Fe3O4@dopa (dopa = dopamine hydrochloride) functionalized Mn(III) Schiff base complex: A promising magnetically separable heterogeneous catalyst for oxidative transformations

Article abstract of DOI:10.1080/00958972.2017.1386786

A chiral Schiff base complex has been prepared by treating (R)-1,2-diaminopropane with 3,5-dichlorosalicylaldehyde in ethanol, followed by addition of manganese chloride hexahydrate to generate a homogeneous catalyst, [MnL(Cl)(H₂O)] (HMN). Crystal structure of the complex reveals its mononuclear nature. Circular dichroism (CD) studies indicate that the ligand and its corresponding complex contain an asymmetric center. The catalytic activity of HMN toward epoxidation of alkenes, oxidation of alcohols and oxidation of alkanes has been investigated in the presence of iodosylbenzenediacetate (PhI(OAc)₂), in acetonitrile. In the present work we found yields to be much higher compared to our previous approaches. For further adaptation, we attached our efficient homogeneous catalyst with surface modified magnetic nanoparticles (Fe₃O₄@dopa) and thereby obtained a new magnetically separable nanocatalyst Fe₃O₄@dopa@MnLCl (FDM). This catalyst has been characterized and its oxidation ability assessed in similar conditions as those used for the homogeneous catalyst. Enantiomeric excess in epoxide yield reveals retention of chirality of the active site of Fe₃O₄@dopa@MnLCl. The catalyst can be recovered by magnetic separation and recycled several times without significant loss of catalytic activity.

Full text of DOI:10.1080/00958972.2017.1386786

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Journal of Coordination Chemistry
ISSN: 0095-8972 (Print) 1029-0389 (Online) Journal homepage: http://www.tandfonline.com/loi/gcoo20
Fe3O4@dopa (dopa = dopamine hydrochloride)
functionalized Mn(III) Schiff base complex: A
promising magnetically separable heterogeneous
catalyst for oxidative transformations
Aratrika Chakraborty & Tanmay Chattopadhyay
To cite this article: Aratrika Chakraborty & Tanmay Chattopadhyay (2017) Fe3O4@dopa (dopa
= dopamine hydrochloride) functionalized Mn(III) Schiff base complex: A promising magnetically
separable heterogeneous catalyst for oxidative transformations, Journal of Coordination Chemistry,
70:19, 3293-3307, DOI: 10.1080/00958972.2017.1386786
To link to this article: http://dx.doi.org/10.1080/00958972.2017.1386786
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Date: 07 November 2017, At: 15:21

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Journal of Coordination Chemistry, 2017
Vol. 70, no. 19, 3293–3307
https://doi.org/10.1080/00958972.2017.1386786
Fe3O4@dopa (dopa = dopamine hydrochloride)
functionalized Mn(III) Schiff base complex: A promising
magnetically separable heterogeneous catalyst for oxidative
transformations
Aratrika Chakrabortya and Tanmay Chattopadhyayb
adepartment of Chemistry, university of Calcutta, Kolkata, india; bdepartment of Chemistry, Panchakot
mahavidyalaya, sarbari, Purulia, india
ABSTRACT
ARTICLE HISTORY
received 18 may 2017
accepted 31 august 2017
A chiral Schiff base complex has been prepared by treating (R)-
1,2-diaminopropane with 3,5-dichlorosalicylaldehyde in ethanol,
followed by addition of manganese chloride hexahydrate to generate
a homogeneous catalyst, [MnL(Cl)(H2O)] (HMN). Crystal structure
of the complex reveals its mononuclear nature. Circular dichroism
(CD) studies indicate that the ligand and its corresponding complex
contain an asymmetric center. The catalytic activity of HMN toward
epoxidation of alkenes, oxidation of alcohols and oxidation of alkanes
has been investigated in the presence of iodosylbenzenediacetate
(PhI(OAc)2), in acetonitrile. In the present work we found yields to
be much higher compared to our previous approaches. For further
adaptation, we attached our efficient homogeneous catalyst with
surface modified magnetic nanoparticles (Fe3O4@dopa) and thereby
KEYWORDS
mn(iii)-schiff base
complex; nanocatalyst;
magnetic separation;
iodosylbenzenediacetate;
reusability
obtained
a new magnetically separable nanocatalyst Fe3O4@
dopa@MnLCl (FDM). This catalyst has been characterized and its
oxidation ability assessed in similar conditions as those used for the
homogeneous catalyst. Enantiomeric excess in epoxide yield reveals
retention of chirality of the active site of Fe3O4@dopa@MnLCl. The
catalyst can be recovered by magnetic separation and recycled several
times without significant loss of catalytic activity.
CONTACT tanmay Chattopadhyay
tanmayc2003@gmail.com
supplemental data for this article can be accessed at https://doi.org/10.1080/00958972.2017.1386786.
© 2017 informa uK limited, trading as taylor & francis Group

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3294 A. CHAKRABORTY AND T. CHATTOPADHYAY
1. Introduction
Research in the field of catalysis mainly focused on augmenting catalytic activity and selec-
tivity. Recovery of the catalyst was not a serious issue. But for “green chemistry”of catalytic
reactions, the recovery and reuse of catalysts emerges as an important factor [1–6].
Magnetically separable nanocatalysts are an emergent topic of research [7–9] especially
when these magnetically separable nanocatalysts are tuned with metal complexes [10,11].
Magnetically separable nanoparticles have excellent catalytic activity mainly due to their
higher surface area, appreciable mechanical stability, increasing contact between catalyst
and reactants, and also good dispersion in a reaction mixture. Magnetic nanoparticles have
advantages, such as insolubility in reaction mixture, paramagnetic nature, and facile sepa-
ration of the nanocatalyst at the end of the reaction by simply using a magnet thus avoiding
catalyst filtration. Also these nanoparticles resist aggregation of nanocatalyst which increases
the durability of the catalysts, recycling, and reusability [12]. Much effort has been expended
for synthesis of magnetic separable nanoparticles and their applications in catalysis [13–15].
Current research focuses on greener synthesis of nanoparticles and their applications [16].
The use of nanoparticles as catalysts in organic transformations provide high surface area-
to-volume ratios [17, 18]. Homogeneous units anchored on nanoparticle supports heightens
catalytic activity since the overall system still behaves as if dissolved in a solution. The choice
of nanoparticles can also deliver additional functionality into the catalytic system [19].
Synthesis of iron oxide nanoparticles is cost effective coupled with simple synthetic
approaches, and since suitable ligands can easily be anchored on their surface [20–23], they
offer excellent potential as a catalyst support. Mn(III) complexes with Schiff-base ligands
displaying N2O2 coordination have been reported as soluble and efficient catalysts in olefin
epoxidations [24–27]. In our recent work we illustrated the epoxidation capability by various
manganese and iron Schiff base complexes [28, 29]. Earlier we had explored only epoxidation
capability by our synthesized catalyst. In the present work we report the synthesis of a new
magnetically separable nanocatalyst, Fe3O4@dopa@MnLCl (FDM). Our previous efforts to
explore the oxidation capabilities of our synthesized MSNps were futile hence in our present
approach the main motive was to explore the versatility of our synthesized nanocatalyst.
The ligand backbone of our catalyst in the present work is similar to our previously reported
work [28]. We employ poorly explored oxidizing agent iodosobenzene diacetate (PhI(OAc)2).
PhI(OAc)2, a precursor toward synthesis of PhIO, is highly soluble in most organic solvents,
easy to handle and also commercially available thus making it a suitable candidate for various
oxidation reactions [30]. Here we alter the metal center as shown in Scheme 1. Altering the
metal center provides comparatively greater yield compared to previous catalyst and hence
subjected it to various organic transformations such as epoxidation, alcohol oxidation and
alkane oxidation; besides alkene epoxidation our catalyst was very effective in oxidizing
alcohols and alkanes. Our synthesized nanocatalyst was easily recovered and reused for
more than five times without any loss of catalytic activity or selectivity.
2. Experimental
2.1. Physical methods and materials
All chemicals were obtained from commercial sources and used as received. Solvents were
dried according to standard procedure and distilled prior to use.

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JOURNAL OF COORDINATION CHEMISTRY
3295
Scheme 1. schematic diagram for the formation of [mnl(Cl)(h2o)].
Elemental analyses (carbon, hydrogen and nitrogen) were performed using a Perkin-Elmer
240C elemental analyzer. Infrared spectra (4000–500 cm−1) were recorded at 27 °C using a
Perkin-Elmer RXI FT-IR spectrophotometer with KBr pellets. Electronic spectra (800–200 nm)
were obtained at 27 °C using a Shimadzu UV-3101PC with methanol as solvent and reference.
Thermal analyses (TG-DTA) were carried out on a Mettler Toledo (TGA/SDTA851) thermal
analyzer in flowing dinitrogen (flow rate: 30 cm3 min−1). Field Emission Scanning Electron
Microscope (FE-SEM) measurements were carried out with a JEOL JSM-6700F field-emission
microscope. Transmission Electron Microscope (TEM) measurements were carried out with
a JEOL (Japan) JEM2100 high-resolution transmission electron microscope. X-ray powder
diffraction (XRPD) was performed on a XPERT-PRO diffractometer with monochromated
Cu-Kα radiation (40.0 kV, 30.0 mA) at room temperature. A vibrating sample magnetometer
1
(EV-9, Microsense, ADE) was utilized for obtaining the magnetization curves. The H NMR
spectra were recorded on a Bruker AC300 spectrometer. CD spectra were measured with a
Jobin Ivon CD 6 spectrophotometer. Conductance of methanolic solutions of complexes
were measured using a SYSTRONICS 306 conductivity meter. A vibrating sample magneto-
meter (EV-9, Microsense, ADE) was utilized for obtaining magnetization curves. Thermo
Scientific iCAP 7400 ICP-OES was utilized for detection of leaching of manganese ions from
the catalyst.
2.2. Syntheses of [MnL(Cl)(H2O)] (HMN)
Ethanolic solution (10 mL) of (R)-1,2-diaminopropane (0.74 g, 1 mmol) was added slowly to
20 mL ethanolic solution of 3,5-dichlorosalicylaldehyde (0.382 g, 2 mmol). Immediate pre-
cipitation of HL took place. Ethanolic solution (10 mL) of manganese chloride hexahydrate
(0.135 g, 0.5 mmol) was added to the suspension of HL. Yellow precipitate of HL disappeared
readily to give a deep brown solution. The solution was stirred overnight. Black micro crystals
were formed from DMF after a few days. Yield: 90% (with respect to L). Anal. calcd for HMN:
C, 38.78; H, 2.68; N, 5.32. Found: C, 37.55; H, 2.27; N, 5.49. IR (KBr): ν(C=N) 1614.63 cm−1; ν
(skeletal vibration) 1527.40 cm−1; UV/vis (CH3CN): λmax/nm = 272, λmax/nm = 377.