A dendritic TiO2-Co(ii) nanocomposite based on the melamine catalyzed one-pot aerobic photocatalytic synthesis of benzimidazoles

Article abstract of DOI:10.1039/c8nj00351c

A novel heterogeneous visible light active nanophotocatalyst was produced via modification of a nanocrystalline TiO₂ surface with a cobalt(ii) complex of melamine dendrimer under ultrasonic agitation, and was characterized by different techniques such as FT-IR, XPS, EDS, ICP-AES, TGA, DRS, UV-Vis, TEM and SEM. A red-shift of the band-edge and a significant reduction of the band-gap (2.7 eV) were revealed by UV-DRS. The as-prepared dendritic nanophotocatalyst significantly promoted the aerobic synthesis of benzimidazole derivatives with biological and pharmaceutical interest by a one-pot three-step process from the photooxidation of alcohols to aldehydes followed by cyclocondensation with 1,2-phenylenediamine with good to excellent yield. This process has the potential to enable a more sustainable benzimidazole synthesis. The combination of photocatalytic and catalytic reactions presented here may help to develop a new strategy towards the development of photocatalysis-based organic synthesis.

Full text of DOI:10.1039/c8nj00351c

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A dendritic TiO2–Co(II) nanocomposite based
on the melamine catalyzed one-pot aerobic
Cite this:DOI: 10.1039/c8nj00351c
photocatalytic synthesis of benzimidazoles†
Ameneh Eskandari, Maasoumeh Jafarpour, * Abdolreza Rezaeifard * and
Mehri Salimi
A novel heterogeneous visible light active nanophotocatalyst was produced via modification of a
nanocrystalline TiO2 surface with a cobalt(II) complex of melamine dendrimer under ultrasonic agitation,
and was characterized by different techniques such as FT-IR, XPS, EDS, ICP-AES, TGA, DRS, UV-Vis, TEM
and SEM. A red-shift of the band-edge and a significant reduction of the band-gap (2.7 eV) were
revealed by UV-DRS. The as-prepared dendritic nanophotocatalyst significantly promoted the aerobic
synthesis of benzimidazole derivatives with biological and pharmaceutical interest by
a one-pot
three-step process from the photooxidation of alcohols to aldehydes followed by cyclocondensation
with 1,2-phenylenediamine with good to excellent yield. This process has the potential to enable a
more sustainable benzimidazole synthesis. The combination of photocatalytic and catalytic reactions
presented here may help to develop a new strategy towards the development of photocatalysis-based
organic synthesis.
Received 20th January 2018,
Accepted 19th March 2018
DOI: 10.1039/c8nj00351c
rsc.li/njc
the efficiency of reactions is still a challenge in modern
catalytic science.12 One of the most critical issues for practical
1. Introduction
Dendrimers are well-defined synthetic polymeric architectures application of photocatalytic processes is the low catalytic
with low polydispersity and controlled surface functionalities activity under visible light irradiation (l 4 400 nm), the main
that include an interior core, branching layers (generations) component of solar irradiance, which is achieved extensively by
and a multivalent shell and make useful building blocks and semiconductor materials such as titanium dioxide.13–17
carriers for molecules and ions at nanoscales.1,2 Melamine-
Nanosized TiO2, one of the most promising photocatalysts,
based dendrimer amines (MDAs) first reported in 2000 by has been the focus of various investigations on the photocatalytic
Simanek and Zhang3 are such ideal dendrimer ligands due to degradation of organic pollutants, dissociation of water, solar
their intensively binding amine sites. Recently, the synthesis energy conversion, and disinfection.18 However, due to its large
and characterization of dendrimers containing 1,3,5-triazine band gap of 3.2 eV, TiO2 can only be excited with a small UV
units have received considerable attention due to their ease of fraction of solar light, making it impractical for use under natural
synthesis, as well as their low cost and the chemoselective sunlight. The high rate of electron–hole recombination also
reactivity of the starting material, 2,4,6-trichlorotriazine.3–8 results in a low quantum yield and poor efficiency of photo-
Moreover, dendrimer-encapsulated metallic particles (DEMs), catalytic reactions. In order to address these concerns, there
due to the more activation of the surface of the metallic have been various attempts to sensitize TiO2 for much larger
particles, can exhibit better catalytic activities compared to visible light regions. One of these investigations has been the
the other types of ligand-stabilized particles.9–12 For separation surface organic modification of TiO2, which was proved to be
of DEMs from the reaction mixture and simple recycling, efficient to make TiO2 active in the visible range.
heterogenizing DEMs by immobilizing them on solid supports
The surface organic modification of TiO2 with dendritic polymers
has received increased attention. Furthermore, the design and can make TiO2 particles disperse better without reducing their
fabrication of novel visible-light-driven catalysts that promote active sites.19
Also, the dendritic polymer modified TiO2 hybrid photo-
catalysts have a large number of functional groups at the
periphery of the dendritic polymers that can be modified with
Catalysis Research Laboratory, Department of Chemistry, Faculty of Science,
University of Birjand, Birjand, 97179-414, Iran. E-mail: mjafarpour@birjand.ac.ir,
visible light absorbed molecules which enables the dendritic
rrezaeifard@birjand.ac.ir
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c8nj00351c polymer modified TiO2 hybrid photocatalysts to exhibit high
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Scheme 1 One-pot aerobic condensation of benzylic alcohols and 1,2-phenylenediamine catalyzed by Co(II)–TD@TiO2.
power conversion efficiency and high molar extinction coefficient Assignments of the products were made by 1H NMR and 13C
in the visible range.20
NMR spectroscopy.
In this line of research, herein, we wish to develop a new
visible light photocatalyst by the surface modification of nano-
crystalline TiO2 with a cobalt melamine dendrimer complex.
The catalyst demonstrated an efficient and environmentally-
friendly method toward the aerobic photooxidation of benzylic
alcohols to aldehydes and then can be coupled with a diamine,
in a second reaction process, where the carbon–nitrogen bond
is oxidized to afford the imidazole cycle (Scheme 1). An addi-
tional advantage of this catalytic system is the facile and
efficient reusability of the solid catalyst.
Reusability of the catalyst
To a mixture of benzyl alcohol (1 mmol), 1,2-phenylenediamine
(1.2 mmol) and Co(II)–TD@TiO2 (2 mol%), was added NHPI
(5 mol%, 0.008 g) and the reaction mixture was stirred under
1 atm. O2 (7–10 mL minÀ1) and visible light at 80 1C for 3 h.
After the completion of the reaction, EtOAc (5 mL) was added to
the mixture and then the Co(II)–TD@TiO2 nanocatalyst (solid
phase) was separated by centrifugation followed by decantation
(3 Â 5 mL EtOAc). The isolated solid phase (Co(II)–TD@TiO2)
was dried under reduced pressure and reused for the next runs.
2. Experimental
3. Results and discussion
3.1. Synthesis and characterization of the Co(II)–TD@TiO2
Note: see the step by step preparation of the Co(II)–TD@TiO2
catalyst in the ESI.†
nanohybrid
In the first step of this investigation, TiO2 nanoparticles of
approximately 18–20 nm diameter were prepared by the PC
sol–gel method.21 The nanoparticles were subsequently reacted
General procedure for the synthesis of benzimidazoles from
benzyl alcohols and o-phenylenediamines
To a mixture of benzylic alcohols (1 mmol), 1,2-phenylene- with 3-aminopropyltrimethoxysilane (APTS) according to the
diamines (1.2 mmol) and Co(II)–TD@TiO2 (2 mol%, 0.001 g), literature22 to produce AP-TiO2 needed for growing dendritic
was added NHPI (N-hydroxyphthalimide) (5 mol%, 0.008 g) and molecules. The reaction of cyanuric chloride (CC) with AP-TiO2
the mixture was stirred under 1 atm. O2 (7–10 mL minÀ1) and was carried out at room temperature to substitute one chlorine
visible light at 80 1C under solvent free conditions for the atom of cyanuric chloride to afford CC1–TiO2, which upon
required time. The progress of the reaction was monitored by reaction with diethylenetriamine, gave G1. The same procedure
TLC (eluent: n-hexane/EtOAc, 1.5 : 1). After the completion of was applied to increase the dendrimer generation, CC2–TiO2
the reaction, the mixture was cooled to room temperature and and G2 (Scheme 2). The G2-dendrimer with NH2 groups on its
the catalyst was separated by filtration and washed with EtOAc outer periphery is suitable for the encapsulation of Co(II). The
(5 mL). The desired product (liquid phase) was extracted by as-prepared Co(II)–TD@TiO2 was analyzed by ICP-AES and the
plate chromatography, and eluted with n-hexane/EtOAc (10/2). Co loading was found to be 1.93 wt% based on Co.
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Scheme 2 Preparation route for Co(II)–TD@TiO2.
The FT-IR spectrum of TiO2 showed major bands at 450– (Fig. S1a, ESI†).23,24 Fig. S1b (ESI†) (AP-TiO2) shows a strong
775 cmÀ1, which are attributed to the stretching vibrations of peak at about 1000–1122 cmÀ1 corresponding to the Si–O bond
the Ti–O groups. The broad peaks at 3400 and 1638 cmÀ1 and a broad O–H stretching band at 3200–3400 cmÀ1 25
.
correspond to the surface adsorbed water and hydroxyl groups The bands at 1614–1630 cmÀ1 (CQN) in the FT-IR spectra of
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CC1–TiO2, G1, CC2 and G2 (Fig. S1c–f, ESI†) confirm the 3.2. One-pot condensation of benzylic alcohols with 1,2-
presence of a triazine fragment on the nano-titanium dioxide. phenylenediamines using the Co(II)–TD@TiO2 catalyst
Also, the characteristic bands of the C–Haliph bond appear
In a one-pot reaction, as a useful reaction condition for the
at 1480 and 2990 cmÀ1. The appearance of a weak band at
synthesis of benzimidazoles, the reaction of benzyl alcohol
416 cmÀ1 was rationalized to the stretching vibrations of the
Co–N bond confirming the complexation of Co with G2-coated
the presence of NHPI was chosen as a model. No product was
TiO2 (Fig. S1g, ESI†).
XPS of Co(II)–TD@TiO2 was performed to detect the elements
Next, the influence of several critical reaction conditions on the
contained in the nanodendrites and their oxidation states
(Fig. S2, ESI†). The binding energies of the particles were calibrated
with the C 1s photoelectron peak as a reference (285 eV). The
(1 mmol) with 1,2-phenylenediamine (1.2 mmol) under air in
obtained in the absence of the catalyst under these conditions.
condensation of benzyl alcohol (1 mmol) with 1,2-phenylene-
diamine (1.2 mmol) such as solvent nature, temperature,
solvent amount, catalyst amount, NHPI amount and oxidant
respective peaks for Ti 2p, Co 2p, N 1s and O 1s confirm the
nature was investigated using the Co(II)–TD@TiO2 under visible
presence of these elements in the title nanocatalyst (Fig. S2a, ESI†).
Fig. S2b–e (ESI†) show the high resolution narrow-scan XPS spectra
light irradiation (Fig. S5, ESI†).
First, several commonly used solvents were used for the reaction
while other reaction conditions were maintained unchanged.
Polar solvents, such as ethanol, methanol, acetonitrile,
ethylacetate (EtOAc), water, acetic acid and a mixed solvent of
of the Ti 2p, Co 2p, N 1s and O 1s peaks of the sample, respectively.
The peak of Ti 2P3/2 is located at B458 eV and that of Ti 2P1/2 is
at 464 eV. Also, the two signals positioned at B782.4 and 800 eV
represent the Co 2P3/2 and Co 2P1/2. So, cobalt is present mainly
H2O and EtOAc, afforded 10–60% yields of the corresponding
as Co(II).26–28 The binding energy at 531 eV is attributed to the
benzimidazole (even no product in AcOH). Fig. S5a (see the
contribution of oxygen (O 1s).26–29 The peak at 399.50 eV was
ESI†) shows excellent yield and selectivity under solvent free
conditions.
It should be noted that the highest catalytic performance of the
Co(II)–TD@TiO2 was observed under bubbling O2/70 1C/heater/
assigned to the nitrogen (N 1s) coordinated to the Co(II) ion in
the dendritic structure.30
Also, the elemental composition of the Co(II)–TD@TiO2 was
clarified by EDS (Fig. S3, ESI†), which was in agreement with
visible light irradiation (Fig. S5b, ESI†).
the XPS results of the nanohybrid.
Also, the efficiency of the reaction was dependent on the
Further characterization of the Co(II)–TD@TiO2 was performed
NHPI amount, so the reaction did not proceed in the absence of
by TGA (Fig. S4, ESI†), TEM and SEM. The TEM and SEM
NHPI under any conditions (Fig. S5d, ESI†). NHPI has been
reported to act as a free radical oxidation catalyst.31,32 Therefore,
images of the nanocatalyst showed well-defined spherical
nanoparticles of the dendritic Co(II)–TD@TiO2 with sizes ranging
a radical mechanism may be suggested for the oxidation steps
of the system using O2, similar to the previous reports in the
from 65–70 nm (Fig. 1).
presence of cobalt complexes.33–39 This was further supported
by a retarded reaction in the presence of radical scavengers such
as 2,6-di-tert-butyl-4-methylphenol under the same conditions.
Furthermore, the reaction was carried out in the presence of
other common oxidants such as air, H2O2, TBHP, UHP and Oxones
under the same conditions. The best results were obtained under
continuous O2 (5–7 mL minÀ1) as the oxidant, 2 mol% Co(II)–
TD@TiO2 and 5 mol% NHPI at 80 1C within 3 h under visible
light irradiation and solvent free conditions.
With the optimized conditions in hand, the photocatalytic
performance of the Co(II)–TD@TiO2 for structurally and elec-
tronically diverse benzylic alcohols and 1,2-phenylenediamines
in the oxidative-coupling reaction was investigated.
As shown in Table 1, benzylic alcohols and 1,2-phenylene-
diamines containing electron-withdrawing groups are inactive
in this catalytic system (entries 3d and 3j).
3.3. Photocatalytic activity
In order to investigate the photocatalytic activity of the Co(II)–
TD@TiO2 on the benzimidazole synthesis, we compared the photo-
catalytic activity of the Co(II)–TD@TiO2 with those of the other nano-
oxometals such as MoO3,21 g-Fe2O3 and TiO221 and their nanocom-
posites such as TD@TiO2, SMNP,40 TD@SMNP, Co(II)–TD@SMNP,
TD@MoO3, and Co(II)–TD@MoO3 in the benzimidazole synthesis
Fig. 1 TEM and SEM images of the Co(II)–TD@TiO2 nanocatalyst.
under the same conditions and the results are depicted in Fig. 2.
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a,b,c
Table 1 Synthesis of benzimidazole derivatives from benzyl alcohols and o-phenylenediamines catalyzed by Co(II)–TD@TiO2
a
Reaction conditions: 1000 : 1200 : 50 : 20 molar ratio for alcohol : 1,2-phenylenediamine : NHPI : catalyst, solvent-free conditions under a contin-
uous stream of O2 (5–7 mL minÀ1) at 80 1C after 3 h. The products were identified by NMR spectroscopy.21 c The reported yields are the yields of
b
the isolated products.
Fig. 3 Dependence of the catalytic activity of the Co(II)–TD@TiO2 for the
Fig. 2 Comparison of the catalytic activity of the Co(II)–TD@TiO2 with
those of the other nanocomposites and nanooxometals in the benzimidazole
synthesis of benzyl alcohol (1 mmol) and 1,2-phenylenediamine (1.2 mmol),
under solvent-free conditions using NHPI (5 mol%) under a continuous stream
of O2 (5–7 mL minÀ1) and visible light at 80 1C after 3 h.
synthesis of benzimidazole on the irradiation wavelength. The numbers
with percentages show the contribution of the light irradiation effect.
Reaction conditions: benzyl alcohol (1 mmol), 1,2-phenylenediamine
(1.2 mmol), NHPI (5 mol%), catalyst (2 mol%), O2 (5–7 mL minÀ1) at
80 1C for 3 h under solvent-free conditions (see lamp models in the ESI†).
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Fig. 4 Diffuse reflectance UV-vis spectra of (a) G2, and (b) Co(II)–TD@TiO2.
The results presented in Fig. 2 show that the catalytic light gave the greater contribution of irradiation to the overall
efficiency of the Co(II)–TD@TiO2 for the condensation of benzyl conversion rate. The results clearly show a light dependence,
alcohol and 1,2-phenylenediamine under the optimized conditions which is due to the photocatalytic effect of the TiO2 core on the
was higher than those of the other catalysts. This observation maybe efficiency of the Co(II)–TD@TiO2 for benzimidazole synthesis.
because of the oxidative activity of the Co(II) centers combined
with the photocatalytic activity of the TiO2 core which also acts along with a tangent drawn for the band gap calculation are shown
as a support. in Fig. 4. The red-shift of the band-edge of the Co(II)–TD@TiO2
The UV-Vis-DR spectra of the as-prepared G2 and Co(II)–TD@TiO2
Fig. 3 shows the relative contributions of light and thermal to wavelengths higher than 400 nm makes it favorable for
processes to the conversion efficiencies. Herein, we calculated photocatalytic applications under visible light radiation. The
the contributions of light irradiation to the conversion efficiency band gap energies of G2 and the Co(II)–TD@TiO2 were calculated
by subtracting the conversion of the reaction in the dark from from these spectra using the Kubelka–Munk formula and Tauc
the overall conversion observed when the system was irradiated, plot.41 The observations revealed a significant reduction in the
with both reactions occurring at identical reaction conditions. It band-gap of the Co(II)–TD@TiO2 nanoparticles (2.69 eV) compared
should be noted that the conversion of the reaction in the dark with those of G2 (3.1 eV) and TiO2 (3.2 eV).42 So, according to the
is regarded as the contribution of the thermal effect.
obtained results, the cobalt dendrimer complex promotes the
When the light sources were visible light (l = 366–400 nm, visible absorption of TiO2, as we can see in Fig. 4, which can
P = 15 W), an LED (l = 505 nm, P = 12 W), sun light (P = 18 W), improve its application under visible light. Thus, the use of the
room light lamps (fluorescent lamp, l = 400–650 nm, P = 40 W)
and UV light (l = 200–290 nm, P = 15 W) the light contributions
were 40, 50, 57, 66, and 70%, respectively. We can see that UV
Fig. 5 Recycling of the catalytic system for the synthesis of 2-
phenylbenimidazole under optimized conditions.
Fig. 6 FT-IR spectra of (a) fresh Co(II)–TD@TiO2 and (b) after 4 reuses.
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Table 2 The comparison of the catalytic activity of the Co(II)–TD@TiO2 nanocatalyst with those of other previously reported catalysts for 2-
phenylbenzimidazole synthesis
Entry
Catalyst
Amount catalyst
Conditions
Time (h)
Conv. (%)
Ref.
a
1
2
3
4
5
6
7
8
Co(II)–TD@TiO2
Pt@TiO2
Au–CeO2
2 mol%
80 mgr
SF/O2 (80 1C)
3
24
24
24
4
5
5
90
89
85
88
90
72
85
72
hn (l 4 300 nm)/N2/(30 1C)
Trifluorotoluene/O2 (90 1C)
Trifluorotoluene/O2 (90 1C)
EtOAc (0–25 1C)
43
44
44
45
46
47
48
0.5 mmol%
0.5 mmol%
2.0 equiv.
1.1 equiv.
0.1 mmol
2.5 mol%
Pd–MgO
T3Pb
IBXc
DMSO (25 1C)
RuCl2(PPh3)3
Ru(PPh3)3(CO)H2
Toluene (10 mL)/Ar (215 1C)
Xantphos (2.5 mol%), crotononitrile (2.2 equiv.),
piperidinium acetate (15 mol%)
KOtBu, 110 1C, diglyme
H2O2 (6 mmol), (70 1C)
8
9
10
[Ir(cod)2,6-DiAmPy(iPr)2]
MoO3–SiO2
1.4 mol%
20 wt%
24
7
85
91
49
50
a
b
c
This work. Propylphosphonic anhydride. 2-Iodoxybenzoic acid.
Co(II)–TD@TiO2 nanoparticles synthesized for visible-light photo- organic products are the strengths of the presented work. Therefore,
catalytic applications can be interesting.
this process has the potential to enable a more sustainable
benzimidazole synthesis. The combination of photocatalytic
and catalytic reactions presented here may help to develop a
new strategy towards the development of photocatalysis-based
organic synthesis.
3.4. Catalyst recovery and reuse
The recovery and reusability of the catalyst are very important
for commercial applications as well as green process considerations.
In this context, after the completion of the reaction, ethyl
acetate as a green solvent was added, and the catalyst was
separated by centrifugation and decantation of the reaction
mixture, dried in a vacuum oven at 40 1C and then reused. No
catalytic activity was observed in the filtrate solution, while the
aerobic 2-phenylbenzimidazole synthesis proceeded well by
using the recovered heterogeneous catalyst. These observations
indicated that the catalyst is stable under the reaction conditions,
and can be recovered and reused (Fig. 5).
Conflicts of interest
There are no conflicts to declare.
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