Visible light-induced direct conversion of aldehydes into nitriles in aqueous medium using Co@g-C3N4 as photocatalyst

Article abstract of DOI:10.1016/j.catcom.2018.10.031

Unprecedented Co@g-C₃N₄ catalyzed visible light driven efficient conversion of a variety of aldehydes into corresponding nitriles is reported. Operational simplicity, excellent yield of pure products (87–94%), ambient reaction condition, using aqueous methanol as solvent, visible-light photocatalysis are the salient features of envisaged methodology for direct conversion of aldehydes into nitriles. Furthermore, reusability of Co@g-C₃N₄ was checked up to five runs and it was noticed that there was no substantial change in morphology as well as the catalytic efficiency of catalyst.

Full text of DOI:10.1016/j.catcom.2018.10.031

Accepted Manuscript
Visible light-induced direct conversion of aldehydes into nitriles
in aqueous medium using Co@g-C3N4 as photocatalyst
Fooleswar Verma, Prashant Shukla, Smita R. Bhardiya,
Manorama Singh, Ankita Rai, Vijai K. Rai
PII:
S1566-7367(18)30562-4
DOI:
https://doi.org/10.1016/j.catcom.2018.10.031
CATCOM 5542
Reference:
To appear in:
Catalysis Communications
Received date:
Revised date:
Accepted date:
6 August 2018
24 October 2018
29 October 2018
Please cite this article as: Fooleswar Verma, Prashant Shukla, Smita R. Bhardiya,
Manorama Singh, Ankita Rai, Vijai K. Rai , Visible light-induced direct conversion of
aldehydes into nitriles in aqueous medium using Co@g-C3N4 as photocatalyst. Catcom
(2018), https://doi.org/10.1016/j.catcom.2018.10.031
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ACCEPTED MANUSCRIPT
Visible light-induced direct conversion of aldehydes into
nitriles in aqueous medium using Co@g-C N as
3
4
photocatalyst
a
b
a
a
b
Fooleswar Verma, Prashant Shukla, Smita R. Bhardiya, Manorama Singh, Ankita Rai, Vijai
a
K. Rai *
a
Department of Chemistry, Guru Ghasidas Vishwavidyalaya, Bilaspur (C.G.)-495009, INDIA
b
School of Physical Sciences, Jawaharlal Nehru University, New Delhi, 110067, INDIA
Corresponding Author
*
Dr. Vijai K. Rai
Department of Chemistry,
Guru Ghasidas Vishwavidyalaya,
Bilaspur (C.G.)-495009, INDIA
Ph: +91-75-87178627
Email: vijaikrai@hotmail.com

ACCEPTED MANUSCRIPT
ABSTRACT
Unprecedented Co@g-C N catalysed visible light driven efficient conversion of a variety of
3
4
aldehydes into corresponding nitriles is reported. Operational simplicity, excellent yield of pure
products (87-94%), ambient reaction condition, using aqueous methanol as solvent, visible-light
photocatalysis are the salient features of envisaged methodology for direct conversion of
aldehydes into nitriles. Furthermore, reusability of Co@g-C3N4 was checked up to five runs and
it was noticed that there was no substantial change in morphology as well as the catalytic
efficiency of catalyst.
Key words: Visible-light; Aldehydes; Nitriles; Aqueous Medium; g-C3N4, Photocatalysis.

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Abbreviations
g-C3N4, graphitic carbon nitride.

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1
.
Introduction
Visible light induced organic transformations are efficient and green but quite limited, due to
poor absorbance of visible light by organic functionalities [1]. To overcome this problem, design
and synthesis of visible light active photocatalyst would be a great contribution in organic
synthesis [2]. Graphitic carbon nitride (g-C N ) with graphite-like structure is an extremely
3
4
appealing host material for interruption of metal in its cavity which improves its catalytic
behavior [3]. Metal-doped g-C3N4, especially Co@g-C3N4 shows extremely high catalytic
activity for organic reactions along with its reusability [4].
Nitriles represent a versatile structural motif in a variety of pharmaceuticals, bioactive natural
products, agrochemicals, dyes, and functional materials [5]. Aromatic nitriles are important
building blocks for the synthesis of many natural products, polymer, textiles and for functional
group transformations viz., benzoic acid derivatives, amines, aldehydes, ketones, imines, and
heterocycles [6]. The classical approach for nitrile synthesis usually involves dehydration of
amide or oxime [7-8] including Sandmeyer, [9-10] Rosenmund-von Braun [11-12] and cyanide-
halide exchange reactions [13-14]. Recently, improved synthesis of nitriles has been reported
using different catalysts viz., sodium azide, NH4OAc, HMDS, CF3-BHA, TMSN3, TfOH, TFA,
ZrCl4/Pd/L, Zn(CN)2 (Scheme 1) [15-21]. Major challenges of the reported methods are one or
more among harsh reaction conditions, use of hazardous reagents and toxic solvents.

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Scheme 1. Difference strategy for synthesis of nitrile via aldehyde.
Aldehydes are commercially available substrates that afford nitriles directly without any
necessary isolation of oxime. Different N-sources have been employed with aldehydes for nitrile
synthesis in literature [22]. However, these methods involve highly toxic sodium azido (Schmidt
reaction), stoichiometric use of iodine or ammonia coupled with transition-metal catalysis and
undergoes complicated operations with low functional group compatibility. Therefore, search for
novel and efficient catalytic methods for direct synthesis of nitriles from aldehydes with various
functional group compatibility is still attractive [23, 8] and challenging target in organic

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synthesis [24-27]. To fill this literature gap and in continuation of our interest in developing new
organic transformations [28-31], we report herein visible light photocatalytic nitrile synthesis
from aldehyde and hydroxylamine in aqueous medium at room temperature (Scheme 1h).
1
.
Experimental
1
.1.
Preparation of Co@g-C N4
3
For preparation of photocatalyst, urea was calcinated at 500 °C to get converted as yellowish
graphitic carbon nitride (g-C3N4) and then it was used as support for Co-ion deposition [32]. The
solution of g-C3N4 (Sol A, 425 mg) in aqueous methanol (50%, 200 mL) and cobalt (II) acetate
Co(OAc)2).4H2O (Sol B, 317 mg, 15 wt% of g-C3N4, Co 75 mg) was prepared in aqueous
methanol (50%, 50 mL) through sonication for 2 h (Model no.- 6.5 L200, PCI Analytics). Then,
Sol B was added in Sol A and stirred for 6 h at rt. Prepared nanoparticle was centrifuged and
washed with water and methanol, centrifuged and dried under vacuum at 50 °C.
1
.2.
Characterization methods of Co@g-C3N4
The prepared Co@g-C3N4 catalyst was characterized by FTIR, X-ray diffraction (XRD),
photoluminescence (PL), scanning electron microscopy (SEM), energy-dispersive X-ray
spectroscopy (EDX), transmission electron microscopy (TEM) and X-ray photoelectron
spectroscopy (XPS).
SEM images exposed the two-dimensional arrangement of Co@g-C3N4 sheet along with a fine
agglomeration (ESI, Fig. 1). Carbon and nitrogen and a small amount of Co metal in Co@g-
C N were studied by EDX spectra (ESI, Fig. 1; inset). The X-ray diffraction (XRD) pattern has
3
4
shown the graphitic-like phase of C3N4 in g-C3N4 and Co@g-C3N4. The strong peaks at 27.6°
communicated to typical (002) interlayer stacking (JCPDS No. 87-1526) and confirms the
dissimilarity in g-C3N4 and Co@g-C3N4 (Fig. 1). A transmission electron microscopy (TEM)

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image of Co@g-C3N4 has also confirmed the graphitic- like structure and it supported the XRD
analysis very well (Fig. 2).
Fig. 1. XRD spectra of Co@g-C3N4, and g-C3N4.

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Fig. 2. TEM image of Co@g-C3N4; inset: SAED pattern.
Blue luminescence was observed in photoluminescence spectra of g-C N and Co@g-C N
4
3
4
3
because of transition among lone pair (LP) states in the valence band and the π* antibonding
states in the conduction band. The PL spectrum of g-C3N4 and Co@g-C3N4 (ESI, Fig. 3) shows
strong blue luminescence at ∼465 nm and ∼460 nm, respectively. The states consisting of the
3
2
sp C−N σ band, the sp C−N π band, and the LP state of the bridge nitride atom are accountable
for the luminescence from g-C3N4. The absorption in the visible region for g-C3N4 helps in
investigation of its visible light photocatalysis.
The Fourier transform infrared (FTIR) spectrum exhibits evidently exposed broad bands in the
−1
3
000−3500 cm region that could be recognized for O−H and N−H stretching. The absorption in
−1
the range of 1200−1700 cm is assigned to the characteristic stretching modes of g-C3N4
−1
heterocycles. At ∼806 cm absorption band is considered to be the out-of-plane skeletal bending
−1
modes of triazine and the band at ∼1640 cm is recognized to the C=N stretching vibration
mode (ESI, Fig. 2).
Moreover, the X-ray photoelectron spectroscopy (XPS) measurements were performed to
observe the chemical composition and chemical position of the constituent elements in the
Co@g-C3N4 sample (ESI, Fig. 4). The XPS survey spectra depicts that C, N, and Co elements
were detected on the Co@g-C3N4 sample (ESI, Fig. 4a), indicating the loading of Co on g-C3N4.
The peaks of Co 2p3/2 and Co 2p1/2 were located at 769.3 and 789.2 eV, respectively, along with
the presence of the two strong satellite peaks of cobalt at 773.3 and 792.3 eV, indicating the
presence of Co (II) in Co@g-C3N4, without the formation of any other oxidation state (ESI, Fig.
4
b). The C 1s spectra peaks at 285.7 and 287.1 eV were assigned to the C=N coordination and
2
sp -hybridized carbon in N-containing aromatic rings (N-C=N), respectively (ESI, Fig. 4c). The

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N 1s spectra shown can be deconvoluted into four peaks. The peaks centred at 398.1 and 398.7
2
eV are derived from the sp -hybridized nitrogen atoms in C=N-C groups and the amino function
groups, respectively. The peak at around 399.4 eV was attributed to the charging effects or
positive charge localization in heterocycles, while that at 400.2 eV is attributed to the tertiary
nitrogen in N-C3 groups or in H-N-C2 (ESI, Fig. 4d).
2
.
Results and discussion
2
.1.
General method for the synthesis of nitrile 3:
A round bottom flask was charged with a mixture of benzaldehyde 1 (1.0 mmol), NH2OH.HCl 2
1.5 mmol), Co@g-C3N4 (20 mg) in H2O/MeOH (1:1, 5 mL) and stirred under the visible light
(
condition at room temperature for 14-20 h. After completion of the reaction (monitored by TLC),
the catalyst was filtered and added EtOAc (10 mL). Remaining organic layer was washed with
brine (2×5 mL) and distilled water (1×10 mL) and dried over anhydrous sodium sulfate. Solvent
was evaporated under reduced pressure to afford the crude residue, which was further purified by
flash chromatography, EtOAc/n-hexane: 10:90 to obtain the analytically pure product 3.
At the outset, 4-chlorobenzaldehyde 1a (1.0 mmol) and hydroxylamine hydrochloride 2a (1.5
mmol) were chosen as model substrate along with a wide range of solvents such as MeOH, H2O,
EtOH, DMF and MeCN (Table 1). Initially, a mixture of 4-chlorobenzaldehyde 1a (1.0 mmol),
hydroxylamine hydrochloride 2a (1.5 mmol) and g-C3N4 (20 mg) was stirred in MeOH at room
temperature under visible light, which did not result any conversion even after 24 h.
Subsequently, we turned our attention to check the role of metal interrupted g-C3N4 hence
different metal-doped g-C3N4 catalyst viz., Ni@g-C3N4, Cu@g-C3N4, Fe@g-C3N4 and Co@g-
C3N4 (15 wt %) were screened (Table 1). Under the same reaction condition, Ni@g-C3N4 (20
mg) was used as catalyst and surprisingly, 54% of corresponding nitrile 3a was isolated only in

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1
8 h at room temperature. However, Co@g-C3N4 (20 mg) was found the best among all the
screened M@g-C3N4 catalysts for the present conversion (Table 1, entries 1-5).
Next, different polar protic and aprotic solvents were screened such as MeOH, H2O, EtOH,
DMF, MeCN, H2O/MeOH (1:1) for this conversion. Methanol showed good result with 75 %
yield in 14 h, however, H O/MeOH (1:1) afforded the best result among all screened solvents
2
(Table 1, entries 5-14). While optimizing the decrease in yield was observed on decreasing the
catalyst amount from 20 to 15 mg (Table 1, entries 14 and 15), while increasing the catalyst
loading from 20 to 25 mg did not increase the yield of nitrile (Table 1, entries 14 and 16).
Finally, 20 mg of photocatalyst Co@g-C3N4 with H2O/MeOH (1:1) in presence of LED light at
room temperature was found the common protocol for the present conversion (Table 1, entries
1
4).
A control experiment was performed to observe the effect of visible light in the present
a
Table 1. Optimization of reaction conditions for synthesis of nitrile 3a.
b,c
Entry
Catalyst (mg)
Solvent
MeOH
MeOH
MeOH
MeOH
MeOH
Time (h)
24
Yield (%)
1
2
3
4
5
6
7
8
9
@g-C3N4 (20)
---
54
66
52
75
60
45
54
60
Ni@g-C3N4 (20)
Cu@g-C3N4 (20)
Fe@g-C3N4 (20)
Co@g-C3N4 (20)
Co@g-C N (20)
18
16
20
14
H O
2
24
3
4
Co@g-C3N4 (20)
Co@g-C3N4 (20)
Co@g-C3N4 (20)
EtOH
14
DMF
14
CH3CN
18

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1
1
1
1
1
1
0
1
2
3
4
5
6
Co@g-C3N4 (20)
Co@g-C3N4 (20)
Co@g-C3N4 (20)
Co@g-C3N4 (20)
Co@g-C N (20)
H2O/MeOH (4:1)
H2O/MeOH (1:4)
H2O/MeOH (3:2)
H2O/MeOH (2:3)
14
14
14
14
14
14
14
75
77
81
74
92
75
92
H O/MeOH (1:1)
2
3
4
Co@g-C3N4 (15)
Co@g-C3N4 (25)
H2O/MeOH (1:1)
H2O/MeOH (1:1)
1
a
Reaction conditions: 4-chlorobenzaldehyde 1a (1 mmol), hydroxylamine hydrochloride 2a (1.5 mmol), 7-volt white
b
c
LED light, r.t. in air. Yield of isolated and purified products. All compounds gave C, H and N analyses within ±
0
1
13
.35 %, and satisfactory spectral ( H NMR, C NMR and EIMS) data.
investigation (Scheme 2a-d). Four set of reactions (2a-d) were performed separately, by taking
a (1.0 mmol) and NH2OH.HCl 2a (1.5 mmol), Co@g-C3N4 (20 mg) in water/MeOH (1:1). The
results clearly reveal that the reaction is only successful in the presence of visible light (Scheme
d) and no product formation was observed in absence of visible light either at room temperature
Scheme 2a) or at elevated temperature (Scheme 2b). The role of visible light was also confirmed
1
2
(
by using radical scavenger TEMPO and it was found that the yield of product was dropped
significantly (Scheme 2c), which ultimately confirms the radical pathway in the envisaged
method (Scheme 2c). Thus, on the basis of these controlled (Scheme 2a-d), it was concluded that
the reaction is photocatalytic.
Next, to check any difference between reaction from aldehyde with hydroxylamine and pre-
synthesized aldoxime, we have set two additional reactions directly from pre-synthesized
aldoxime at room temperature, one in the presence of visible light (Scheme 2e) and other in
absence of visible light (Scheme 2f). We found that similar results in terms of yield and
conversion rate in both case whether substrate was aldoxime (Scheme 2e,2f) or aldehyde and
NH2OH.HCl (Scheme 2a, 2d). Furthermore, we also investigated the role of catalyst in the
formation of oxime from aldehydes 1a and 1u. We did not isolate any oxime in absence of

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catalyst, however, the corresponding oximes 4a and 4u were isolated in presence of Co@g-C3N4
1
and further confirmed by their H NMR spectra and melting point [33]. Thus, conversion of
aldehyde 1 into nitrile 3 was successful using Co@g-C3N4 as photocatalyst under visible light
Scheme 2. Control experiment for synthesis of nitrile 3a.

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Scheme 3. Substrate scope of nitrile 3.
and this protocol was employed throughout the investigation to check the general validity of the
optimized reaction condition in controlled experiment.
Initially, we took butyraldehyde and employed the present optimized reaction condition, but
the yield of corresponding nitrile was not acceptable (32%). Next, a variety of substituted
aromatic aldehydes afforded the corresponding nitriles 3a-x and total 24 compounds have been
synthesized in good to excellent yields with maximum 94% of 3e and 3h (Scheme 3). Aldehyde
1
tolerated both EWG (F, Cl, Br, CF3, CN, NO2, Ac) as well as EDG (Me, OMe, OH, NHAc,
t
Bu) groups smoothly at the various position of phenyl ring to afford good to excellent yield of
corresponding nitrile 3 (Scheme 3). All the synthesized compounds were characterized by their
1
13
H and C NMR spectroscopy.

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Scheme 4. Photocatalytic cycle for conversion of aldehyde 1 into nitrile 3.
The formation of nitrile 3 is rationalized by initial N-nucleophilic attack of hydroxylamine 2
on carbonyl carbon of aldehyde 1 resulting oxime 4, formed in situ, which further undergoes
photocatalytic conversion into corresponding nitrile 3 (Scheme 4). This conclusion is based on
the observation that the representative oxime 4a and 4u could be isolated [S7-S8, ESI],
1
characterized by H NMR spectra [33] and further be converted into the corresponding nitrile 3a
and 3u in quantitative yield (Scheme 4). Formation of aldoxime from aldehyde is promoted by
the catalyst, Co@g-C3N4 via (i) increasing the polarity of carbonyl group due to vacant d-orbital
of Co (II) and thus facilitating the N-nucleophilic attack of hydroxylamine, (ii) dehydration
through removal of acidic H by basic nitrogen of g-C3N4. The visible light activates the catalyst,
Co@g-C3N4, which presumably converts oxime 4 into radical cation 5. The radical cation 5 then
undergoes chain termination by elimination of OH radical to afford the target compound nitrile 3
[34]. The eliminated OH radical from 6 finally get terminated into H2O by combining with
hydrogen radical and thus, the catalyst re-inters in the catalytic cycle (Scheme 4). The catalyst
was recovered by centrifuge, washed with ethanol and reprocessed for further reactions and it

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could be recycled and reused more than five times with good yield without any significant loss of
its activity (Fig. 5, S5, ESI). Furthermore, SEM (Fig. 6, S6, ESI) confirms that there was no
substantial change in morphology and surface of the catalyst, while XRD pattern (Fig. 7, S6,
ESI) reveals the same structure of catalyst before and after use in five consecutive reactions [35].
3
. Conclusions
Thus, we disclose an original visible light induced green and efficient method for conversion
of nitriles from aldehydes, which is affected by unprecedented use of Co@g-C3N4 as
photocatalyst in aqueous medium under ambient condition. Selectivity, stability, and
recyclability over several reactions, heterogeneous nature, efficient at ambient conditions are
salient features of the designed photocatalyst, which make the envisaged method advantageous
than earlier reported method for synthesis of such fine chemicals and would be the best
alternative to be further exploited for academia and industries.
Appendix A. Supplementary data
Supplementary
data
to
this
article
can
be
found
online
at
https://
doi.org/10.1016/j.catcom.00000000
Acknowledgment
We sincerely thank the DST, New Delhi (File No. SR/FT/CS-99/2010) for partial financial
support and to AIRF JNU, New Delhi, SAIF, Punjab University, Chandigarh, NIT Raipur, and
IIT Kanpur for providing microanalyses and spectra.

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Highlights
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Co@g-C3N4 catalyzed efficient conversion of aldehydes into corresponding nitriles.
Co@g-C3N4 was characterized by FTIR, XRD, photoluminescence, SEM, EDX, TEM
and XPS.
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Reactions were performed under visible-light condition at room temperature.
Substrate tolerated both electron withdrawing and electron releasing groups.
Excellent yield (87-94%) using aqueous methanol as solvent are salient features.