The document summarizes the development of an iron(II) bipyridine complex grafted to nanoporous graphitic carbon nitride (Fe(bpy)3/npg-C3N4) as a photocatalyst for the oxidative coupling of benzylamines using molecular oxygen and visible light. The photocatalyst showed excellent activity and the benefits of easy recovery and recycling without loss of activity. Characterization methods like SEM, TEM, XRD and XPS confirmed the immobilization of the iron complex on the carbon nitride support without affecting its structure. The photocatalyst exhibited enhanced surface area and visible light absorption compared to the individual components. It represents an efficient and reusable system for the oxidative reaction under

1. Green Chemistry
PAPER
Cite this: Green Chem., 2016, 18,
2514
Received 4th September 2015,
Accepted 22nd December 2015
DOI: 10.1039/c5gc02090e
www.rsc.org/greenchem
A [Fe(bpy)3]2+
grafted graphitic carbon nitride
hybrid for visible light assisted oxidative coupling
of benzylamines under mild reaction conditions
Arvind Kumar,a
Pawan Kumar,a
Chetan Joshi,a
Srikanth Ponnada,b
Abhishek K. Pathak,c
Asgar Ali,a
Bojja Sreedhard
and Suman L. Jain*a
The present paper describes the use of a readily synthesized, environmentally benign, reusable and non-
toxic iron based nanocomposite i.e iron(II) bipyridine grafted to graphitic carbon nitride (Fe(bpy)3/npg-
C3N4) as a photocatalyst, molecular oxygen as an oxidant and a household white LED as a light emitting
source for the oxidative coupling of benzylamines under mild reaction conditions. The developed hetero-
genized homogeneous photocatalyst showed excellent activity with the added benefits of facile recovery
and efficient recycling ability without any significant loss in catalytic activity.
Introduction
Oxidation of amines to the corresponding imine compounds
is an important chemical transformation as imines find exten-
sive applications as synthetic intermediates in the preparation
of several bioactive molecules. Besides, from the stoichio-
metric oxidants such as 2-iodoxybenzoic acid, and N-tert-butyl-
phenylsulfinimidoyl chloride, a number of catalytic systems
based on transition metals mainly ruthenium based catalysts
such as RuCl3, [RuCl2(RCH2NH2)2(PPh3)2], Ru-porphyrin, Ru-
hydroxyapatite, Ru2(OAc)4Cl and Ru/Al2O3 using molecular
oxygen as the sole oxidant have been reported for the oxidation
of amines.1–3
In contrast to these thermochemical methods
which usually require elevated temperatures, photochemical
reactions provide better conversions under ambient tempera-
ture conditions. In addition, for the effective utilization of
solar energy, it is necessary to develop a material that can
work under visible light. In this context, Su and co-workers
reported the use of mesoporous graphite carbon nitride (mpg-
C3N4) as the photocatalyst to activate O2 for the selective oxi-
dation of benzylic alcohols and amines with visible light.4
Although this material exhibited excellent catalytic perform-
ance under visible light irradiation, high oxygen pressure (0.5
MPa) and the use of trifluorotoluene as a solvent make the
developed method of less practical utility. Furukawa et al.5
reported selective oxidation of amines to the corresponding
imines using niobium oxide as a catalyst and molecular
oxygen as an oxidant. Apart from these semiconductor photo-
catalysts, recently, Berlicka et al.6
reported porphyrin- or por-
phycene-mediated photo-oxidation of primary amines to
N-benzylidene benzylamines in excellent conversions under
mild reaction conditions. Although these dyes exhibited excel-
lent catalytic performance under visible light irradiation, the
homogeneous nature and non-recycling ability were the
obvious drawbacks. Transformation of homogeneous metal
complexes to heterogeneous forms via immobilization to
photoactive supports constitutes an effective approach to
provide better photocatalytic activity with the added benefits
of facile recovery and recycling of the catalyst. In addition, the
development of low cost, non-toxic metal based photo-catalysts
is the prime need in present day chemistry. Considering these
issues, we have considered iron metal which is easily available,
relatively non-toxic and can be established as the ideal catalyst
for various applications from both environmental and econ-
omic perspectives.
Recently, metal free semiconductors like polymeric carbon
nitride which can efficiently work under visible light have
emerged to be efficient and environmentally benign photocata-
lysts.7
Immobilization of metal complexes or doping of
different metals such as Ag, Pt, Pd etc. on carbon nitride8,9
enhances its photocatalytic activity. Moreover, the regular
arrangement of nitrogen atoms in the carbon structure not
only increases its electronic and catalytic properties but also
creates electrons and holes after visible light absorption. The
fascinating chemical, thermal, structural and electronic pro-
perties of carbon nitride also make it a promising support for
a
Chemical Sciences Division, CSIR-Indian Institute of Petroleum, Dehradun-248005,
India. E-mail: suman@iip.res.in; Fax: +91-135-2660202; Tel: +91-135-2525788
b
Department of Applied Chemistry, Indian School of Mines, Dhanbad, India
c
Physics and Engineering of Carbon, CSIR-National Physical Laboratory,
New Delhi-110012, India
d
Inorganic and Physical Chemistry Division, CSIR-Indian Institute of Chemical
Technology, Hyderabad-500607, India
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2. immobilization of metal complexes for enhanced photo-
catalytic activity.10
Herein, we report an efficient and reusable iron(II) bipyri-
dine complex11
grafted nanoporous carbon nitride (Fe(bpy)3/
npg-C3N4) photocatalyst for oxidative coupling of benzyl-
amines using molecular oxygen as the oxidant under visible
light at ambient temperature (Scheme 1). The grafting of
[Fe(bpy)3]+2
ions in nanoporous carbon nitride (npg-C3N4) pro-
vides both the benefits i.e. enhanced photo activity owing to
the synergistic effect of both components and facile recovery,
recycling of the catalyst. To the best of our knowledge, this is
the first report on heterogenized molecular photocatalysts for
oxidative coupling of benzylamines under visible light.
Results and discussion
Synthesis and characterization of the photocatalyst
Nanoporous graphitic carbon nitride (npg-C3N4) was syn-
thesized by a known method as reported by Xu et al.12
by
heating dicynamide and thiourea at a programmed tempera-
ture. The synthesized iron(II) complex was immobilized to
nanoporous carbon nitride (npg-C3N4) by taking advantage of
π–π interactions. The presence of nitrogen atoms on the sheets
provides an electron rich surface for the immobilization of the
Fe(bpy)3PF6 complex as shown in Scheme 2.
Scanning electron microscopy was used to explore the
surface morphology of npg-C3N4 and Fe(bpy)3/npg-C3N4
(Fig. 1). The prepared npg-C3N4 showed crumpled and
enfolded thin sheets similar to graphene. The framework of
C3N4 contains nitrogen as a substituted heteroatom having a
similar π-conjugated system as in graphitic planes which is
formed due to the sp2
hybridization between carbon and nitro-
gen atoms (Fig. 1a). For Fe(bpy)3/npg-C3N4 (Fig. 1b), the
crumpled nature of carbon nitride sheets was found to be
increased and elaborated due to the non-covalent π–π inter-
action between the complex molecules and the surface
(Fig. 1b). Moreover, the EDX pattern of Fe(bpy)3/npg-C3N4 indi-
cates the presence of iron in the synthesized photocatalyst
(Fig. 1c and d). SEM elemental mapping shows homogeneous
distribution of iron complex units on the surface of npg-C3N4
in the synthesized Fe(bpy)3/npg-C3N4 composite (Fig. 1e and f).
The sizes and morphologies of the representative samples
are determined by TEM (Fig. 2). TEM images of both samples
at 200 (Fig. 2a and e) and 50 nm (Fig. 2b and f) scale show
crumpled sheets with a nanoporous graphitic structure.
HR-TEM images (Fig. 2c and g) reveal crystallite fringes of
carbon nitride having 0.32 nm d-spacing value, which corres-
ponds to the 002 carbon plane of npg-C3N4. Furthermore, the
SAED pattern (Fig. 2d and h) shows a broad ring due to the
diffraction of the 002 plane; however its lower intensity is prob-
ably due to the amorphous nature of the materials. Based on
the TEM analysis, it can be concluded that the immobilization
step does not affect the morphology; hence before and after
the grafting of the iron complex the morphology of the catalyst
remained intact (Fig. 2a–h).
Fig. 3 shows the FTIR spectra of the iron(II) bipyridyl
complex, npg-C3N4 and composite Fe(bpy)3/npg-C3N4. The
FTIR spectrum of the homogeneous iron(II) complex shows
peaks at 1604 and 1444 cm−1
which can be attributed to the
aromatic C–C and C–N stretching vibration of the bipyridine
ring. The peaks appearing in the range of 900 cm−1
are due to
Scheme 1 Oxidative coupling of benzylamines.
Scheme 2 Synthesis of Fe(bpy)3/npg-C3N4.
Fig. 1 FE-SEM images of (a) npg-C3N4, (b) Fe(bpy)3/npg-C3N4; EDX
pattern of: (c) npg-C3N4; (d) Fe(bpy)3/npg-C3N4 and elemental mapping
for: (e) C; (f) Fe.
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3. the deformation aromatic ring vibration of the bipyridine unit.
The peak at 690 cm−1
was assumed to be due to the Fe–N
stretching mode (Fig. 3a).13
The npg-C3N4 sample reveals a
sharp peak at 815 cm−1
, which corresponds to the character-
istic breathing mode of the triazine units. The band at
1637 cm−1
is attributed to CvN stretching vibration mode,
whereas the bands at 1240, 1315, and 1413 cm−1
are associated
with aromatic C−N stretching. A broad band in the range of
3000–3700 cm−1
is attributed to the stretching mode of –NH2
or due to the N–H group vibrations of carbon nitride
(Fig. 3b).14
After immobilization of the iron complex to
npgC3N4, the FTIR spectrum of the composite Fe(bpy)3/npg-
C3N4 reveals some peaks of the iron complex at 1639, 1417 and
1245 cm−1
which confirm the presence of Fe(bpy)3 units in the
composite.
The crystal structure of the representative samples was
determined by XRD (Fig. 4). The XRD pattern of npg-C3N4
shows a characteristic peak at 27.4° which can be indexed as
the (002) diffraction plane having 0.32 nm interlayer distance.
This is mainly due to the stacking of graphite like conjugated
triazine aromatic sheets and matches well with JCPDS 87-1526
for npg-C3N4 (Fig. 4a).15
Immobilization of the iron complex
on npg-C3N4 does not influence the phase structure of the
carbon nitride; however the intensity of the peak has slightly
decreased which is mainly due to the lower loading of the
complex to the carbon nitride support (Fig. 4b).
In order to analyze the surface chemical properties and to
confirm the immobilization of the iron complex to the npg-
C3N4 support, XPS analyses of npg-C3N4 and Fe(bpy)3/npg-
C3N4 were carried out (Fig. 5). The survey scan of npg-C3N4
shows peaks at 284 and 400 eV due to the presence of C and
N, respectively and a small peak at 537 eV due to the adsorbed
O (Fig. 5a), while for the Fe(bpy)3/npg-C3N4 an additional
signal at 710 eV due to iron is detected (Fig. 5b). The high
resolution XPS spectrum in the C 1s region of both i.e. npg-
C3N4 and Fe(bpy)3/npg-C3N4 shows two characteristic peak
components due to the C–C and NvC–N2 at 284.4 and 288.3 eV
respectively (Fig. 5c and d).16
The XPS spectra in the N 1s
region of npg-C3N4 give two peak components i.e. at 398.1 eV
Fig. 2 TEM and HR-TEM images of npg-C3N4 and Fe(bpy)3/npg-C3N4,
respectively; (a, e) at 200 nm scale bar; (b, f) at 50 nm scale bar; (c, g) at
5 nm scale bar; (d, h) SAED pattern.
Fig. 3 FTIR spectra of: (a) Fe(bpy)3(PF6)2; (b) npg-C3N4 and (c) Fe(bpy)3/
npg-C3N4.
Fig. 4 XRD pattern of: (a) npg-C3N4 and (b) Fe(bpy)3/npg-C3N4.
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4. due to CvN–C and at 400.3 eV the peak is related to the ter-
tiary nitrogen (N–(C)3) atoms (Fig. 5e).17
The immobilization of
the iron complex on npg-C3N4 does not influence the chemical
environment of the nitrogen atoms and the values remain
unchanged. Based on XPS analysis it can be estimated that
there is no covalent interaction between complex units and
carbon nitride (Fig. 5f). Furthermore, two new peaks at 710.1
and 724.2 eV due to the Fe2p3/2 and Fe2p1/2, respectively in the
Fe2p region confirmed that iron is presented in the +2 oxi-
dation state in the synthesized composite (Fig. 5g).18
The surface textural properties of the synthesized samples
are determined by the adsorption and desorption of the nitro-
gen gas on the surface of the material with the help of the BET
theory of multilayer adsorption and desorption (Fig. 6). The
BET surface area (SBET), total pore volume (Vp) and mean pore
diameter (rp) of npg-C3N4 are found to be 6.14 m2
g−1
,
0.15 cm3
g−1
, and 99.38 nm, respectively; however for Fe(bpy)3/
npg-C3N4 these values are found to be 27.5 m2
g−1
, 0.25 cm3
g−1
and 36.54 nm respectively. An increase in the surface area
of the photocatalyst from 6.14 to 27.5 m2
g−1
after immobili-
zation of the iron complex is mainly due to the intercalation of
complex units between the sheets.19
The optical properties of the synthesized samples i.e.
Fe(bpy)3(PF6), npg-C3N4 and Fe(bpy)3/npg-C3N4 are investigated
by UV-Vis spectroscopy. The absorbance spectrum of the
Fe(bpy)3(PF6) photocatalyst as shown in Fig. 7a exhibits an
intense peak at λmax 286 nm due to the interligand π → π* tran-
sition in the bipyridine ligand, whereas a less intense peak
observed at 525 nm originates due to the metal d(π) → π*
MLCT transition (Fig. 6a).20
Furthermore, pure npg-C3N4
shows an absorption spectrum similar to a typical semicon-
ductor absorption spectrum between 200–450 nm originating
due to the charge transfer from the populated valence band of
the nitrogen atom (2p orbitals) to the conduction band of the
carbon atom (2p orbitals) of carbon nitride. The sharp peak at
244 nm is due to the aromatic π → π* transition whereas
another peak at 377 nm appeared due to the nitrogen non-
bonding orbital to the aromatic nonbonding (n → π*)
transition (Fig. 7b). Compared with pure npg-C3N4, the syn-
thesized composite Fe(bpy)3/npg-C3N4 shows a gradual red
shift and a sharp peak at 533 nm due to the iron(II) tris-bipyri-
dine MLCT transition which confirms the successful attach-
ment of the complex to carbon nitride (Fig. 7c).21
The thermal stability of npg-C3N4 and Fe(bpy)3/npg-C3N4 is
determined by thermo gravimetric analysis (Fig. 8). The TGA
graph of npg-C3N4 shows a weight loss between 550 to 720 °C
which can be attributed to the burning of npg-C3N4 (Fig. 8b).22
A similar weight loss pattern in this range is also observed in
the Fe(bpy)3/npg-C3N4 photocatalyst (Fig. 8c). In addition, the
thermogram of Fe(bpy)3/npg-C3N4, exhibits a weight loss in the
Fig. 5 XPS survey scan of: (a) npg-C3N4; (b) Fe(bpy)3/npg-C3N4; (c, d)
high resolution XPS spectra in the C 1s region; (e, f) in the N 1s region of
npg-C3N4 and Fe(bpy)3/npg-C3N4 respectively; (g) in the Fe 2p region of
Fe(bpy)3/npg-C3N4.
Fig. 6 N2 adsorption desorption isotherm and pore size distribution of:
(a) npg-C3N4 and (b) Fe(bpy)3/npg-C3N4.
Fig. 7 UV-Vis absorption spectra of: (a) Fe(bpy)3(PF6)2; (b) npg-C3N4
and (c) Fe(bpy)3/npg-C3N4.
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5. range of 300 to 380 °C which can be assumed to be due to the
loss of bipyridine units of the iron complex from the surface of
carbon nitride (Fig. 8c).
Catalytic activity
To demonstrate the photocatalytic activity of the synthesized
photocatalyst, photo oxidation of benzylamines to the corres-
ponding imines was carried out using molecular oxygen as an
oxidant with acetonitrile as a solvent under visible light
irradiation at room temperature (Scheme 1). At first, the reac-
tion conditions were optimized by choosing benzylamine as
the model substrate. The results of the optimization experi-
ments are summarized in Table 1. Blank reaction was carried
out in the absence of the photocatalyst and no product was
obtained even after 24 h visible light irradiation. Further, to
establish the significant role of visible light, the reactions were
carried out in the dark under otherwise identical conditions.
The reaction in the dark was found to be very slow and
afforded very poor product yield using Fe(bpy)3(PF6)2, npg-
C3N4 and Fe(bpy)3/npg-C3N4 as photocatalysts (Table 1, entry
1). However, in the presence of visible light under identical
conditions, excellent conversion to the corresponding imine
was obtained (entry 3). Further, in order to investigate the
effect of solvent, oxidation of benzylamine was performed in
different solvents such as dichloromethane (DCM), methanol
(MeOH), ethanol (EtOH), tetrahydrofuran (THF), acetonitrile
(CH3CN), and N,N-dimethylformamide (DMF under the
described reaction conditions (Table 1, entries 2–7). Among
the various solvents studied, acetonitrile (CH3CN) was found
to be the best for this transformation (Table 1, entry 3).
Although the heterogeneous Fe(bpy)3/npg-C3N4 photo-
catalyst afforded slightly lower yield than the homogeneous
Fe(bpy)3(PF6)2 one (Table 1, entry 3), the facile recovery and
recycling ability make the heterogeneous catalyst more advan-
tageous from the practical viewpoint. To demonstrate the
scope of the synthesized catalytic system, its photocatalytic
activity was explored for different substituted benzylamines
under optimized reaction conditions using visible light at
room temperature (Table 2). As is shown, all the substrates
containing either electron-donating or electron-withdrawing
groups were selectively and efficiently converted to their corres-
ponding N-benzylidene benzylamines in good to excellent
yields (Table 2, entries 2–6). However, among the various sub-
strates, benzyl amines having electron-donating groups exhibi-
ted higher activity due to the ease of formation of the imine
intermediate (Table 2, entries 2 and 3) and afforded higher
product yield in comparison with the substrates having elec-
tron withdrawing groups (Table 2, entries 4–6).
Next, we evaluated the recyclability of the developed hetero-
geneous catalyst. After the reaction, the photocatalyst was
recovered by simple filtration, washed with acetonitrile, dried
and reused in subsequent experiments for six runs (Fig. 9). No
significant loss was observed in the activity of the recycled
photocatalyst and the product yield remained almost
unchanged even after six recycling experiments, which con-
firmed that the catalyst is highly stable and the true hetero-
geneous nature of Fe(bpy)3/npg-C3N4. Further the iron metal
content of the Fe(bpy)3/npg-C3N4 photocatalyst after six re-
cycling experiments was found to be 0.52 wt% as determined
with ICP.
Although the exact mechanism for the above transform-
ation is not clear at this stage however based on our obser-
vation and previously published reports,23
a plausible
mechanism is shown in Scheme 3. Carbon nitride, having a
distorted π-conjugated system due to the presence of nitrogen
atoms, works as a semiconductor. According to the previous
literature,24
the band gap value of carbon nitride is 2.70 eV
which can induce visible light mediated electron transfer.
However the fast rate of electron and proton recombination
prevents efficient electron transfer to the oxygen atom. Immo-
bilization of the iron complex on the carbon nitride framework
Fig. 8 TGA graph of: (a) Fe(bpy)3(PF6)2; (b) npg-C3N4; (c) Fe(bpy)3/
npg-C3N4.
Table 1 Oxidative coupling of benzylamine under different reaction
conditionsa
Entry Condition
Yieldb
(%)
[Fe(bpy)3](PF6)2] npg-C3N4
Fe(bpy)3/
npg-C3N4
1 Dark CH3CN 6 0 3
2 Light DCM 32 3 24
3 Light CH3CN 98 7 94
4 Light MeOH 74 9 70
5 Light EtOH 72 8 67
6 Light THF 65 7 57
7 Light DMF 67 8 62
a
Reaction conditions: benzylamine 1 mmol; photocatalyst, 10 mol%;
solvent (10 ml), light source: white cold LED λ > 400 nm, time 8 h;
power at a reaction vessel 70 W m−2
. b
Isolated yield.
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6. forms a heterojunction between the npg-C3N4 π-electron sheet
and the metal complex.25
With the absorption of visible light,
the iron complex Fe(II) gets excited and transforms into the
excited triplet state Fe(II)* via intersystem crossing. This
excited triplet state can transfer electrons to the conduction
band of carbon nitride.26
Molecular oxygen abstracts electrons
from the conduction band of carbon nitride and gets trans-
formed into singlet molecular oxygen (1
O2) which further
reacts with benzylamine to form a peroxide intermediate
which eventually releases hydrogen peroxide to form the
imine intermediate. Finally, the imine formed reacts with
another free molecule of benzylamine in the system to yield
the corresponding N-benzylidene benzylamine as depicted in
Scheme 3.
Conclusions
In conclusion, we have demonstrated a novel hybrid Fe(bpy)3/
npg-C3N4 photocatalyst synthesized via grafting of the iron(II)
trisbipyridine complex to the nanoporous graphitic carbon
nitride (npg) support through π–π interactions for visible light
driven oxidative coupling of benzylamines using molecular
oxygen as an environmentally benign oxidant at ambient
temperature. The heterogenization of the iron(II) complex by
supporting it onto the npg-C3N4 support not only enhances
the photocatalytic activity but also makes it easily recoverable
and recyclable for subsequent runs. Furthermore, unlike
ruthenium, rhodium and other expensive metals, the low cost
iron complex is found to be more economical and attractive
for the above transformation giving high to excellent yields of
the products under mild conditions. We believe that the
developed heterogenized homogeneous photocatalyst having
superior activity can be further used to develop visible light
driven more diverse photochemical transformations.
Table 2 Light driven oxidative coupling of benzylamines using Fe(bpy)3/npg-C3N4 as the photocatalysta
Entry Reactants Products T/h Yieldb
(%) TOF/h−1
1. 8 92 11.5
2. 7 94 13.4
3. 7.5 91 12.1
4. 8.5 86 10.1
5. 8.5 81 9.5
6. 10 85 8.5
a
Reaction conditions; amine compound, 1 mmol; photocatalyst, 10 mol%; in 10 ml acetonitrile under visible light irradiation with a white cold
LED λ > 400 nm, power at a reaction vessel 70 W m−2
. b
Isolated yield.
Fig. 9 Results of recycling experiments.
Scheme 3 Plausible mechanism of oxidative coupling of benzylamine
using the Fe(bpy)3/npg-C3N4 photocatalyst.
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7. Experimental
Materials
Iron(II) chloride (98%), 2,2′-bipyridine (99%), ammonium hexa-
fluorophosphate (99.9%), dicyanamide (99%), thiourea (99%)
and organic amines were purchased from Aldrich and used as
received. All other chemicals were of A.R. grade and used
without further purification.
Techniques used
Rough surface morphology of materials was determined with
the help of field emission scanning electron microscopy by
using an FE-SEM (Jeol Model JSM-6340F). Ultrafine surface
morphologies of the obtained samples were determined by
high resolution transmission electron microscopy on an FEI-
Tecnai G2
Twin TEM operating at an acceleration voltage of
200 kV. For the sample preparation a very dilute aqueous sus-
pension of the material was deposited on the carbon coated
TEM grid. FT-IR spectra of the compounds were collected on
a Perkin-Elmer spectrum RX-1 IR spectrophotometer having a
potassium bromide window. The XRD pattern for determining
the phase purity and crystallinity of the materials was carried
out on a Bruker D8 Advance diffractometer at 40 kV and
40 mA with Cu Kα radiation (λ = 0.15418 nm). UV-Vis absorp-
tion spectra of the iron(II) bipyridine complex in acetonitrile
and solid UV of other samples were recorded on a Perkin
Elmer lambda-19 UV-VIS-NIR spectrophotometer using a
10 mm quartz cell, using BaSO4 as the reference. Surface pro-
perties like BET surface area (SBET), BJH porosity, mean pore
diameter etc. of samples were examined by using the N2
adsorption–desorption isotherm at 77 K by using a VP, Micro-
meritics ASAP2010. The thermal degradation pattern of the
synthesized materials was determined by thermogravimetric
analysis (TGA) using a thermal analyzer TA-SDT Q-600. The
analysis was carried out in the temperature range of 40 to
800 °C under nitrogen flow with a 10 °C min−1
heating rate.
1
H NMR and 13
C NMR of the iron complex and reaction pro-
ducts were collected at 500 MHz by using a Bruker Advance-II
500 MHz instrument. For determining the metal content of
material, ICP-AES analysis of samples was performed by
using an inductively coupled plasma atomic emission spectro-
meter (ICP-AES, DRE, PS-3000UV, Leeman Labs Inc., USA).
Samples for ICP-AES were made by digesting a calculated
amount of the sample with nitric acid followed by filtration
and diluting it up to 10 mL volume by adding deionized
water.
Synthesis of Fe(II)(bpy)3(PF6)2 complex.11
The required iron(II)
tris bipyridine complex was prepared by following the litera-
ture procedure. In a typical synthesis, iron(II) chloride
(0.80 mmol, 0.1 g) was dissolved in a minimum amount of
water and a separate solution of bipyridine (2.56 mmol, 0.4 g)
was made in a minimum amount of ethanol. Both the solu-
tions were mixed together by stirring followed by the addition
of an aqueous solution of ammonium hexafluorophosphate to
obtain a deep red color precipitate of the Fe(II)(bpy)3(PF6)2
complex.
Synthesis of nanoporous carbon nitride (npg-C3N4).12
For
the synthesis of npg-C3N4, dicyandiamide (1 g) and thiourea
(4 g) were ball milled for 50 min at 300 rpm. The obtained
mixture was heated in a muffle furnace by a programmed
heating rate from room temperature to 300 °C, the heating rate
was 8 °C min−1
; from 300 to 500 °C, the heating rate was 2 °C
min−1
; from 500 to 550 °C, the heating rate was 1 °C min−1
;
finally it was heated at 550 °C for 4 h.
Synthesis of Fe(bpy)3/npg-C3N4 .27
The Fe(II)(bpy)3(PF6)2
complex (25 mg) was dissolved in 10 mL of THF and then 1 g
of nanoporous carbon nitride was added to make a suspen-
sion. The resulting suspension was stirred at room tempera-
ture until the solvent was evaporated. Thus the obtained solid
was heated at 373 K for one hour to yield the Fe(bpy)3/npg-
C3N4 photocatalyst. The iron loading in the synthesized photo-
catalyst was found to be 0.56 wt% (6.8 μmol per g cat) as deter-
mined by ICP-AES analysis.
Photocatalytic oxidative coupling of benzylamines
The photocatalytic activity of the synthesized Fe(bpy)3(PF6)2,
npg-C3N4 and Fe(bpy)3/npg-C3N4 photocatalysts was evaluated
by using a 20 watt white cold LED light (model no.
HP-FL-20W-F-Hope LED Opto-Electronic Co., Ltd, λ> 400 nm).
In a 50 mL round bottomed flask 1 mmol of amine, photo-
catalyst (10 mol%) and 10 mL of acetonitrile were added. The
obtained reaction mixture was irradiated under visible light by
stirring for 12 h in the presence of molecular oxygen. The pro-
gress of the reaction was monitored by TLC. After completion
of the reaction, the photocatalyst was removed by filtration
and the solvent was removed under reduced pressure. The
crude product was further purified by column chromatography
on silica gel by using ethyl acetate and hexane (9 : 1) as the
eluent.
Acknowledgements
The authors are thankful to Director IIP for granting per-
mission to publish these results. AK and PK are thankful to
UGC and CSIR, New Delhi, respectively for providing research
fellowships. CJ kindly acknowledges CSIR, New Delhi for pro-
viding technical HR under XII five year projects. The analytical
department is kindly acknowledged for the analysis of
samples.
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