An ionic liquid supported on magnetite nanoparticles as an efficient heterogeneous catalyst for the synthesis of alkyl thiocyanates in water

Article abstract of DOI:10.1080/17415993.2021.1885673

The present study describes a convenient method to synthesize alkyl thiocyanates from alkyl halides with the use of a novel nanomagnetic-supported organocatalyst (MNP@PEG-ImCl). The new supported ionic liquid is fully characterized by field-emission scanning electron microscopy (FESEM), Fourier-transform infrared (FT-IR), energy dispersive X-ray analysis (EDAX), X-ray powder diffraction (XRD), transmission electron microscopy (TEM), vibrating sample magnetometry (VSM) as well as thermogravimetric analysis (TGA) techniques. It is noteworthy that we observed easy separation of the catalyst from the reaction mixture by a simple magnetic decantation and its reutilization many times without any appreciable loss of activities.

Full text of DOI:10.1080/17415993.2021.1885673

Journal of Sulfur Chemistry
ISSN: (Print) (Online) Journal homepage: https://www.tandfonline.com/loi/gsrp20
An ionic liquid supported on magnetite
nanoparticles as an efficient heterogeneous
catalyst for the synthesis of alkyl thiocyanates in
water
Mehdi Fallah-Mehrjardi & Soheil Sayyahi
To cite this article: Mehdi Fallah-Mehrjardi & Soheil Sayyahi (2021) An ionic liquid
supported on magnetite nanoparticles as an efficient heterogeneous catalyst for the
synthesis of alkyl thiocyanates in water, Journal of Sulfur Chemistry, 42:3, 335-345, DOI:
10.1080/17415993.2021.1885673
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JOURNAL OF SULFUR CHEMISTRY
021, VOL. 42, NO. 3, 335–345
2
https://doi.org/10.1080/17415993.2021.1885673
An ionic liquid supported on magnetite nanoparticles as an
efficient heterogeneous catalyst for the synthesis of alkyl
thiocyanates in water
a,b
and Soheil Sayyahic
Mehdi Fallah-Mehrjardi
a
b
Department of Chemistry, Payame Noor University (PNU), Tehran, Iran; Research Center of Environmental
c
Chemistry, Payame Noor University, Ardakan, Iran; Department of Chemistry, Mahshahr Branch, Islamic
Azad University, Mahshahr, Iran
ABSTRACT
ARTICLE HISTORY
The present study describes a convenient method to synthesize
alkyl thiocyanates from alkyl halides with the use of a novel
nanomagnetic-supportedorganocatalyst(MNP@PEG-ImCl). Thenew
supported ionic liquid is fully characterized by field-emission scan-
ning electron microscopy (FESEM), Fourier-transform infrared (FT-
IR), energy dispersive X-ray analysis (EDAX), X-ray powder diffrac-
tion (XRD), transmission electron microscopy (TEM), vibrating sample
magnetometry (VSM) as well as thermogravimetric analysis (TGA)
techniques. It is noteworthy that we observed easy separation of the
catalyst from the reaction mixture by a simple magnetic decanta-
tion and its reutilization many times without any appreciable loss of
activities.
Received 18 May 2020
Accepted 25 January 2021
KEYWORDS
Green synthesis; alkyl
thiocyanate; alkyl halide;
ionic liquid; phase transfer
catalyst
CONTACT Mehdi Fallah-Mehrjardi
m.fallahmehrjerdi@pnu.ac.ir
Department of Chemistry, Payame Noor
University, 19395-3697 Tehran, Iran
Supplemental data for this article can be accessed here. https://doi.org/10.1080/17415993.2021.1885673
©
2021 Informa UK Limited, trading as Taylor & Francis Group

336
M. FALLAH-MEHRJARDI AND S. SAYYAHI
1. Introduction
It is well known that sulfur-containing architectures exhibit significant functions in general
organic synthesis and thus are utilized in the pharmaceutical industry as well as materials
science [1]. Organic thiocyanates are the members of a chemical class of organic chalco-
gen cyanates where O, Se, S, or Te, heteroatoms are bound via a single bond to the organic
substituent such as alkyl or aryl, and simultaneously to a CN group. As a result of the fre-
quent use of the XCN functional group especially as a leaving group, alkyl thiocyanates are
frequently regarded as organic pseudo-halides. They have been considered to be beneficial
functional motifs found in diverse natural products as well as bio-active compounds and
have widespread utilization as the synthetic intermediate for accessing different worthwhile
sulfur-containing compounds. Hence, a considerable number of attempts have focused
on innovative methodologies in order to incorporate a thiocyano-group into organic
molecules. Most frequently these compounds are prepared from alcohols [2], thiols [3]
and isothiocyanates [4], but nucleophilic substitution reaction of thiocyanate anion with
alkyl halides is still the simplest and most common procedure for their synthesis [5–12].
It is obvious that various electrophilic as well as nucleophilic thiocyanating and cyanat-
ing agents have been provided and may be utilized in such transformations. Nonetheless,
a main caveat of a majority of them is to apply excessively strong oxidants which low-
ers chemoselectivity and regioselectivity, and results in a narrow substrate scope. Hence,
it is very crucial to devise a novel efficacious thiocyanation reaction to synthesize SCN
containing organic molecules.
It is widely accepted that ionic liquids (ILs) are a major alternative to organic solvents
that have the potential to contribute to the development of green chemistry. These liquids
have several properties, like lower melting points, reduced vapor pressure, lower coordi-
nating abilities, as well as very good chemical and thermal stability that have enhanced
their stature as candidates for replacement of the popular organic solvents in diverse chem-
ical procedures [13]. Imidazolium-based ionic liquids exhibit greater catalytic activities
and have attracted significant interest as eco-friendly catalysts in organic transformations
[
14,15]. However, biphasic ionic liquid-organic systems require large amounts of IL which
makes them unattractive based on economic considerations since ILs still are expensive
solvents. In addition, the high viscosity of ILs can induce mass transfer limitations if the
chemical reaction is fast, in which case the reaction takes place only within the narrow
diffusion layer and not in the bulk of the IL catalyst solution. Furthermore, the chemical
industry still prefers to use heterogeneous catalyst system because of the easy separa-
tion [16,17]. One of the main ways to eliminate these disadvantages is immobilization
of ILs with solid supports such as polymers [18], amorphous silica [19], mesoporous sil-
ica [20], carbon nanotubes [21], and magnetic nanoparticles [22]. Some drawbacks of
homogeneous ILs including high cost, low hydrothermal stability, and undesirable cata-
lyst recoverability, hindering their catalytic applications. So, it is very desirable to develop
new supports to immobilize ionic liquids.
It has been found that the supporting technique and the kind of support has a large effect
on the features and utilizations of the SILs. Nowadays, magnetic nanoparticles (MNPs)
are often developed as prominent catalyst supports as a result of their respective vast area
to volume ratio, low cost, facile synthesis, easy functionalization and simple separation
using an external magnet [23]. Because of these advantages experts in the field have utilized

JOURNAL OF SULFUR CHEMISTRY
337
magnetic nanoparticles, especially magnetite nanoparticles for immobilizing homogenous
ionic liquids. The nanomagnetic supported ILs combine advantages of the ionic liquids
with those of heterogeneous catalysts like enhanced design features, higher solubility of
the catalytic sites, convenient handling, and recycling and separation [24–26].
Hence, the present research reports the synthesis and characterization of a novel ionic
liquid supported on magnetic nanoparticles and their utilization as an efficacious phase-
transfer catalyst for the synthesis of alkyl thiocyanates from the alkyl halides in water.
2. Results and discussion
In this work, the MNP@PEG-OH nanoparticles are procured based on the devised proce-
dure [27]. First, we used thionyl chloride in the presence of pyridine to replace hydroxyl
groups with chlorine. Then, nucleophilic substitution reaction of MNP@PEG-Cl with
1
-methylimidazole obtained a novel supported ionic liquid, MNP@PEG-ImCl (Scheme 1).
Next, different physico-chemical characterization procedures like SEM, FT-IR, TEM,
EDAX, TGA, VSM as well as XRD were used to confirm the structure of the resultant
nanomagnetic organocatalyst. Figure S1 represents the FT-IR spectra of MNP@PEG-
ImCl, Fe O @SiO , and Fe O . As seen from the figure, FT-IR spectrum of Fe O
3
4
2
3
4
3
4
−
1
presents a powerful band at 578 cm belong to Fe-O vibration and a broadband at
−
1
−1
3
200 cm –3600 cm that is ascribed to the –OH groups surfaces. Moreover, FT-IR
−
1
spectrum of Fe O @SiO exhibited three other bands about 460, 980, and 1080 cm
3
4
2
that have relationship with the Si–O–Si bending, asymmetric stretching vibrations as
well as symmetric stretching. The observed peaks confirmed the presence of a silica
coating surrounding the Fe O magnetic NPs. In addition, introducing the ionic liquid
3
4
into Fe O @SiO surface has been verified by C – H bending and stretching bands at
3
4
2
−
1
−1
1
400–1500 cm and 2900–2950 cm , respectively. Furthermore, the bands that approx-
imate 1550 cm –1650 cm have been attributed to imidazole ring stretching vibrations
−
1
−1
and imidazole C =N bending.
Figure S2 presented the XRD pattern of the MNP@PEG-ImCl catalyst and Fe O
3
4
magnetic nanoparticles in the 2θ region of zero –80°. As can be seen, six typical
peaks at 2θ = 30.23°, 35.59°, 43.30°, 53.78°, 57.25°, and 62.83°in the XRD pattern of
Scheme 1. Preparation of MNP@PEG-ImCl nanocomposite.

338
M. FALLAH-MEHRJARDI AND S. SAYYAHI
MNP@PEG-ImCl nanoparticles, where is perfectly consistent with the Fe O nanoparti-
3
4
cles pattern (2θ = 30.31°, 35.69°, 43.32°, 53.81°, 57.26°, and 62.92°). The broad peak from
2
θ = 20–26° in the XRD pattern of the catalyst is attributed to amorphous silica [28].
The TGA analysis of MNP@PEG-ImCl was performed via heating the specimen to
−1
800°C at the rate equal to 10°C min in air (Figure S3). The thermogram curve exhib-
ited two main weight-loss phases: the first one below 200°C was assigned to a loss of the
physically adsorbed water and dehydration of the OH groups’ surfaces; the second weight
loss at above 340°C is probably correlated to the thermal-crystal phase transformation
from Fe O to γ -Fe O [29] as well as decomposition of organic substances grafted on
3
4
2
3
the Fe O @SiO surface.
3
4
2
In order to study the structure, size, and surface morphology of the MNP@PEG-ImCl
nanoparticles, FESEM and TEM characterization were carried out and are shown in Fig-
ures S4 and S5. It could be seen an approximately spherical form of the particles with the
average size ranged from 26–71 nm diameter. Also, the core–shell structure that had dark
and brighter dots that correspond to the Fe O core as well as the catalyst organic part, can
3
4
be observed in image of TEM (Figure S5).
EDAX pattern of the MNP@PEG-ImCl NPs approved the presence of the key elements
like carbon (24.8 wt%), iron (21.6 wt%), oxygen (21.6 wt%), chlorine (9.0 wt%), silicon
(
18.2 wt%) and nitrogen (4.8 wt%) (Figure S6).
The magnetic features of the catalyst NPs have been examined at the room temperature
Figure S7). With regard to the chart, saturation magnetization values have been 57, 41
(
and 14 emu/g for Fe O , Fe O @SiO and MNP@PEG-ImCl NPs, respectively. Notably,
3
4
3
4
2
magnetism reduction for the catalyst has been caused by coating the magnetic NPs via the
nonmagnetic materials, linker moiety, SiO shell, imidazolium ring as well as PEG chain.
2
Then, the catalytic activity of MNP@PEG-ImCl for synthesizing alkyl thiocyanates was
investigated. Hence, many experimentation have been devised and the most acceptable
output has been achieved in the situation that benzyl bromide (1 mmol) reacted with potas-
sium thiocyanate (1.5 mmol) in the presence of MNP@PEG-ImCl (0.02 g) in 3 mL water at
the room temperature. It is noteworthy that the reaction did not take place in the absence
of the catalyst, and in the presence of Fe O or Fe O @SiO , the reaction did not progress
3
4
3
4
2
even after 3 h, whereas in the presence of the MNP@PEG-ImCl catalyst, it obtained benzyl
thiocyanate in the quantitative yield after one hour (Table 1, Entry 1). Notably, any evi-
dence has been not observed to form isothiocyanate as the reaction by-product and thus
the corresponding product has been observed in its pure form with no additional purifi-
1
3
cations. Furthermore, C-resonance of -NCS and -SCN groups at about 145 and 110 ppm
have been the characteristic of isothiocyanate and thiocyanate functionality [30].
Generality and scope of this procedure have been then checked at optimal conditions
using a variety of alkyl halides in the presence of the catalyst (Table 1). With regard to
the results, it is possible to utilize the above procedure to prepare phenacyl thiocyanates
(
Table 1, Entries 11 and 12).
According to our predictions of the effects of steric hindrance on S 2 reaction rate, it is
N
possible to effectively convert the primary alkyl halides into the related alkyl thiocyanates
in relatively short reaction time, whilst the secondary and tertiary alkyl halides cannot be
well converted, even at longer reaction times and higher temperatures (Table 1, entries
1
4–16).

JOURNAL OF SULFUR CHEMISTRY
339
Table 1. MNP@PEG-ImCl catalyzed alkyl thiocyanates synthesis under aqueous medium.
KSCN(1.5 mmol)
MNP@PEG−ImCl(0.02g)
RX −−−−−−−−−−−−−−→ RSCN
H2O,r.t.
Entry
1
Alkyl halide
Product
Time (min)
60
Yield (%)a,b
91
2
3
4
60
60
50
88
85
87
5
6
7
50
45
45
89
90
87
8
9
60
87
50
55
84
86
1
0
1
1
45
82c
85c
12
50
1
3
4
40
90
1
600
25d
15
600
30d
(
continued).

340
M. FALLAH-MEHRJARDI AND S. SAYYAHI
Table 1. Continued
Entry
Alkyl halide
Product
Time (min)
Yield (%)a,b
16
–
600
–d
a
Isolated yield.
bAll product are known and structures have been specified via comparing their spectral and physical outputs with the
ones of authentic specimens [5–12].
c
Reaction has been done at 40°C.
Reaction has been done at 70°C.
d
Scheme 2. The catalytic activity of MNP@PEG-ImCl.
The remarkable feature of the mentioned catalyst might be attributed to the specific
features of the magnetic NPs and exist polyethylene glycol and imidazolium ring in
the structure of catalyst that enhanced its organophilicity and gave it the phase-transfer
catalytic feature.
It seems that the catalyst accelerates the reaction in two ways. The chlorine ion-pair
along with the imidazolium ring is replaced with thiocyanate anion and transferred to the
organic phase. On the other hand, the polyethylene glycol part of the catalyst activates the
thiocyanate anion for nucleophilic attack by forming a complex with the potassium cation,
much like crown ethers (Scheme 2).
In case of the use of a supported catalyst, one of the essential issues has been considered
to be the possibility that a number of active sites are capable of migrating from the solid
support to the liquid phase wherein such leached specimens can handle a considerable
portion of the catalytic activities [31]. So, the catalyst has been added to an aqueous solution
of benzyl bromide (1 mmol) as well as KSCN (1.5 mmol). Following the shaking of the
reaction mixture for thirty minutes at the room temperature, we removed the catalyst with
the use of the external magnet and allowed to continue the reaction. As shown in Figure
1
, after the separation of the nanoparticles no progress in the reaction was detected. It
confirmed that the ionic liquid species have not been leached out during the course of the
reaction.
For investigating the catalyst reusability, benzyl bromide reaction with KSCN has been
selected as the model and has been run based on the optimal conditions for five subsequent
runs. Following the reaction completion on all runs, we separated catalyst from the reaction

JOURNAL OF SULFUR CHEMISTRY
341
Figure 1. Leaching test results (1.0 mmol benzyl bromide, 1.5 mmol KSCN as well as 0.02 g MNP@PEG-
ImCl in H2O at the room temperature).
mixture with the use of an external magnet and used acetone or methanol to wash it; then, it
has been reused for another run. Outputs did not show any significant change in structure
or decrease in catalytic activity. Moreover, reaction time elevated with a mild slope from
60 to 72 min (Figure 2). Also, the weight of the nanocatalyst remained virtually unchanged
after each run.
3. Conclusion
In this study, MNP@PEG-ImCl was successfully synthesized and characterization using
FT-IR, EDAX, TEM, FESEM, TGA, VSM, and XRD. The nanoparticles were then used in
synthesizing of alkyl thiocyanates from alkyl halides in an aqueous medium at the room
temperature. Such a protocol offered multiple benefits like short reaction time, the prod-
ucts’ higher yield, environmentally friendly reaction medium, the catalyst recyclability and
simplified separation.
4. Experimental section
4.1. Generals
Each chemical material utilized in the present study has been bought from Sigma-Aldrich
and Merck companies and employed with no additional purifications. Moreover, we pro-
cured MNP@PEG as well as Fe O , Fe O @SiO under the suggested procedures [27,32].
3
4
3
4
2
We used TLC on the silica gel polygram SILG/UV 254 plates to monitor the reaction.
All products have been characterized in terms of physical features and compared to the
authentic specimens. Melting points have been observed in the open capillary tubes and
measured with an electro-thermal 9200 apparatus. FT-IR spectra have been registered on
the KBr pellets over a Shimadzu IRPresting-21 spectrometer in ranges between 4000 and
−1
4
00 cm and the XRD patterns of the specimens have been taken on a PANalytical X-
ray diffracto-meter Model X’Pert Pro. Furthermore, EDAX as well as FESEM have been

342
M. FALLAH-MEHRJARDI AND S. SAYYAHI
Figure 2. Recovery and reusability of the catalyst.

JOURNAL OF SULFUR CHEMISTRY
343
analyzed with a Zeiss-Sigma VP for micro-analysis and mapping MNP@PEG-ImCl NPs.
The TEM images of the catalyst have been also registered with the use of a Zeiss-em10c
and thermal stability of MNP@PEG-ImCl composite has been specified by a TA-Q600
thermo-gravimetric analysis (TGA) in the temperature ranges between 25 and 800°C at
−1
the heating rate of 10°C min in air atmosphere. Vibrating sample magnetometer (Megh-
natis Kavir; Kashan: Iran) has been used to investigate the NPs’ magnetic features at the
ambient temperature.
4
.2. Preparing the MNP@PEG-ImCl
The as prepared MNP@PEG-OH (2 g) [28] was dispersed in 100 mL toluene by ultra-
sonic irradiation and pyridine (2 mL, 0.025 mol) as well as then, thionyl chloride (3 mL,
0
.025 mol) were added in the next step. The resultant mixture was stirred and refluxed for
six hours and the particles have been separated by an external magnet. After that, we used
ethanol (2 × 10 mL) to wash them and dried them in an oven at 60°C for twelve hours. Next,
1
5
-methylimidazole (4 g, 0.05 mol) was poured into a suspension of MNP@PEG-Cl (2 g) in
0 mL dichloromethane. Consequently, the mixture was shaken under reflux conditions
for twelve hours. Finally, the magnetic nanoparticles were collected by an external magnet
and washed several times with EtOH and CH Cl . Finally, an oven has been employed to
2
2
dry it at 60°C for twelve hours.
4
.3. The general-process to synthesize the alkyl thiocyanates catalyzed by
MNP@PEG-ImCl
In a round bottom flask, the alkyl halide (1 mmol) and KSCN (1.5 mmol) mixture in water
(
3 mL) were reacted in the presence of MNP@PEG-ImCl (0.02 g) at the ambient tem-
perature for a suitable time (Table 1). Following the completion of the reaction that was
monitored via TLC analysis (n-hexane:ethyl acetate; 7:1), the mixture was passed throuhg a
filter in the presence of a magnet for separating the catalyst and then EtOAc (3 × 5 mL) was
added to extract the filtrate. Moreover, we dried the combined organic layers over anhy-
drous Na SO and filtered and removed in vacuo. Finally, the desired alkyl thiocyanates
2
4
have been observed in the higher isolated yields.
5. Selected spectroscopic data
Benzyl thiocyanate 1, 2: IR: υ = 2153 (SCN) (Figure S8). 3C-NMR (100 MHz, CDCl3):
1
δ = 38.3 (CH ), 112.1 (SCN), 128.9 (2×CH), 129.0 (2×CH), 129.1 (CH), 134.5 (C) (Figure
2
S9).
1
-Naphthylmethyl thiocyanate 8: IR: υ = 2151 (SCN) (Figure S10). 13C-NMR (100
MHz, CDCl ): δ = 37.5 (CH ), 112.2 (SCN), 122.9 (CH), 126.4 (CH), 126.8 (CH), 126.9
3
2
(
CH), 128.1 (CH), 129.2 (CH), 129.6 (CH), 130.2 (C), 130.6 (C), 134.0 (C) (Figure S11).
-(4-bromophenyl)-2-thiocyanato ethanone 11: IR: υ = 2157 (SCN) (Figure S12).
C-NMR (100 MHz, CDCl ): δ = 42.6 (CH ), 111.4 (SCN), 129.8 (2×CH), 130.3 (C),
1
1
3
3
2
1
32.6 (C), 132.7 (2×CH), 189.8 (C=O) (Figure S13).

344
M. FALLAH-MEHRJARDI AND S. SAYYAHI
1
-Octyl thiocyanate 13: IR: υ = 2155 (SCN) (Figure S14). 13C-NMR (100 MHz,
CDCl ): δ = 14.1 (CH ), 22.6 (CH ), 26.2 (CH ), 29.2 (CH ), 31.8 (CH ), 33.9 (CH ),
3
3
2
2
2
2
2
3
9.0 (CH ), 70.8 (CH ), 111.3 (SCN) (Figure S15).
2 2
Acknowledgment
The researchers appreciate the Research Council of Payame Noor University (PNU) for financially
supporting the present study.
Disclosure statement
No potential conflict of interest was reported by the author(s).
Funding
The researchers appreciate the Research Council of Payame Noor University (PNU) for financially
supporting the present study.
ORCID
Mehdi Fallah-Mehrjardi http://orcid.org/0000-0001-9780-6235
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