Carbon Nitride Grafted Cobalt Complex (Co@npg-C3N4) for Visible LightAssisted Esterification of Aldehydes

z Sustainable Chemistry
Carbon Nitride Grafted Cobalt Complex (Co@npg-C3N4) for
Visible LightÀAssisted Esterification of Aldehydes
Anurag Kumar,[a, b]
Pawan Kumar,[a, b]
Abhishek Kumar Pathak,[c]
Appala Naidu Chokkapu,[d]
and Suman L. Jain*[a]
Azide containing bipyridine complex of cobalt was grafted to
the propargylated nanoporous graphitic carbon nitride (npg-C3
N4) via click reaction to obtain heterogenized photocatalyst
which could efficiently provide direct esterification of aldehydes
under visible light irradiation at room temperature. The use of
click reaction as grafting strategy provided covalent attachment
of the cobalt complex to support which not only provided
higher loading but also precluded the leaching. Furthermore,
the presence of carbon nitride support exhibited synergistic
effect to enhance the reaction rate. In addition, the milder basic
nature of nitrogen containing graphitic support provided
efficient ester synthesis without the need for an external base.
The synthesized photocatalyst was found to be quite robust
which could easily be recovered and reused several times
without significantly losing activity.
Introduction
Photocatalysis is a science of employing catalyst for speeding
up the chemical reactions utilizing sunlight.[1]
The use of
sunlight owing to its abundance, environmentally benign
nature and sustainability is gaining considerable interest in
recent decades.[2,3]
More particularly, the visible light driven
photocatalytic reactions has become an ideal approach for
achieving green and economically viable chemical synthesis.[4]
In this regard, metal complexes which involve in single-
electron-transfer (SET) processes with organic substrates upon
photoexcitation with visible light have been well documented
in the literature.[5,6]
So far, the most commonly employed visible
light photocatalysts are polypyridyl complexes of ruthenium
and iridium that have extensively used for plethora of organic
transformations.[7]
However, homogeneous nature, limited accessibility and
high cost of these photoredox catalysts make the developed
methodologies of less practical importance.[8]
Thus, the syn-
thesis of low cost metal complexes and their immobilization to
photoactive supports[9]
can solve both the problems of being
non-recyclability and high cost.Although, this research area of
transforming potential homogeneous photocatalysts to hetero-
geneous forms via immobilization to photoactive supports is of
immense futuristic importance but less explored up till now.
Recently, our group has done significant work in the area of
immobilization of homogeneous molecular photocatalysts on
semiconductor supports to transform them in heterogeneous
forms for developing photocatalytic methodologies for organic
transformations.[10]
For example, recently we reported thesyn-
thesis of highly efficient, visible-light active and recyclable TiO2-
immobilized ruthenium polyazine complex for the C-H activa-
tion of tertiary amines.[11]
Nanostructured carbon nitrides (CNs), a class of compounds
having general formula of C3N4 are gaining worldwide interest
to be used in a variety of applications.[12]
Interestingly, CNs are
considered to be green materials as they are composed of
carbon and nitrogen only, and can be prepared from low cost
easily accessible feedstocks. So far, various nanoarchitectural
CNs including one-dimensional nanorods, two dimensional
nanosheets, and three-dimensional mesoporous structures
have been synthesized by using different strategies for
example, hard/soft template, solvo-thermal, exfoliation or
supramolecular synthetic methods.[13]
Owing to the various
fascinating properties such as high surface area, interesting
optical properties and lower band gap graphitic carbon nitrides
have been successfully used in the photocatalytic applica-
tions.[14]
In this regard, Bhunia et al[15]
reported the use of
crystalline carbon nitrides as efficient photocatalyst for hydro-
gen evolution from water splitting. Jun et al[13]
reported macro-
scopic assemblies of low-dimensional carbon nitrides for
enhanced hydrogen evolution. Besides these reports, a signifi-
cant amount of work has been done on the modifications of
the carbon nitride photocatalysts for visible light activity using
various approaches such as dye sensitization, copolymerization,
[a] A. Kumar, Dr. P. Kumar, Dr. S. L. Jain
Chemical Sciences Division
CSIRÀIndian Institute of Petroleum
Mohkampur, DehradunÀ248005 India
Fax: (+)91-135-2660202
E-mail: suman@iip.res.in
[b] A. Kumar, Dr. P. Kumar
Academy of Scientific and Industrial Research
New Delhi India
[c] A. K. Pathak
Analytical Sciences Division
CSIRÀIndian Institute of Petroleum
Mohkampur, Dehradun-248005 India
[d] A. N. Chokkapu
Division of Advanced Materials and Devices
CSIR-National Physical Laboratory
Delhi, India
Supporting information for this article is available on the WWW under
https://doi.org/10.1002/slct.201602031
Full PapersDOI: 10.1002/slct.201602031
3437ChemistrySelect 2017, 2, 3437–3443 2017 Wiley-VCH Verlag GmbH Co. KGaA, Weinheim

hybridization and doping with metal atoms (e. g., Fe, Zn, Cu) or
non-metal atoms (e. g., S, B, P).[16-19]
Recently Sanny et al[20]
reported a bimetallic catalyst on a graphitic carbon nitride
(AgPd@g-C3N4) for conversion of biomass-derived levulinic acid
under visible light. In a subsequent report they described a
photoactive VO@g-C3N4 catalyst for the C-H activation and
selective oxidation of alcohols to the corresponding aldehydes
and ketones.[21–22]
However, immobilization of metal complexes to CN support
via covalent attachment comparatively less explored. Very
recently, we have reported graphitic carbon nitride supported
([Fe(bpy)3]2+
) photocatalyst for oxidative coupling of benzyl
amines under mild reaction conditions.[23]
Esters are important building blocks[24]
which have found
extensive applications in both chemical industry as well as
academic laboratories.[25]
The conventional approach for their
synthesis involves the reaction of activated carboxylic acids or
their acid-catalyzed condensation with alcohols in presence of
acid or basic catalysts.[26]
The direct conversion of aldehydes
into the corresponding esters is known,[27]
but most of these
methods require either stoichiometricamount of heavy metallic
oxidants which generate huge amounts of undesired waste or
thermal catalytic methods using homogenous metal complexes
and metal salts in conjunction with an oxidant.[28]
The direct
conversion of aldehydes to esters using sunlight is rarely known
in the literature.[29-30]
In this context, recently, Zhang et al.[31]
reported the direct photocatalytic conversion of aldehydes to
esters using supported gold nanoparticles (Au/Al2O3) under
visible light irradiation. Sanny et al[32]
reported an efficient
methodology for direct oxidative esterification of alcohol via
photocatalytic C-H activation by using VO@g-C3N4 as photo-
catalyst.
In continuation to our on-going studies on photocatalytic
transformations[33-35]
herein, we report a simple, efficient and
high yielding protocol for photocatalytic conversion of alde-
hydes to esters using cobalt complex immobilized to nano-
porous graphitic carbon nitride(Co@npg-C3N4) synthesized via
click reaction[36]
under visible light at room temperature. The
use of click reaction as grafting strategy offered several
advantages including covalent anchoring of metal complex,
higher catalyst loading, leach-proof catalyst and higher reaction
rates. In addition the milder basic nature of nitrogen containing
graphitic support provided efficient ester synthesis without the
need for an external base.[37]
Results and Discussion
3.1 Synthesis and characterization of catalyst
The required nanoporous graphitic carbon nitride (npg-C3N4)
support was prepared by heating of dicyandiamide and
thiourea at programmed temperature.[38]
The obtained support
was subsequently functionalized with –OH groups by harsh
oxidation with hydrogen peroxide.[39]
The presence of –OH
functionalities at the surface of npg-C3N4 sheets provide sites
for the attachment of propargyl groups.[40]
Before the reaction
with propargyl alcohol, the –OH groups of support were
converted into chloro groups by thionyl chloride, which
subsequently reacted with propargyl alcohol to give propargy-
lated npg-C3N4 (PA@npg-C3N4). The next step involves the
copper catalyzed [3+2] azide-alkyne click reaction between
azide groups of cobalt complex and propargylated npg-C3N4 to
get covalently anchored cobalt complex immobilized npg-C3N4
(Co@npg-C3N4) as depicted in (Scheme 2).
The fine structure of materials was determined with the
help of HR-TEM (Figure 1). The HR-TEM images of npg-C3N4 and
Co@npg-C3N4 at 100 nm scale show rough and crumpled
graphitic sheets like structure (Figure 1a). The image at 10 nm
scale shows crystallite fringes for both samples i.e. npg-C3N4
and Co@npg-C3N4(Figure 1b). Further resolution at 2 nm scale
bar shows that crystallite fringes are corresponded to the 0.32
nm interplaner d-spacing of 002 plane of npg-C3N4 (Figure 1c).
Presence of crystallite fringes with similar interplaner d-spacing
confirms that sheets remain intact during immobilization step.
STEM elemental mapping of Co@npg-C3N4 catalyst shows
that elements such as C, N and Co were evenly distributed in
the composite material. Appearance of all the desired elements
such as C, N, O and Co in EDX pattern of Co@npg-C3N4 confirms
the successful immobilization of cobalt complex to support.
The presence of copper was observed due to the copper
present in carbon coated copper TEM grid.
FTIR spectra of cobalt complex, npg-C3N4, and Co@npg-C3
N4is given in the supporting information (Figure S1). Cobalt
Scheme 1. Esterification of aldehydes with alcohol by using Co@npg-C3N4 as
photocatalyst.
Scheme 2. Synthesis of Co@npg-C3N4 photocatalyst via [3+2] azideÀalkyne
cycloaddition “click reaction”.
Full Papers

complex shows various vibration bands at 1024, 1202, 1474,
1608 cmÀ1
specific to bipyridine ring. The band at 2021 cmÀ1
is
observed due to the vibration of azide ligand present in the
complex (Figure S1a. The FTIR spectrum of as-synthesized
graphitic carbon nitride (npg-C3N4) reveals characteristic peak
at 822 cmÀ1
due to the bending vibration of triazine ring
mode.[41]
The peaks at 1633 cmÀ1
and in the range of 1200–
1600 cmÀ1
are attributed to C=N stretch and C-N stretch of
aromatic rings[42,43]
respectively. The peaks at 3000–3700 cmÀ1
are assumed due to the combined vibration of –OH stretching
mode of moisture and NÀH stretching of uncondensed amine
groups presented on the surface of carbon nitride (Figure S1b).
After the attachment of cobalt complex on carbon nitride
(Co@npg-C3N4), appearance of some new peaks and disappear-
ance of characteristic peak of –N3 at 2021 cmÀ1
confirms the
successful immobilization via click reaction (Figure S1c).
Crystallinity and phase structures of the synthesized
materials were determined by XRD analysis (Figure 3). XRD
diffraction pattern of nanoporous graphitic carbon nitride (npg-
C3N4) revealed an intense broad peak at the 2q value of 27.48,
indexed to 002 planes with 0.32 nm interlayer distance which
matches well with JCPDS card No. 87–1526.[44-46]
This diffraction
was appeared due to the stacked structure of graphite-like
conjugated triazine aromatic sheets (Figure 3a). After covalent
immobilization of cobalt bipyridine complex the intensity of
peak was reduced slightly, this is most likely due to the
intercalation of cobalt complex between the sheets of graphitic
carbon nitride (Figure 3b).[47]
The peak due to cobalt complex
cannot be observed due to the low loading as well as complex
is present in the molecular form.
The surface chemical composition of Co@npg-C3N4 photo-
catalyst was determined by XPS analysis. The primary survey
scan of Co@npg-C3N4 shows peaks at 284, 410, 530, 779 eV and
940 eV due to C1s, N1s, O1s, Co2p and OKLL respectively
confirms the presence of all desired elements in the photo-
catalyst (Figure S2). High resolution XPS spectrum of Co@npg-
C3N4 in the C1s region revealed two characteristic peak
components at 287.5 and 291.1 eV due to the C–C and N=C–
N2 types of carbon respectively (Figure 4a).[23,48]
Wide scan in
N1s region of Co@npg-C3N4 shows two peak components at
399.2 and 399.9 eV originated due to secondary nitrogen C=
N–C and tertiary nitrogen (N–(C)3) respectively (Figure 4b).[47,49]
XPS spectra in O1s region show two peak components at 530.2
and 531.2 eV due to C-OH and C3-N+
-OÀ
types of oxygen
which conforms the oxidation of carbon nitride support with
hydrogen peroxide under harsh conditions (Figure 4c).[39]
Two
characteristic peaks at 781.4 and 795.8 eV due to Co 2p3/2 and
Figure 1. TEM images of npg-C3N4 and Co@npg-C3N4 a, b) at 100 nm, c, d) at
10 nm showing fringes, e, f) at 2 nm scale bar showing interplaner
dÀspacing.
Figure 2. STEM Elemental mapping of Co@npg-C3N4 a) electron image, b)
carbon, c) nitrogen, d) cobalt, and e) EDX pattern showing elemental
composition .
Figure 3. XRD Pattern of a) npg-C3N4, b) Co@npg-C3N4.
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Co 2p1/2 confirms that cobalt complex is present in Co+3
state
in the photocatalyst (Figure 4d).[50]
Raman spectroscopy is a powerful tool to characterize the
structure and electronic properties of carbon nitride. The
Raman spectrum of npg-C3N4 shows broad merged band in the
range of 1260À1900 cmÀ1
which is assumed due to the
overlapping of peaks at 1400 cmÀ1
, 1600 cmÀ1
and 1380 cmÀ1
,
corresponding to N (N-C=N), N (-CN) and sp2
C respectively
(Figure S3a).[51]
After covalent immobilization of cobalt complex
on oxidized carbon nitride there is no significant change in the
Raman spectrum observed, which confirms that sheet structure
of the support remained intact during immobilization. Further
no peak due to cobalt complex was observed that may be due
to low concentration of complex in material in comparison to
bulk material (Figure S3b). The sharp peak at 225 cmÀ1
was
originated due to Si support used for deposition of sample.
Surface specific properties of the synthesized materials i.e.
BET surface area (SBET), total pore volume (Vp) and mean pore
diameter (rp) are determined with the help of N2 adsorption
desorption isotherm (Figure S4). It can be seen from Figure S4a
the loop of N2 adsorption desorption isotherm for npg-C3N4
was of type-(IV) which reveal mesoporous nature of material.
After immobilization of cobalt complex in Co@npg-C3N4 photo-
catalyst the loop of adsorption desorption remains of type–
(IV)[52]
which confirms that immobilization procedure did not
change the mesoporous nature of the material (Figure S4b).
The BET surface area (SBET), total pore volume (Vp) and mean
pore diameter (rp) of npg-C3N4 was found to be 15.64 m2
gÀ1
,
0.19 cm3
gÀ1
, and 6.84 nm, respectively and after immobiliza-
tion of Co complex in Co@npg-C3N4, these values were found
to be 28.48 m2
gÀ1
, 0.13 cm3
gÀ1
and 21.25 nm respectively.
The increase in surface area from 15.64 to 28.48 m2
gÀ1
was
assumed due to the intercalation of Co complex units between
the nanoscopic carbon nitride sheets.
The electronic and optical properties of the photocatalysts
were determined by UV-visible spectroscopic analysis (Fig-
ure S5). The UV-visible spectrum of npg-C3N4 shows a absorp-
tion spectrum to typical semiconductor spectrum between
200–450 nm, originating from the charge transfer from a
populated valence band of nitrogen atom (2p orbital) to a
conduction band of carbon atom (2p orbital) of carbon nitride
(Figure S5b). It clearly show two absorption peaks centered at
260 and 370 nm, and additional absorption band in the visible
range from 400 to 550 nm, which is in accordance with the
literature.[53]
These absorptions are mainly related to the p–p*
or n–p* electronic transitions, respectively. The absorption
band tailing from 410 to 500 nm suggests a slight visible light
absorption capacity of carbon nitride in the visible light region.
But in case of cobalt bipyridine complex, there was some
decrease in absorbance due to shortage in n–p* transition.
After the attachment of cobalt complex with graphitic carbon
nitride, the material shows better absorbance in the visible
region.
In order to check light absorption capacity of photocatalyst
we plotted Tauc plot for optical band gap determination
(Figure 5). The band gap of npg-C3N4 was found to be 2.43 eV
(Figure 9a). After covalent attachment of cobalt complex in
Co@npg-C3N4 the band gap value was red shifted to 2.29 eV
which was assumed due to the higher visible light absorbance
of cobalt complex. It can be concluded from graph that hybrid
composite (Co@npg-C3N4) has narrow combined band gap and
can absorb well in visible region.
The thermal stability of the synthesized materials was
determined by thermogravimetric analysis (TGA) as given in
Figure 6. TGA thermogram of npg-C3N4 shows a gradual weight
loss between 550 to 700 8C due to the degradation of npg-C3
N4 sheets at elevated temperature (Figure 6a).[54]
For cobalt
complex a sharp weight loss was observed at 250 8C that was
due to breakdown of bipyridyl units in complex (Figure 6b). For
Co@npg-C3N4 catalyst two weight losses were observed (Fig-
ure 6c). The first small weight loss around 300À350 8C was
Figure 4. High resolution XPS spectra of Co@npg-C3N4 ina) C1s, b) N1s, c)
O1s and d) Co 2p region.
Figure 5. Tauc plot for band gap determination of a) npg-C3N4, b) Co@npg-C3
N4.
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assumed due to the degradation of organic/ ligand moieties of
Co complex while another major weight loss around 600À850
8C was due to the degradation of graphitic carbon nitride
sheets. In contrast to pristine cobalt complex, the weight loss
in Co@npg-C3N4 for cobalt complex was observed at somewhat
higher temperature that can be defined due to the formation
of new coordination complex bearing tetrazole ligand moiety
via click reaction between azide group of cobalt complex and
alkyne in propargylated npg-C3N4 (PA@npg-C3N4).
The photo-catalytic activity
Photocatalytic activity of the synthesized Co@npg-C3N4 photo-
catalyst is tested for esterification of aromatic aldehydes with
aliphatic alcohols i.e. methanol and ethanol at room temper-
ature under visible light irradiation. The results of these
experiments are summarized in Table 1 and 2. For the
optimization studies we have chosen benzaldehyde as model
substrate. In the absence of any photocatalyst there was no
reaction occurred even after prolonged time of visible light
irradiation. Cobalt homogeneous complex (identical amount as
present in the Co@npg-C3N4 catalyst) in the absence of light
did not provide any product and unreacted reactants could
recover at the end. However in the presence of visible light
35.2 % yield of the desired product was obtained. Following
the similar pattern npg-C3N4 and Co@npg-C3N4 did not give
any reaction product in the absence of visible light, which
confirmed that the reaction was truly photocatalytic in nature.
In the presence of visible light under otherwise identical
conditions, the yield of product while using npg-C3N4 and
Co@npg-C3N4 as photocatalyst was found to be 46.5 and 94.5
% respectively.
Further scope of the reaction was explored by treating a
variety of aldehydes having electron donating and withdrawing
groups with alcohols such as methanol and ethanol under
above mentioned optimized reaction conditions. The results of
these experiments are summarized in Table 2. As shown, the
substrates bearing electron donating groups (methoxy, methyl)
were found to be more reactive and afforded nearly 94À95 %
yield of the corresponding product. However the substrates
having electron withdrawing groups (-Br, -Cl, -NH2, -NO2 etc)
were found to be less reactive and afforded lesser product
yields in 80À82 % range.
Further, we tested the recycling of the catalyst in order to
confirm the heterogeneous nature and robustness of the
developed photocatalyst under described experimental con-
ditions. After completion of the reaction the catalyst was
separated via centrifugation, washed with methanol, dried at
50 8C and reused for subsequent run. The recovered photo-
catalyst was tested for six runs and it was observed that
photocatalyst remained almost equally efficient for six runs as
shown in Figure 7. Furthermore, the developed photocatalyst
Figure 6. TGA diagram ofa) npg-C3N4,b) Co complex 1,c) Co@npg-C3N4.
Table 1. Optimization of reaction parameters on the esterification of
aldehydes.a
S. No Catalyst Visible light Conversion Yield
1.
2.
3.
4.
À
Co complex
npg-C3N4
Co@npg-C3N4
Yes
À
Yes
À
Yes
À
Yes
À
À
36.4
À
48.6
À
96.0
À
À
35.2
À
46.5
À
94.5
a
Reaction conditions: benzaldehyde (1.0 mmol), methanol (5 mL), room
temperature, time 12hvisible light (l400 nm); b
determined with GC;
c
Isolated yield. Figure 7. Recycling experiment for esterification of aldehyde by Co@npg-C3
N4.
Full Papers

did not show significant leaching during the experiments which
was ascertained by ICPÀAES analysis of the recovered photo-
catalyst after sixth run. The cobalt content in the recovered
photocatalyst was found to be 1.18 wt% which was nearly
same to fresh synthesized photocatalyst (1.20 wt%). This study
confirmed that the developed photocatalyst was highly stable
and truly heterogeneous in nature without any significant loss
in activity.
Next, to establish the superiority of the click reaction as
immobilization strategy over direct immobilization, we synthe-
sized cobalt complex immobilized to npg-C3N4 by direct
method as follows: cobalt complex (10 mg) was mixed with
npg-C3N4 support (100 mg) in ethanol (10 ml) for 24 h, the
resulting solid was separated by centrifugation and subjected
to ICPÀAES analysis for determining the metal loading. The
loading of the cobalt was found to be 0.24 wt% which was
indeed very lower as compared to the catalyst synthesized by
click reaction (1.2 wt% Co). These results supported the
superiority of the click reaction as the immobilization strategy
for immobilization of homogeneous catalyst. In addition, after
third cycle in case of direct immobilization a significant loss in
activity was obtained where as in click, the catalyst showed
consistent activity at least for six runs.
Conclusions
We have demonstrated the successful synthesis of covalently
anchored cobalt complex to nanoporous graphitic carbon
nitride via click reaction between azide functionalities of the
cobalt complex and alkyne functionalities of the support. The
synthesized photocatalyst was used for the visible light
mediated esterification reaction of aromatic aldehydes with
alcohols. After 12 h of visible light irradiation high to excellent
yields of the esters was isolated by using various aldehydes
conjugates. The higher stability and enhanced performance
was assumed due to the covalent attachment of cobalt
complex on carbon nitride surface and continuous flow of the
photogenerated electrons from cobalt complex to the con-
duction band of carbon nitride, respectively. After completion
of the reaction, photocatalyst could easily be recovered by
centrifugation and was recyclable for several runs without any
significant loss in photocatalytic performance.
Supporting information summary
Detailed experimental procedures for the synthesis of the
photocatalysts as well as characterization of photocatalysts by
FTIR (Figure S1), XPS survey scan (Figure S2), Raman spectra
(Figure S3), Adsorption desorption isotherm (Figure S4), UVÀ
Vis spectra (Figure S5) and 1
H NMR spectra of the products are
given in supporting information.
Acknowledgements
Authors would like to thanks Director of the Institute for
granting permission to publish these results. AK is thankful to
CSIR for providing research fellowship for conducting research.
Analytical department of Institute is acknowledged for analysis
of samples. DST New Delhi is kindly acknowledged for
providing funding in the project GAPÀ3122.
Conflict of Interest
The authors declare no conflict of interest.
Keywords: carbon nitride · click reaction · cobalt ·
esterification · photocatalysis
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Table 2. Photocatalytic conversion of aldehydes to ester by using Co@npg-
C3N4 catalyst
Entry Reactant Product Conv.
(%)b
Yield
(%)c
TOF
(hÀ1
)
1
97.0
97.4
96.2
96.0
8.0
8.0
2
97.7
97.4
97.2
97.0
8.1
8.0
3
96.0
96.4
96.0
96.0
8.0
8.0
4
91.0
92.4
91.2
92.0
7.6
7.6
5
90.2
90.0
88.6
87.0
7.4
7.3
6
88.0
87.8
86.3
86.2
7.2
7.1
7
87.0
87.4
86.2
86.0
7.1
7.2
8
78.6
76.4
76.8
75.0
6.4
6.3
9
84.2
80.5
80.2
80.0
6.8
6.6
a
Reaction conditions: aldehyde (1.0 mmol, alcohol (5 mL), photocatalyst
(0.1g), room temperature, time 12h, visible light (l400 nm); b
determined
by GC; c
Isolated yield
Full Papers

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Submitted: December 22, 2016
Accepted: April 10, 2017
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