous studies,[7,16,17] we only consider CH3CN as solvent.
Table 2 presents the conversion, turnover number (TON) and
isolated yield for the epoxidation of the alkenes. These data
suggest some considerations. First, according to the reaction
yields, manganese catalysts (yields between 78 and 92%) seem
to be much more efficient than the corresponding iron ones
(yields between 28 and 56%). Second, ligand L3 gives rise to
the highest yields with both metals (complexes 3 and 6). Third,
the effect of the various substituents at the backbone of the
3.2 | Optimization of conditions for catalytic
epoxidation with PhIO
We have followed the same procedure for all homogeneous
catalysed epoxidation reactions using catalysts 1–6. In an ini-
tial experiment, (Z)‐stilbene (0.30 mmol) (as representative),
2 (0.01 mmol) and PhIO (0.30 mmol) were mixed in 15 ml
of acetonitrile and stirred for 2 h at room temperature. TLC
TABLE 2 Epoxidation of alkenes catalysed by complexes 1–6 in CH3CN with PhIOa
Catalyst
1
Substrate
Conversion (%)
TONb
Yield (%)c
ee (%)d
(E)‐Stilbene
(Z)‐Stilbene
Styrene
89
87
83
13.4
13.1
12.5
88
38
41
42
85 (cis: trans = 70: 30)
80
2
3
4
5
6
(E)‐Stilbene
(Z)‐Stilbene
Styrene
90
86
80
13.5
13.0
12.0
87
35
37
40
83 (cis: trans = 66: 34)
78
(E)‐Stilbene
(Z)‐Stilbene
Styrene
95
94
93
14.3
14.1
13.9
92
—
—
—
91 (cis: trans = 60: 40)
89
(E)‐Stilbene
(Z)‐Stilbene
Styrene
50
55
37
7.5
8.3
5.6
48
25
27
35
Tracee
Tracee
20
53 (cis: trans = 80: 20)
33
(E)‐Stilbene
(Z)‐Stilbene
Styrene
47
51
35
7.1
7.7
5.3
43
49 (cis: trans = 70: 30)
28
(E)‐Stilbene
(Z)‐Stilbene
Styrene
57
59
50
8.6
8.9
7.5
55
—
—
—
56 (cis: trans = 65: 35)
47
aCatalyst (0.01 mmol), alkene (0.30 mmol), PhIO (0.30 mmol) and CH3CN (15 ml) stirred at room temperature for 2 h in air.
bTON = moles of substrate converted per mole of catalyst per hour.
cIsolated epoxide yield.
dDetermined by 1H NMR (300 Hz) in the presence of Eu(hfc). Configuration not determined.
eConcentration < 5%.

6
CHATTOPADHYAY ET AL.
ligand is quite similar. Complexes 3 and 6 did not have chiral-
ity; thus we observed highest enantiomeric excess product for
catalysts 1 and 4. The order of the experimental catalytic
efficiency has been rationalized by theoretical calculations,
as explained below. The epoxidation process has also been
monitored using UV–visible spectroscopy. We have obtained
observations similar to those we reported earlier.[16,17] The
results clearly suggests the generation of higher valent
MnV═O and FeV═O species.
conservation rule. The only way such a reaction could happen
would be through the assistance of another simultaneous
reaction where the reverse spin change happened, that is,
reactant spin multiplicity in the auxiliary reaction should be
3 and that of products 5. In this assumption, the Gibbs energy
of the reaction in equation (1) is very similar for the three
complexes with different substituents at the non‐aromatic
chain of the ligand, as displayed in the second column of
Table S4 (supporting information). A small preference
for the complex with two methyl substituents, 3, is
observed. The other reliable possibility for this oxidation
to happen is the reaction taking place in the triplet state,
which involves assuming that reactants are excited by about
20.5 kcal mol−1 to their triplet state before the beginning of
the oxidation process. From this excited state they evolve to
the stable triplet products. Last column in Table S4
(supporting information) indicates that, in this assumption,
the complex with the cyclohexane substituent, 2, is thermo-
dynamically preferred, and closely followed by 3, due to their
most negative Gibbs reaction energy. It is interesting to note
that the large energy released during the reaction at the triplet
state can easily compensate the initial energy input needed to
excite the reactants.
3.4 | Theoretical investigation
Initial Mn(III) complexes 1–3, as well as Fe(III) ones
discussed below, are assumed to be oxidized by PhIO ((a)
in Figure S7, supporting information) to render the final
oxidized MnV═O species, 1‐oxo, 2‐oxo and 3‐oxo, and PhI
((b) in Figure S7, supporting information), along with two
released water molecules. The complex oxidation schemati-
cally represented by the following equation is an intermediate
step in the epoxidation of alkenes:
Complex þ PhIO⇔complex‐oxo þ PhI
(1)
Quantum chemical calculations were performed to see
which catalysts render the most stable oxidized complexes,
assuming that they have the best chance of providing higher
epoxide yields.
Figure S9 (supporting information) displays the B3LYP/6–
31 + G(d,p) optimized structures of the initial Fe(III) (with
high, 6, and low, 2, spin multiplicity) and final FeV═O
(with high, 4, and low, 2, spin multiplicity) complexes along
with some selected geometrical parameters. Compared to Mn
complexes, initial Fe(III) ones could be described as shorter,
due to smaller axial distances, and wider, due to larger equato-
rial distances. For all the initial iron complexes the metal is
closer to oxygen atoms than to nitrogen ones in the equatorial
plane. For those with high spin, the O─Fe─O angle (109°) is
approximately30°larger thantheN─Fe─Noneandaxialwater
ligands are about 2.21 Å apart from the metal, but this distance
reduces to 2.02 Å for low‐spin ones. For the final FeV═O
complexes the distance between Fe and axial oxygen is
1.610 Å and both O─Fe─O and N─Fe─N become of
similar size (85° on average) for both spin multiplicities.
Table S5 (supporting information) collects the absolute
electronic and Gibbs energies of all these species in
acetonitrile solution. As for Mn complexes, initial high‐spin
Fe complexes (spin multiplicity =6) are more stable than the
low‐spin ones (spin multiplicity =4), but now the difference
between them amounts to 11 kcal mol−1 on average. However,
final iron oxidized complexes present similar absolute energy,
as previously reported for analogous systems.[7] It is
noteworthy that theoretical geometry of 5 in the stable
high‐spin state is in agreement with experimental X‐ray
measurements previously discussed.
Figure S8 (supporting information) displays the B3LYP/
6–31 + G(d,p) optimized structures of the initial Mn(III)
(with high, 5, and low, 3, spin multiplicity) and final MnV═O
(with high, 3, and low, 1, spin multiplicity) complexes along
with some selected geometrical parameters. For all initial Mn
(III) complexes the metal ion is closer to oxygen atoms than
to nitrogen ones in the equatorial plane. The hexacycle sub-
stituent of 2 makes both N atoms equivalent but the asymme-
try caused by methyl and dimethyl substituents produces a
slight enlargement of the Mn─N2 distance in 1 and 3.
Besides, the O─Mn─O angle (95°) is approximately 12°
larger than the N─Mn─N one for all species. For the initial
Mn complexes with high‐spin multiplicity axial water ligands
are about 2.36 Å apart from the metal but this distance
reduces to 2.04 Å for low‐spin multiplicity initial complexes.
For the final MnV═O complexes the distance between Mn
and axial oxygen is 1.641 and 1.53 Å, on average, for high‐
and low‐spin complexes, respectively. Table S3 (supporting
information) collects the absolute electronic and Gibbs ener-
gies of all these species in acetonitrile solution, and shows
that high‐spin complexes are more stable than low‐spin ones
for the initial and the final manganese complexes, although
for the initial species the difference in Gibbs energy between
both spin states is much larger (20.5 kcal mol−1 on average)
than in the final ones (7.5 kcal mol−1 on average). Consider-
ing the most stable complexes, the spin multiplicity of the
reaction would vary from 5 for the reactants to 3 for the
oxidized Mn complexes, which is forbidden by the spin
Table S6 (supporting information) shows that both mech-
anistic hypotheses previously explained (iron complex oxida-
tion by PhIO happening assisted by an auxiliary reaction
whose spin multiplicity changes from 4 to 6 or initial excita-
tion of Fe complexes to a doublet state to yield products in a

CHATTOPADHYAY ET AL.
7
doublet excited state) point to the complex with a dimethyl
substituent at the non‐aromatic chain of the ligand, 6, as the
most active one closely followed by 5. Again, the large
amount of energy released in the process taking place in the
excited doublet state largely compensates the initial input of
energy the complexes need to reach the doublet state (about
11 kcal mol−1).
3.5 | Preparation and characterization of
Fe3O4@dopa@MnL1 and Fe3O4@dopa@FeL1
A
schematic diagram for the preparation of
Fe3O4@dopa@MnL1 and Fe3O4@dopa@FeL1 is presented
in Scheme 2. Magnetic nanoparticles Fe3O4 and modification
of their surface were realized following the same procedure as
we earlier reported.[7] For the preparation of
Fe3O4@dopa@MnL1 and Fe3O4@dopa@FeL1, 1 g of
MnL1ClO4 or FeL1ClO4 was added to
a dispersed
acetonitrile solution of amine‐functionalized nano‐Fe3O4
(Fe3O4@dopa, 500 mg), stirring the mixture for 14 h at room
temperature. The product was allowed to settle, washed
several times with acetonitrile and dried under vacuum at
50°C for 3 h. Newly prepared Fe3O4@dopa@MnL1 and
Fe3O4@dopa@FeL1 were characterized as described in
the following.
FT‐IR spectra of MnL1ClO4, Fe3O4@dopa@MnL1,
FeL1ClO4, and Fe3O4@dopa@FeL1 are presented in
Figure 3. We observed very similar peaks for Fe3O4 and
Fe3O4@dopa related to Fe─O bond and to the vibration of
the benzene ring present in dopamine moiety.[7] Several
new peaks are generated for Fe3O4@dopa@MnL1 and
Fe3O4@dopa@FeL1, along with characteristic peaks as for
Fe3O4. The peaks in the range 1530–1540 cm−1 may be
assigned to the skeleton vibration of the complex. The sharp
peaks in the ranges 1620–1626 and 1645–1660 cm−1 may be
due to the presence of two types of C═N (imine bond) vibra-
tion of the incorporated complex moiety. The FT‐IR spectra
unambiguously prove that the desired surface modification
of nanoparticles has been successfully done.
The degree of crystallinity of magnetic Fe3O4,
Fe3O4@dopa, Fe3O4@dopa@MnL1 and Fe3O4@dopa@FeL1
was checked using PXRD measurements (Figure 4A). The
PXRD data match well with the standard Fe3O4
sample. The same peaks are observed among the PXRD
FIGURE 3 FT‐IR spectra: (A) [MnL1]ClO4; (B) Fe3O4@dopa@MnL1; (C)
[FeL1]ClO4; (D) Fe3O4@dopa@FeL1
patterns of Fe3O4@dopa, Fe3O4@dopa@MnL1 and
Fe3O4@dopa@FeL1, indicating that the resultant nanoparti-
cles contain pure Fe3O4 with a spinel structure and that the
grafting process does not induce any phase change of
Fe3O4 as previously found by our group for similar sys-
tems.[7] The solid‐state UV spectra of Fe3O4, Fe3O4@dopa,
Fe3O4@dopa@MnL1
and
Fe3O4@dopa@FeL1
are
depicted in Figure 4(B). The UV spectra of Fe3O4 and
Fe3O4@dopa are very similar to those reported earlier.[7]
MnL1ClO4 and FeL1ClO4 spectra have broad shoulders at
SCHEME 2 Schematic representation of the preparation of magnetically
separable nanocatalysts, Fe3O4@dopa@ML1 (M = Mn(III)/Fe(III))

8
CHATTOPADHYAY ET AL.
FIGURE
5
TGA
curves
of
Fe3O4‐NPs,
Fe3O4@dopa,
Fe3O4@dopa@MnL1 and Fe3O4@dopa@FeL1
(A)
(B)
FIGURE 4 (A) PXRD spectra and (B) solid‐state UV spectra of Fe3O4,
Fe3O4@dopa, Fe3O4@dopa@MnL1 and Fe3O4@dopa@FeL1
ca 420 and 530 nm, respectively (Figure S1 and S2,
supporting information). An increase of the absorbance
clearly suggests the conjugation of MnL1 and FeL1 with
Fe3O4@dopa.
Thermogravimetric analysis (TGA) was carried out for the
verification of the successful surface modification of magnetic
Fe3O4 and grafting of MnL1ClO4 and FeL1ClO4 on the surface
of Fe3O4@dopa (Figure 5). The weight loss of Fe3O4 and
Fe3O4@dopa is very similar as we reported earlier.[7] We
observe 28 and 31% weight loss for Fe3O4@dopa@MnL1 and
Fe3O4@dopa@FeL1, respectively. TGA results undoubtedly
suggest that the enhanced weight loss is due to the increasing
quantity of attached organic moiety from Fe3O4@dopa to
Fe3O4@dopa@MnL1 and to Fe3O4@dopa@FeL1, similarly
to the results described by our group earlier.[7]
FIGURE
6
SEM images of (A) Fe3O4@dopa@MnL1 and (B)
Fe3O4@dopa@FeL1
SEM images of Fe3O4@dopa@MnL1ClO4 and
Fe3O4@dopa@FeL1ClO4, presented in Figure 6, show mor-
phologies quite different from those of Fe3O4‐NPs and
Fe3O4@dopa we reported in a previous work.[7] Both show
agglomerated particles of larger size with respect to those
reported and this alteration in morphology confirms the
surface modification process.
TEM
images
of
Fe3O4@dopa@MnL1
and
Fe3O4@dopa@FeL1 are shown in Figure 7. A close inspec-
tion of these images reveals that the magnetic nanoparticles
are quasi‐spherical with an average diameter of less than
10 nm. Here MnL1ClO4 and FeL1ClO4 are grafted on the
surface of Fe3O4@dopa.

CHATTOPADHYAY ET AL.
9
(A)
(B)
FIGURE
8
EDX spectra of (A) Fe3O4@dopa@MnL1 and (B)
Fe3O4@dopa@FeL1
Fe3O4@dopa@FeL1 of between 50 and 200 mg was used
per 3 mmol of (E)‐stilbene. Enhancement in the yield of
epoxide was observed when the amount of catalyst was
increased from 50 to 100 mg but the yield remained the same
with further increase of catalyst amount up to 200 mg. The
reaction has also been studied by varying the amounts of
terminal oxidant (PhIO) and time. Optimum epoxidation
yield was obtained by using 3 mmol of PhIO and 6 h of
stirring. At the end of the reaction, the catalyst was
magnetically separated out and reused for further epoxida-
tion. Here it is noteworthy that we did not find any leaching
of catalyst during the epoxidation reaction. Again, the
Fe3O4@dopa@MnL1/Fe3O4@dopa@FeL1‐catalysed epoxi-
dation of three different alkenes, (E)‐stilbene, (Z)‐stilbene
and styrene, in CH3CN with PhIO was studied and the results
are presented in Table 3.
FIGURE
7
TEM images of (A) Fe3O4@dopa@MnL1 and (B)
Fe3O4@dopa@FeL1
The energy‐dispersive X‐ray (EDX) spectra of
Fe3O4@dopa@MnL1 and Fe3O4@dopa@FeL1 nanocatalysts
are depicted in Figure 8. Fe and O signals are generated from
the Fe3O4 nanoparticles and C signal is generated from dopa-
mine. The well‐defined peaks of iron and manganese in the
EDX spectra of nanocomposites confirm the presence of
MnL1 and FeL1 around the magnetic core. The copper (Cu)
signals come from the coating material of the instrument.
The magnetization behaviour of the Fe3O4‐NPs,
Fe3O4@dopa, Fe3O4@dopa@MnL1 and Fe3O4@dopa@FeL1
nanoparticles under an applied magnetic field is presented in
Figure 9. The curves exhibit very similar phenomenon as we
have observed earlier. Firstly, the decrease in the values of the
saturation magnetization (Ms) from Fe3O4 nanoparticles
(58.11 emu g−1) to Fe3O4@dopa (39.22 emu g−1) and to the
3.6 | Epoxidation properties of Fe3O4@dopa@MnL1
and Fe3O4@dopa@FeL1
We evaluated the optimum reaction conditions in order to
achieve the maximum epoxide yield from the heterogeneous
catalyst, as already done for the homogeneous system. For
this purpose a variable weight of Fe3O4@dopa@MnL1 and
final
Fe3O4@dopa@MnL3
and
Fe3O4@dopa@FeL6

10
CHATTOPADHYAY ET AL.
TABLE 3 Epoxidation of alkenes catalysed by Fe3O4@dopa@MnL1 and Fe3O4@dopa@FeL1 in CH3CN with PhIOa
Catalyst
Substrate
Conversion (%)
Yield (%)b
ee (%)c
Fe3O4@dopa@MnL1
(E)‐Stilbene
(Z)‐Stilbene
Styrene
84
81
78
80
35
37
39
77 (cis: trans = 68: 32)
75
Fe3O4@dopa@FeL1
(E)‐Stilbene
(Z)‐Stilbene
Styrene
45
50
34
41
25
27
35
48 (cis: trans = 75: 25)
31
aCatalyst (100 mg), alkene (2 mmol), PhIO (2 mmol) and CH3CN (15 ml) stirred at room temperature for 2 h in air.
bIsolated epoxide yield.
cDetermined by 1H NMR (300 Hz) in the presence of Eu(hfc). Configuration not determined.
of its size, the net magnetism exhibited by the final nanocatalysts
is sufficiently good for an effective separation from the solution
medium through the application of an external magnetic force.
3.7 | Reusability and characterization
Fe3O4@dopa@MnL1 and Fe3O4@dopa@FeL1
The recycling efficiency of our synthesized magnetically sep-
arable heterogeneous nanocatalysts was also evaluated. A set
of experiments was carried out by performing epoxidation of
FIGURE
9
Magnetic curves of Fe3O4 NPs, Fe3O4@dopa,
Fe3O4@dopa@MnL1 and Fe3O4@dopa@FeL1. Inset: image of the
efficiency of the magnetic separation of the catalyst
nanocatalysts (19.29 and 18.99 emu g−1, respectively) can be
attributed to the gradual increase of diamagnetic organic
materials from Fe3O4 to Fe3O4@dopa@MnL3 and to
Fe3O4@dopa@FeL6. In addition, in comparison with the
bulk magnetite nanomaterials that normally show a saturation
magnetization value of 92 emu g−1, the Ms value of the Fe3O4
nanoparticles is found to be much lower. Since the
magnetization of a particle in an external field is a function
(A)
(B)
FIGURE 10 Reusability test of Fe3O4@dopa@MnL1 (blue) and
Fe3O4@dopa@FeL1 (red) for the epoxidation of (E)‐stilbene
FIGURE 11 TEM images of used (A) Fe3O4@dopa@MnL1 and (B)
Fe3O4@dopa@FeL1

CHATTOPADHYAY ET AL.
11
(E)‐stilbene catalysed by Fe3O4@dopa@MnL1 and
Fe3O4@dopa@FeL1 nanocatalysts with the aim of examining
their activity after five runs (Figure 10). After each run the
catalysts were recovered solely by application of a magnet.
Then these catalysts were washed with acetonitrile to
remove any absorbed products and then dried. The used cat-
alysts were further characterized using FT‐IR spectroscopy
(Figure S10, supporting information) and TEM (Figure 11).
The results clearly indicate that the catalysts are stable with
very good activity in the epoxidation reaction runs.
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and Fe(III) Schiff base complexes for the epoxidation of alkenes
both in homogeneous and heterogeneous catalysis. Density
functional theory calculations confirm experimental results and
suggest that the efficiency of the catalysts is related to the stabil-
ity of MnV═O or FeV═O intermediates which form during the
catalytic process in the presence of PhIO. Here we have adapted
an economically workable and energy efficient catalytic process
using MnL1 or FeL1 (L1 comes from (R)‐1,2‐diaminopropane)
complexes over magnetically separable nanoparticles for the
selective epoxidation of alkenes at room temperature. The enan-
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magnetically separable nanoparticles. The easy operation, the
stability of catalysts, the use of cheap and mild magnetic
nanoparticles as support, the easy recoverability and reusabil-
ity of the catalysts, along with the high epoxide yield make
them an environmentally acceptable and greener alternative
to other reported catalytic systems for alkene epoxidation.
Above all, we can consider that these novel catalytic systems
would find applications in several other industrially significant
catalytic processes as well as in general synthetic organic
transformations due to their high enantiomeric excess yield.
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ACKNOWLEDGMENTS
[30] S.‐C. Cheng, C.‐W. Chang, H.‐H. Wei, G.‐H. Lee, Y. Wang, J. Chin. Chem.
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Financial support by the Science and Engineering Research
Board (a statutory body under DST, New Delhi, India; F. no.
SB/FT/CS‐185/2013 dt. 30‐06‐2014) is gratefully acknowl-
edged by T.C. We also thank DST, New Delhi, for providing
single‐crystal diffractometer facility at the Department of Chem-
istry, University of Calcutta, through DST‐FIST programme.
SUPPORTING INFORMATION
Additional Supporting Information may be found online in
the supporting information tab for this article.
How to cite this article: Chattopadhyay, T.,
Chakraborty, A., Dasgupta, S., Dutta, A., Menéndez,
M. I., and Zangrando, E. (2016), A route to magneti-
cally separable nanocatalysts: Combined experimental
and theoretical investigation of alkyl substituent role
in ligand backbone towards epoxidation ability, Appl
Organometal Chem, doi: 10.1002/aoc.3663
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