Sulfonated carbon-encapsulated iron nanoparticles as an efficient magnetic nanocatalyst for highly selective synthesis of benzimidazoles

Article abstract of DOI:10.1039/c8dt00677f

Surface functionalized carbon-encapsulated iron nanoparticles (CEINs) were found to be a magnetic nanocatalyst for the efficient and highly selective synthesis of benzimidazoles. CEINs were covalently decorated with carboxyl or sulfonyl groups and their catalytic activity was examined. Carboxyl-modified CEINs were obtained via the radical or oxidative treatment, whilst the sulfonated CEINs were obtained using the one-step diazotization approach with sulfanilic acid and isoamyl nitrite. The content of surface acidic groups varied between the obtained materials and was found to be the highest for sulfonyl-modified CEINs. CEINs functionalized with sulfonyl groups were the most efficient and the most selective nanocatalyst for the synthesis of benzimidazoles. Various benzimidazoles were obtained in very high yields (92.5-97.0%). Both metallocene, aliphatic, heterocyclic and aromatic aldehydes substituted with different functional groups were subjected to the synthesis process. The reaction proceeded in a short time, which varied from 25 min to 65 min depending on the aldehyde used. Additionally, the mechanism of the studied catalytic condensation by applying sulfonated CEINs as the catalyst was discussed. Importantly, the developed magnetic nanocatalysts could be easily separated from the reaction mixture using a permanent magnet. The nanocatalysts can be used up to six reaction cycles without any significant loss of their catalytic activity. This work opens up new ways for very efficient and simple synthesis of benzimidazoles-an important class of organic compounds for various biomedical applications.

Full text of DOI:10.1039/c8dt00677f

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DOI: 10.1039/C8DT00677F
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Sulfonated carbon-encapsulated iron nanoparticles as efficient magnetic
nanocatalyst for highly selective synthesis of benzimidazoles
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Artur Kasprzaka*, Michał Bystrzejewskib, Magdalena Poplawskaa
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*e-mail: akasprzak@ch.pw.edu.pl
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a Faculty of Chemistry, Warsaw University of Technology, 00-664 Warsaw, Poland
b Faculty of Chemistry, University of Warsaw, 02-093 Warsaw, Poland
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Abstract
Surface functionalized carbon-encapsulated iron nanoparticles (CEINs) were found to be a
magnetic nanocatalyst for the efficient and highly selective synthesis of benzimidazoles.
CEINs were covalently decorated with carboxyl or sulfonyl groups and their catalytic activity
was examined. Carboxyl-modified CEINs were obtained via the radical or oxidative
treatment, whilst the sulfonated CEINs were obtained using one-step diazotization approach
with sulfanilic acid and isoamyl nitrite. The content of surface acidic groups varied between
the obtained materials and was found to be the highest for sulfonyl-modified CEINs. CEINs
functionalized with sulfonyl groups were the most efficient and the most selective
nanocatalyst for the synthesis of benzimidazoles. Various benzimidazoles were obtained in
very high yields (92.5-97.0%). Both metallocene, aliphatic, heterocyclic and aromatic
aldehydes substituted with different functional groups were subjected to the synthesis process.
The reaction proceeded in a short time, which varied from 25 min to 65 min depending on the
aldehyde used. Additionally, the mechanism of the studied catalytic condensation applying
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sulfonated CEINs as the catalyst was discussed. Importantly, the developed magnetic
DOI: 10.1039/C8DT00677F
nanocatalysts could be easily separated from the reaction mixture using a permanent magnet.
The nanocatalysts can be used up to six reaction cycles without any significant loss of its
catalytic activity. This work opens up new ways for very efficient and simple synthesis of
benzimidazoles – an important class organic compounds for various biomedical applications.
1. Introduction
Carbon nanomaterials, especially graphene related materials and carbon nanotubes, are
considered as the most important class of solid supports for various catalytic systems.1–3 This
phenomenon is associated with the high chemical inertness of carbon materials as well as
their specific physicochemical features, such as unique curvature, high surface area and high
electrical conductivity. The presence of large number of conjugated rings and functional
groups in the structure of carbon materials gives also a possibility of physical adsorption of
various aromatic moieties via π-π-stacking or positively charged chemical individuals via
electrostatic interactions.4–6 Many methods for the non-covalent modification of carbon
nanomaterials for the catalytic purposes were therefore reported. For example, to date,
different types of active species were deposited on graphite and reduced graphene oxide,
especially metallic catalysts, like platinum or palladium.7–9 The application of carbon material
as the solid support enables to increase the activity of the metallic particles and influences the
reaction rate. These phenomena are reasoned with the fact of the (i) increased electron density
on the metallic surface via π-donation, (i) oxidizing properties of carbon nanostructures, as
well as (ii) adsorption of the reactant on the carbon material.10 The adsorption of the reactant
influences the reaction rate, since the contact between a metallic particle and the reactant is
facilitated and the stabilization of a given intermediate on the surface of carbon support is
observed.
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Catalytic activity of native graphene-related materials, such as grapheneDoOxI:i1d0e.103(9G/CO8D)T00677F
and carbon nanotubes (CNTs), was also demonstrated.11–13 Noteworthy, the main issue
regarding the application of GO as a catalyst is its stability. It has been demonstrated that the
physicochemical properties and structure of GO might change when the material is exposed
on radiation (e.g. UV), high temperature or strong acid/base conditions.10,14,15
The applications of magnetic carbon nanoparticles cover many areas of science,
including nanomedicine16–18, water treatment and sustainable chemistry.19–24 Carbon-
encapsulated metal nanoparticles are a class of core-shell magnetic nanomaterials.25 A pure
metallic phase which is encapsulated in graphene-like shell is responsible for relatively high
magnetization of carbon-encapsulated metal nanoparticles.26–30 Such nanostructures are also
commonly termed as nanomagnets. Importantly, the carbon shell not only protects the
metallic core but also enables the covalent attachment of organic ligands, e.g. via radical
processes31–33 or cycloaddition reactions34–36. Such phenomenon opens up avenues to apply
these magnetic nanoparticles in catalysis and to develop further the field of magnetic
nanocatalysis, which has been intensively studied in recent years. The magnetic nanocatalysis
is of a great interest in the heterogenic catalysis because the catalyst can be easily separated
from the reaction mixture when the reaction is finished. This can be reached using simple
permanent magnets. For example, Stark and co-workers showed that carbon-coated cobalt
nanoparticles could act as the solid matrix for covalent37 or non-covalent38 immobilization of
palladium complexes for Suzuki-Miyamura cross-coupling. The palladium catalyst was found
to be very active, as well as it can be easily separated from the reaction mixture and reused.
An emerging field of application of novel types of nanocatalysts is the synthesis of
bioactive compounds. Benzimidazoles are one of the most prominent family of biologically
important molecules. Benzimidazole framework is the pharmacophore for different classes of
bioactive individuals, like fungicides (e.g. carbendazim), antiparasitic drugs (e.g. albendazole)
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or medicines for wide range of disorders (e.g. omeprazole). The synthesis of benzimidazoles
DOI: 10.1039/C8DT00677F
is most commonly based on the condensation reaction between aromatic diamine and
aldehyde. However, the yield of the process and its selectivity are relatively low. The
application of a proper catalyst in this type of reaction has been widely studied. Carbon
materials were reported to be a promising catalyst for the synthesis of benzimidazoles.
Sharghi and co-workers have demonstrated that graphite/base or graphite/acid system can be
used to obtain various benzimidazoles.40 The synthesis yields were higher in comparison to
the catalyst-free reaction. The role of graphite in this process was explained with its oxidizing
properties and the presence of some carboxyl groups on its surface. In 2016, GO was reported
as the acid nanocatalyst for benzimidazole synthesis.41 The synthesis yield was reported to be
between 42% and 89% and depended on the aldehyde and amine used. However, GO had to
be re-oxidized before the next catalytic reaction. This is because the carboxyl groups from GO
were consumed in this process. Graphene oxide cannot be therefore considered as the “real”
catalyst, because its structure changes after the reaction. Noteworthy, magnetic nanocatalysts,
like iron oxide nanoparticles, were also applied for the synthesis of benzimidazoles.42,43
However, the selectivity and the synthesis yield of the reaction was lower in comparison to
the processes catalyzed by GO. In case of application of iron oxide nanoparticles as the
catalyst for the synthesis of benzimidazoles, the nanomaterial can be relatively easily
separated from the reaction mixture. Nevertheless, it was previously demonstrated that the
magnetic features of iron oxides are lower in comparison to pure metallic phase.44,45
Herein, an efficient, simple and highly selective synthetic route to obtain various
benzimidazoles using magnetic carbon-encapsulated iron nanoparticles (CEINs; Fe@C) or
surface decorated CEINs as the nanocatalyst, is reported. A comparative study on the catalytic
activity of (i) CEINs, (ii) carboxyl-modified CEINs and (iii) sulfonyl-modified CEINs, is
presented. An efficient method for the decoration of CEINs with sulfonyl groups, is also
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demonstrated. It is also shown that CEINs-based nanocatalyst can be easily separated from
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the reaction mixture because of its strong ferromagnetic properties. The recyclability study
and proposed reaction mechanism are also presented.
2. Experimental section
2.1
Materials and methods
Ferrocenecarboxaldehyde (98%), 4-nitrobenzaldehyde (98%), 4-chlorobenzaldehyde
(97), heptanal (95%), 4-tert-butylbenzaldehyde (97%), 4-(dimethylamino)benzaldehyde
(98%), 4-hydroxybenzaldehyde (98%), 4-carboxybenzaldehyde (97%), 1,2-phenylenediamine
(99.5%) were purchased from Sigma-Aldrich. (E)-Cinnamaldehyde (>95), 4-tolualdehyde
(97%), 2-pyridinecarboxaldehyde (98%), 1-pyrenecarboxaldehyde (99%) were purchased
from Acros Organics. Sulfanilic acid (99%) was purchased from Fluorochem. Benzaldehyde
(>98%), ethyl acetate (>99%), hexane (>99%), dichloromethane (>99.5%), dietyhlether
(>99.5%), methanol (>99%), ethanol (99.8%) were purchased from Avantor Performance
Materials Poland S.A. All reagents were used as received without purification.
Thin layer chromatography (TLC; silica gel 60 matrix) plates and preparative thin layer
chromatography (PTLC; silica gel 60 matrix, layer thickness 2000 µm) plates were purchased
from Merck.
Sonication was performed using a Bandelin Sonorex RK 100 H ultrasonic probe with a
temperature control (ultrasonic peak output/HF power: 320W/80W; 35 kHz).
Thermogravimetric analysis (TGA) was performed with a TA Q-50 instrument under
nitrogen atmosphere with the heating rate of 10 °Cmin-1.
Fourier transformation infrared (FT-IR) spectra were recorded in a transmission mode
with a Thermo Scientific Nicolet iS5 spectrometer with the spectral resolution of 4cm-1 The
.
samples were mixed with KBr and pressed in a form of pellets.
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1H NMR and C NMR spectra were recorded on a Varian NMR System spectrometer
(500 MHz, 125 MHz) in DMSO-d6 with calibration on the residual peak 2.50 ppm and 39.5
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ppm, for H NMR and C NMR, respectively. The MestRe-C 2.0 software was used for the
simulation of NMR spectra (MestRe-C NMR Data Processing Made Easy 4.9.9.6, 1996–
2006, courtesy F.J. Sardina, Universidad de Santiago de Compostela, Spain).
2.2
Synthesis of the nanocatalysts
Synthesis of Fe@C (NANOCAT-G1), Fe@C-COOH (NANOCAT-G2) and Fe@C-(CH)2-
COOH (NANOCAT-G3)
Carbon-encapsulated iron nanoparticles (CEINs; Fe@C) were synthesized using the carbon-
arc route. The protocol is described in detail elsewhere.25 Fe@C-COOH were obtained via the
oxidative treatment with nitric acid and sulfuric acid, whilst Fe@C-(CH)2-COOH was
obtained using a radical process with succinic acid acyl peroxide.31 The reaction paths are
presented in Scheme 1a-b. Hereafter, these materials are referred as NANOCAT-G1,
NANOCAT-G2 and NANOCAT-G3, for Fe@C, Fe@C-COOH and Fe@C-(CH)2-COOH,
respectively.
Synthesis of Fe@C-Ph-SO3H (NANOCAT-G4)
Isoamyl nitrite was synthesized using the previously reported method.46 NANOCAT-G4 was
obtained using a diazotization approach. The reaction scheme is presented in Scheme 1c.
Prior to the functionalization, Fe@C (200 mg) were sonicated in o-dichlorobenzene (100 mL)
for 1 h. Sulfanilic acid (2.7712 g, 16.0 mmol) and isoamyl nitrite (2.6 mL, 2.2672 g, 32 mmol,
200 mol%) were then added. The reaction mixture was sonicated at 70 °C under argon
atmosphere for 9 h followed by heating at 70 °C overnight. Then, the reaction mixture was
once again sonicated at 70 °C for 9 h and heated at 70 °C overnight. The suspension
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containing dispersed carbon material was filtrated off and washed with acetone (D2O0I: 1m0.1L03)9/aCn8dDT00677F
ethanol (100 mL). The resultant carbon material was suspended in 100 mL of 1M NaOH (100
mL), sonicated for 1 h, filtrated off and washed with distilled water (100 mL). The as-
obtained solid was suspended in 1M HCl (100 mL), sonicated for 1 h, filtrated off and washed
with distilled water (100 mL). Finally, the as-obtained carbon material was washed with
ethanol (20 mL) and dried in an oven at 45 °C for 24 h. About 225 mg of Fe@C-Ph-SO3H
(NANOCAT-G4) was obtained.
NANOCAT-G4 (Fe@C-Ph-SO3H):
FT-IR (KBr): υ = 1630, 1600, 1496, 1520, 1160, 1125, 1036, 1010, 830, 680, 580 cm-1
TGA (content of introduced organic moiety): 28.0 wt%
Content of SO3H groups (back titration)47: 2.11 mmol/g
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Scheme 1. Structure of Fe@C (NANOCAT-G1) and synthesis of (a) Fe@C-COOH
(NANOCAT-G2), (b) Fe@C-(CH)2-COOH (NANOCAT-G3) and (c) Fe@C-Ph-SO3H
(NANOCAT-G4)
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2.3 General procedure for the synthesis of benzimidazoles
The reaction path is presented in Scheme 2. 1,2-Phenylenediamine (43.3 mg, 0.4 mmol) was
dissolved in ethanol (7 mL). The appropriate amount of the nanocatalyst (G1, 2, 3 or 4) was
then added and the mixture was sonicated at 40 °C for 15 min. Aldehyde (1-13, 0.4 mmol,
100 mol%) was subsequentially added and the reaction mixture was sonicated at 40 °C for
appropriate time (see data in Table 4). The progress of the reaction was monitored by TLC
using an appropriate solvent system as the mobile phase. After the completion of the reaction,
the catalyst was separated from the reaction mixture either using a magnet. The supernatant
was separated from the catalyst. The carbon material was then sonicated with ethanol (7 mL)
for 15 min and once again the supernatant was collected. In case of 2-(1-
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pyrenyl)benzimidazole (7) washing of the catalyst was repeated twice. The collected organic
DOI: 10.1039/C8DT00677F
layers were concentrated using a rotary evaporator to afford a crude product. The pure product
was isolated by means of PTLC with the same solvent system as used in TLC. The reaction
yield was calculated based on the mass of the purified product. The structure of the product
was confirmed by H NMR, 13C NMR and FT-IR. The spectral data for the obtained
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benzimidazoles are consistent with the literature. The structures of all obtained
benzimidazoles (1-13) are presented in Table 4.
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Scheme 2. Synthesis of benzimidazoles using the herein developed nanocatalysts. Structures
of NANOCAT-G1-4 are presented in scheme 1. For the structure of the benzimidazoles
obtained and reactions conditions, see Table 4
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2-phenylbenzimidazole (1)
white solid, yield: 95.0%
Rf (30%v/v AcOEt/hex) = 0.76
1H NMR (DMSO-d6, 500 MHz): δH 12.92 (bs, 1H), 8.20-8.18 (m, 2H), 7.67-7.49 (m, 5H),
7.21-7.20 (m, 2H)
13C NMR (DMSO-d6, 125 MHz): δC 151.2, 143.8, 135.0, 130.1, 129.8, 126.4, 122.5, 121.6,
118.8, 111.3
FT-IR (KBr): υ = 2920, 2780, 2695, 1595, 1535, 1435, 1395, 1275, 1120, 960, 750, 700 cm-1
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2-(4-tolyl)benzimidazole (2)
light yellow solid, yield: 96.5%
Rf (30%v/v AcOEt/hex) = 0.46
1H NMR (DMSO-d6, 500 MHz): δH 12.81 (bs, 1H), 8.06-8.06 (m, 2H), 7.64-7.51 (m, 2H),
7.36-7.35 (m, 2H), 7.19-7.18 (m, 2H), 2.38 (s, 3H)
13C NMR (DMSO-d6, 125 MHz): δC 151.2, 143.6, 139.3, 134.7, 129.3, 126.2, 121.1, 121.3,
118.5, 111.0, 20.8
FT-IR (KBr): υ = 2920, 2860, 2750, 1585, 1495, 1430, 1280, 1220, 960, 820, 750, 540 cm-1
2-(4-tert-butylphenyl)benzimidazole (3)
light yellow solid, yield: 95.0%
Rf (30%v/v AcOEt/hex) = 0.59
1H NMR (DMSO-d6, 500 MHz): δH 12.81 (bs, 1H), 8.10-8.08 (m, 2H), 7.64-7.49 (m, 4H),
7.17-7.16 (m, 2H), 1.31 (s, 9H)
13C NMR (DMSO-d6, 125 MHz): δC 152.5, 151.2, 143.8, 134.9, 127.4, 126.2, 125.7, 122.3,
121.5, 118.7, 111.1, 34.6, 30.9
FT-IR (KBr): υ = 2935, 2785, 2700, 1620, 1565, 1420, 1330, 1270, 1220, 1000, 820, 750, 490
cm-1
2-(4-chlorophenyl)benzimidazole (4)
light yellow solid, yield: 96.5%
Rf (30%v/v AcOEt/hex) = 0.51
1H NMR (DMSO-d6, 500 MHz): δH 13.00 (bs, 1H), 8.20-8.18 (m, 2H), 7.64-7.60 (m, 2H),
7.22-7.21 (m, 2H)
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C NMR (DMSO-d6, 125 MHz): δC 150.1, 134.5, 130.8, 129.7, 129.0, 128.8, 1D2O8I:.110,.1103292/C.87D,T00677F
121.8, 118.9
FT-IR (KBr): υ = 2925, 2855, 2740, 1605, 1430, 1325, 1270, 1090, 965, 835, 745, 540 cm-1
2-(4-hydroxyphenyl)benzimidazole (5)
light orange solid, yield: 96.5%
Rf (30%v/v hex/AcOEt) = 0.53
1H NMR (DMSO-d6, 500 MHz): δH 12.64 (bs, 1H), 9.98 (bs, 1H), 8.01-8.00 (m, 2H), 7.59-
7.48 (m, 2H), 7.15-7.14 (m, 2H), 6.92-6.91 (m, 2H)
13C NMR (DMSO-d6, 125 MHz): δC 158.9, 151.6, 135.7, 134.8, 134.7, 130.4, 127.9, 121.6,
121.0, 118.1, 115.5, 110.7
FT-IR (KBr): υ = 3085, 2925, 2810, 2680, 1620, 1460, 1390, 1275, 1180, 1020, 840, 755, 520
cm-1
2-(4-dimethylaminophenyl)benzimidazole (6)
pale yellow solid, yield: 94.5%
Rf (30%v/v hex/AcOEt) = 0.65
1H NMR (DMSO-d6, 500 MHz): δH 12.53 (bs, 1H), 8.01-7.99 (m, 2H), 7.55-7.45 (m, 2H),
7.13-7.11 (m, 2H), 6.84-6.82 (m, 2H), 2.99 (s, 6H)
13C NMR (DMSO-d6, 125 MHz): δC 152.2, 151.2, 129.8, 127.5, 121.4, 121.2, 121.1, 118.0,
117.4, 112.4, 111.8, 110.6, 41.1
FT-IR (KBr): υ = 2920, 2860, 2785, 1610, 1500, 1435, 1370, 1280, 1090, 940, 745, 540 cm-1
2-(1-pyrenyl)benzimidazole (7)
yellow solid, yield: 95.0%
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Rf (30%v/v AcOEt/hex) = 0.49
1H NMR (DMSO-d6, 500 MHz): δH 13.12 (bs, 1H), 9.50-9.49 (m, 2H), 8.56-8.49 (m, 2H),
8.39-8.27 (m, 5H), 8.16-8.10 (m, 2H), 7.86-7.84 (m, 1H), 7.65-7.63 (m, 1H), 7.31-7.28 (m,
2H)
13C NMR (DMSO-d6, 125 MHz): δC 151.1, 144.1, 134.7, 131.6, 130.8, 130.3, 128.6, 128.3,
127.3, 126.6, 125.9, 125.6, 124.8, 124.3, 123.7, 122.7, 121.7, 121.6, 119.1, 111.4
FT-IR (KBr): υ = 3020, 2910, 2845, 1580, 1545, 1425, 1360, 1270, 955, 840, 745 cm-1
2-(ferrocenyl)benzimidazole (8)
orange solid, yield: 92.5%
Rf (50%v/v DCM/ether) = 0.76
1H NMR (DMSO-d6, 500 MHz): δH 12.36 (bs, 1H), 7.54-7.44 (m, 2H), 7.13-7.12 (m, 2H),
5.04 (s, 2H), 4.47 (s, 2H), 4.10 (s, 5H)
13C NMR (DMSO-d6, 125 MHz): δC 152.7, 143.8,134.5, 121.3, 117.7, 110.3, 74.1, 69.1, 67.1
FT-IR (KBr): υ = 2935, 2800, 2695, 1615, 1565, 1425, 1335, 1270, 1215, 1010, 820, 755, 475
cm-1
2-(4-nitrophenyl)benzimidazole (9)
yellow solid, yield: 94.5%
Rf (30%v/v AcOEt/hex) = 0.42
1H NMR (DMSO-d6, 500 MHz): δH 13.29 (bs, 1H), 8.41-8.40 (m, 4H), 7.66-7.64 (m, 2H),
7.27-7.26 (m, 2H)
13C NMR (DMSO-d6, 125 MHz): δC 149.0, 147.8, 136.0, 127.4, 124.3, 123.0, 122.9, 122.8
FT-IR (KBr): υ = 2920, 2860, 2735, 1605, 1515, 1430, 1340, 1275, 1100, 970, 860, 745 cm-1
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2-(4-carboxyphenyl)benzimidazole (10)
yellow solid, yield: 95.0%
Rf (10%v/v MeOH/DCM) = 0.50
1H NMR (DMSO-d6, 500 MHz): δH 13.09 (bs, 2H), 8.30-8.29 (m, 2H), 8.11-8.10 (m, 2H),
7.64-7.62 (m, 2H), 7.24-7.23 (m, 2H)
13C NMR (DMSO-d6, 125 MHz): δC 166.9, 150.2, 133.8, 131.8, 129.9, 129.3, 126.4, 122.7,
122.5, 122.3, 121,0
FT-IR (KBr): υ = 3180, 2920, 2850, 1695, 1615, 1435, 1230, 1125, 1030, 860, 755, 690 cm-1
2-(2-pyridyl)benzimidazole (11)
yellow solid, yield: 95.5%
Rf (10%v/v MeOH/DCM) = 0.70
1H NMR (DMSO-d6, 500 MHz): δH 13.09 (bs, 1H), 8.74-8.72 (m, 1H), 8.34-8.32 (m, 1H),
8.01-7.98 (m, 1H), 7.71-7.70 (m, 1H), 7.55-7.51 (m, 2H), 7.26-7.20 (m, 2H)
13C NMR (DMSO-d6, 125 MHz): δC 150.7, 149.3, 148.5, 143.8, 137.5, 134.9,124.6, 123.1,
121.8, 121.4, 119.2, 112.0
FT-IR (KBr): υ = 3055, 2960, 2845, 1595, 1450, 1405, 1305, 1280, 1125, 1000, 745, 710 cm-1
(E)-2-styrylbenzimidazole (12)
light yellow solid, yield: 96.5%
Rf (40%v/v AcOEt/hex) = 0.62
1H NMR (DMSO-d6, 500 MHz): δH 12.62 (bs, 1H), 7.68-7.65 (m, 4H), 7.45-7.42 (m, 3H),
7.37-7.34 (m, 2H), 7.24 (m, 1H), 7.21 (m, 1H)
13C NMR (DMSO-d6, 125 MHz): δC 150.09, 144.0, 135.7, 134.2, 128.9, 128.8, 127.0, 122.5,
121.5, 118.6, 118.5, 117.7, 111.0
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FT-IR (KBr): υ = 2920, 2730, 1630, 1520, 1410, 1275, 1230, 1030, 960, 745, 515 DcOmI:-110.1039/C8DT00677F
2-hexylbenzimidazole (13)
light yellow solid, yield: 97.0%
Rf (30%v/v AcOEt/hex) = 0.43
1H NMR (DMSO-d6, 500 MHz): δH 12.14 (bs, 1H), 7.45-7.43 (m, 2H), 7.10-7.08 (m, 2H),
2.80-2.77 (t, J = 7.6 Hz, 2H), 1.75-1.73 (m, 2H), 1.28-1.27 (m, 6H), 0.86-0.83 (t, J = 7.1 Hz,
3H)
13C NMR (DMSO-d6, 125 MHz): δC 155.1, 121.0, 120.9, 30.9, 28.5, 28.3, 27.5, 21.9
FT-IR (KBr): υ = 2930, 2850, 2735, 2680, 1540, 1430, 1270, 1020, 935, 740 cm-1
2.4 Recyclability study on NANOCAT-G4
After washing with ethanol, NANOCAT-G4 used in the synthesis of benzimidazoles (2.3)
was dried in 45 °C for 24 h. The resultant carbon material was subjected to the reaction
between 1,2-phenylenediamine (0.4 mmol) and benzaldehyde (0.4 mmol). The procedure was
the same as described in subsection 2.3. After the reaction, NANOCAT-G4 was washed with
ethanol (2x7mL), dried in 45 °C for 24 h and once again subjected to the condensation of 1,2-
phenylenediamine (0.4 mmol) and benzaldehyde (0.4 mmol). The evaluated yield of the
reaction indicated the catalytic activity of the as-regenerated material.
3
Results and discussion
The herein developed method for the synthesis of NANOCAT-G4 employed the one-step
diazotization approach (Scheme 1c). The method for the synthesis of NANOCAT-G4
employed the diazonium salt generated from sulfanilic acid. Afterwards, the radical reaction
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between graphene layer in CEINs and aryl radical took place. In order to provide the highest
DOI: 10.1039/C8DT00677F
yield possible, the process was conducted in organic solvent (o-dichlorobenzene). The
diazonium salt was generated using the isoamyl nitrite. The success of the functionalization
was confirmed by FT-IR (Fig. S1) and TGA (Fig. S2). The content of the introduced organic
moiety was found to be very high (28.0 wt%) and this value corresponded to high surface
acidity (2.11 mmol/g). Importantly, the evaluated surface acidity is comparable to the surface
acidity of commercial cationic-exchange resigns, e.g. Amberlite (2.30-2.50 mmol/g).
3.1 Study on the catalytic activity of the obtained materials
The reaction conditions were optimized during the studies of condensation of
benzaldehyde and 1,2-phenylenediamine, which was chosen as the representative process.
The reaction was carried out in ethanol at 40 °C under ultrasonic irradiation. The obtained
results are summarized in Table 1. The application of each nanocatalyst (NANOCAT-G1-4)
enabled to increase the yield of the process. For the catalyst-free reaction, the yield was found
to be 28.0% only. NANOCAT-G4 was found to be the most efficient and the most selective
nanocatalyst. 2-Phenylbenzimidazole (1) was obtained with a yield of 95.0% in a relative
short time (50 min) and 20 mg of NANOCAT-G4 was used only. It should be noted that such
high yield of this process was not observed even when higher mass of other nanocatalyst
(NANOCAT-G1-3) was applied. The synthesis yield changed in the following order:
NANOCAT-G1 < NANOCAT-G2 < NANOCAT-G3 < NANOCAT-G4. This order can be
reasoned when one compares the differences in the content of acidic groups (Table 2). The
pristine CEINs (NANOCAT-G1) have very low total surface acidity (0.02 mmol/g) which
originates from the oxygen sites that might be partially located on the graphene-like
coatings.31 In practice, for NANOCAT-G1 the main feature that induced its catalytic activity
was the oxidizing and adsorption properties, as like for other carbon materials. The content of
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surface carboxylic groups for NANOCAT-G2 and NANOCAT-G3 was found to be 0.53
DOI: 10.1039/C8DT00677F
mmol/g and 1.42 mmol/g, respectively.31 These values are lower in comparison to the
sulfonated CEINs (NANOCAT-G4), for which the surface acidity was found to be the
highest (2.11 mmol/g). Additionally, sulphonyl groups are known to be more acidic in
comparison to carboxylic groups. The catalytic activity of NANOCAT-G4 was also
compared with the previously reported both carbon-based and other types of heterogeneous
catalysts applied for the studied condensation of benzaldehyde and 1,2-phenyleniediamine
(Table 3). It was concluded that NANOCAT-G4 provided the highest synthesis yield.
Moreover, the herein developed magnetic nanocatalyst can be effectively and easily separated
from the reaction mixture applying a permanent magnet. This feature is visualized in Fig. 1.
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Table 1. Synthesis of 2-phenylbenzimidazole using different nanocatalysts (NANDOOCI: 1A0.1T03-9G/C18D-T00677F
4). Reaction conditions: benzaldehyde (0.4 mmol), 1,2-phenylenediamine (0.4 mmol), ethanol
(7 mL), temperature: 40 °C (ultrasonic irradiation), time: 50 min. See subsection 2.3 for
experimental details
Entry
1
Nanocatalysta
none
Nanocatalyst loading (mg) Yieldb (%)
-
28.0
54.0
58.0
61.0
60.5
63.0
68.5
72.0
73.0
72.0
81.0
82.0
81.0
86.0
95.0
94.5
94.0
2
NANOCAT-G1
NANOCAT-G1
NANOCAT-G1
NANOCAT-G1
NANOCAT-G2
NANOCAT-G2
NANOCAT-G2
NANOCAT-G2
NANOCAT-G3
NANOCAT-G3
NANOCAT-G3
NANOCAT-G3
NANOCAT-G4
NANOCAT-G4
NANOCAT-G4
NANOCAT-G4
10
20
30
40
10
20
30
40
10
20
30
40
10
20
30
40
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
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369
a for the structure of the nanocatalysts see scheme 1
b isolated yields
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Table 2. Content of surface acidic groups in NANOCAT-G1-4
Carbon material
Type of acidic groups
Content of given acidic
groups (mmol/g)
N.A.a
0.02
NANOCAT-G1
(Fe@C)
COOH
COOH
SO3H
0.53
1.42
2.11
NANOCAT-G2
(Fe@C-COOH)
NANOCAT-G3
(Fe@C-(CH2)2-COOH)
NANOCAT-G4
(Fe@C-Ph-SO3H)
a see description in text
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373
374
375
Table 3. Comparison of the catalytic activity of NANOCAT-G4 with previously reported
heterogeneous catalysts for the synthesis of 2-phenylbenzimidazole
Entry Catalyst
Reaction time Yield (%)
Reference
1
2
3
4
NANOCAT-G4
50 min
70 h
95.0
86.0
40.0
67.0
This work
41
GO
40
40
Graphite/p-TsOH
40 min
4 h
Graphite/N,N-
dimethylaniline
42
41
5
6
Fe3O4 nanoparticles
45 min
80.0
85.0
Dowex-20 (ion-exchange12 h
resign)
48
41
41
7
8
9
Indion-190 (ion-exchange4 h
resign)
89.0
53.0
80.0
Indion-652 (ion-exchange4 h
resign)
Amberlite IR-120 (ion-4 h
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exchange resign)
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Fig. 1. (a) Dispersion of NANOCAT-G4 (Fe@C-Ph-SO3H) in ethanol (300 μg/mL) after
sonication for 5 min, (b) magnetic response of NANOCAT-G4 in the presence of
neodymium magnet
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391
The obtained results for the condensation of benzaldehyde and 1,2-phenylenediamine
using NANOCAT-G4 as the highly efficient magnetic nanocatalyst, prompted us to expand a
list of aldehydes that may be subjected to the process. Both substituted aromatic aldehydes, as
well as aliphatic and metallocene aldehydes were included in our study. The results are
summarized in Table 4. For each entry the reaction yield was very high, i.e. between 92.5%
and 97.0%. NMR and FT-IR measurements proved the formation of pure products (Figs. S3-
S41 in ESI Importantly, washing of the catalyst NANOCAT-G4 with ethanol after the
reaction enabled to clean its surface and desorbed the product of the reaction. It should be also
noted that the reaction yield in the case of synthesis of 2-(1-pyrenyl)benzimidazole (7; entry
7, Table 4, reaction time 45 min) was increased when the catalyst was washed with ethanol
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after the reaction two more times (see details in the experimental section). Such phenomenon
DOI: 10.1039/C8DT00677F
was most probably associated with the well-known fact, i.e. relatively strong adsorption of
pyrene compounds onto carbon materials. Single washing of the catalyst was therefore
sufficient for all the products (1-13) except 2-(1-pyrenyl)benzimidazole (7). The reaction time
varied between 25 min and 65 min depending on the aldehyde used. For aromatic aldehydes
substituted with electron donating groups (entries 2-6, Table 4) the reactions proceeded in a
shorter time (30-40 min, 50 min in case of 6) in comparison to the synthesis of 2-
phenylbenzimidazole (1; 50 min). The shortest reaction time was observed in the case of the
synthesis of 2-(ferrocenyl)benzimidazole (8; 25 min). On the other hand, for aromatic
aldehydes which contained electron withdrawing groups (entries 9-11, Table 4), heterocyclic
aldehyde (synthesis of 2-(2-pyridyl)benzimidazole, entry 11, Table 4) and the aliphatic
aldehydes (entries 12-13, Table 4) the reaction time was slightly longer (55-65 min).
Table 4. Synthesis of various benzimidazoles using NANOCAT-G4 as a nanocatalyst.
Reactions conditions: aldehyde (0.4 mmol), 1,2-phenylenediamine (0.4 mmol), NANOCAT-
G4 (20 mg)a, ethanol (7 mL), temperature: 40 °C (ultrasonic irradiation). See subsection 2.3
for experimental details
Entry -R (aldehyde)
Product
Reaction
time (min)
50
Yieldb (%)
1
95.0
2
40
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6
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35
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96.5
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45
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9
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a for the structure of NANOCAT-G4, see scheme 1c
b isolated yields
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3.2 Recyclability study
The catalyst was separated from the reaction mixture, washed with ethanol, dried in oven in
45 °C for 24 h and reused. The catalytic activity of NANOCAT-G4 was then studied in five
cycles after the first reaction (see details in subsection 2.4). The condensation of
benzaldehyde and 1,2-phenylenediamine was selected as the representative process.
NANOCAT-G4 was isolated after each cycle and purified using the above-mentioned
protocol. No significant difference in the reaction yield was observed for five reaction cycles
(Fig. 2). Additionally, no changes in the FT-IR spectra were observed for NANOCAT-G4
between the cycles (Fig. S42).
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Fig. 2. Recyclability study of NANOCAT-G4 for the synthesis of 2-phenylbenzimidazole.
For the reaction scheme, see Table 1.
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3.3 Proposed reaction mechanism
High catalytic activity of NANOCAT-G4 was likely a result of two synergistic interface
effects, namely (i) presence of the acidic groups on the surface of the NANOCAT-G4, and
(ii) physicochemical properties of graphene coating, toward π- π process. As presented in Fig.
3, highly acidic sulfonyl groups protonate the oxygen atom of carbonyl moiety in the aromatic
aldehyde. The electrophilicity of carbonyl carbon atom was then increased, and it facilitated
the activity of the aldehydes toward the reaction with the nucleophile, i.e. 1,2-
phenylenediamine. It is also reasoned that the herein developed process for the synthesis of
benzimidazoles could also be facilitated by the adsorption of the reactants on the surface of
CEINs during the reaction.40
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440
Fig. 3. Proposed reaction mechanism
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4
Conclusions
The catalytic activity of carbon-encapsulated iron nanoparticles (CEINs) and CEINs
functionalized with carboxyl or sulfonyl groups, in the synthesis of benzimidazoles, were
examined. It was found that sulfonated CEINs (NANOCAT-G4) act as the very efficient and
highly selective magnetic nanocatalyst. The reaction yield varied between 92.5% to 97.0%,
whilst reaction time was 25-65 min, depending on the applied aldehyde. Both aliphatic,
aromatic, heterocyclic and metallocene aldehydes were subjected to the reaction with 1,2-
phenylenediamine. The process is simple, and the reaction occurs under mild conditions
(ultrasonic irradiation, 40 °C) The nanocatalyst could be also easily separated from the
reaction mixture using a permanent magnet. The recyclability studies revealed that the
developed nanocatalyst retain its extraordinary activity up to six reaction cycles. By
comparison to the previously reported heterogeneous catalysts for the synthesis of
benzimidazoles, NANOCAT-G4 had the best catalytic performance because of the highest
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reaction yield and the shortest reaction time. Importantly, the developed method DfOoIr: 1t0h.1e039o/nC8eD-T00677F
step synthesis of NANOCAT-G4 is based on the diazotization approach, which is simple and
efficient. For this carbon material both the content of the introduced organic moiety (28.0
wt%) and total acidity (2.11 mmol/g) were remarkably high. This work opens up new avenues
for the development of novel magnetic nanocatalysts, as well as highly selective and efficient
ways for the synthesis of benzimidazoles.
5
Acknowledgments
This work was financially supported by the National Science Center (Poland) through the
grant PRELUDIUM No. 2016/21/N/ST5/00864 and Warsaw University Of Technology.
Authors thanks Dr. G. Z. Żukowska for her assistance in FT-IR experiments.
6
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Page 29 of 29
Dalton Transactions
View Article Online
DOI: 10.1039/C8DT00677F
Various benzimidazoles were obtained applying sulfonated carbon-encapsulated iron
nanoparticles as the nanocatalyst.