Mg-NHC complex on the surface of nanomagnesium oxide for catalytic application

Article abstract of DOI:10.1016/j.catcom.2016.06.007

We have developed a simple and eco-friendly protocol for the synthesis of Mg-NHC complexes. The Mg-NHC complex is prepared by reacting nano-MgO with imidazolium salts. Thus prepared Mg-NHC complex has been realized in two divergent reactions one in C-C bond formation and other in dehydration reaction. Further, the Mg-NHC catalyst has been characterized by SEM, TEM, XPS, Raman Spectroscopy and XRD analysis.

Full text of DOI:10.1016/j.catcom.2016.06.007

Mg-NHC complex on the surface of nano magnesium oxide for catalytic
application
Melad Shaikh, Mahendra Sahu, Pramod Kumar Gavel, Goverdhan Reddy
Turpu, Santimoy Khilari, Debabrata Pradhan, Kalluri V.S. Ranganath
PII:
DOI:
Reference:
S1566-7367(16)30198-4
doi: 10.1016/j.catcom.2016.06.007
CATCOM 4680
To appear in:
Catalysis Communications
Received date:
Revised date:
Accepted date:
20 March 2016
27 May 2016
7 June 2016
Please cite this article as: Melad Shaikh, Mahendra Sahu, Pramod Kumar Gavel, Goverd-
han Reddy Turpu, Santimoy Khilari, Debabrata Pradhan, Kalluri V.S. Ranganath, Mg-
NHC complex on the surface of nano magnesium oxide for catalytic application, Catalysis
Communications (2016), doi: 10.1016/j.catcom.2016.06.007
This is a PDF file of an unedited manuscript that has been accepted for publication.
As a service to our customers we are providing this early version of the manuscript.
The manuscript will undergo copyediting, typesetting, and review of the resulting proof
before it is published in its final form. Please note that during the production process
errors may be discovered which could affect the content, and all legal disclaimers that
apply to the journal pertain.

ACCEPTED MANUSCRIPT
Mg-NHC complex on the surface of nano magnesium oxide for catalytic
application
a
a
a
a
Melad Shaikh , Mahendra Sahu , Pramod Kumar Gavel , Goverdhan Reddy Turpu ,
b
b
Santimoy Khilari , Debabrata Pradhan , Kalluri V.S. Ranganath*
a
School of Physical Sciences, Central University, Bilaspur, India-495009
b
Materials Science Centre, Indian Institute of Technology, Kharagpur,India-721302
Contact Address: rangakvs@gmail.com, Tel: +91-7752-260488
Abstract: We have developed a simple and eco-friendly protocol for the synthesis of Mg-
NHC complexes. The Mg-NHC complex is prepared by reacting nano MgO with imidazolium
salts. Thus prepared Mg-NHC complex has been realized in two divergent reactions one in
CC bond formation and other in dehydration reaction. Further, the Mg-NHC catalyst has
been characterized by SEM, TEM, XPS, Raman Spectroscopy and XRD analysis.
Keywords: Nano MgO, Mg-NHC, C-C bond formation, dehydration, heterogeneous
1

ACCEPTED MANUSCRIPT
1
. Introduction
The high stability of metal-(N-heterocyclic carbene) NHC complexes and the ability to tune
their electronic properties are the attractive features for the development of organometallic
materials [1-4]. The coordination of NHC ligands is not only limited to transition metals, but
also to non-transition metals [5]. In this direction, the Mg-NHC complex was reported by
Arduengo by reacting stable NHCs with dimethyl magnesium [6]. The Mg-NHC complexes
have also been realized by using different organometallic reagents such as
bis(pentamethylcyclopentadienyl) magnesium, dimethylmagnesium [7], mesitylmagnesium
bromide [8], methyl magnesium chloride [9] which leads to the formation of mono and bis
Mg-NHC adducts [9]. However, the organometallic reagents used to produce Mg-NHC are
highly expensive and a lot of waste could be generated after the reaction. Besides the
disadvantage of non-recyclable, Mg-NHCs could not be prepared in large scale, due to
economical and environmental concerns. The successful use of Mg-NHCs in catalysis has
not been reported on the surface of nanomaterials to the best of our knowledge [10].
Nanostructure materials possess a unique property like high surface area and accessibility
of various active facets [11-12]. Because of their chemical activity, robustness and
recyclability, nanocrystalline metal oxides have been using in catalysis, either as simple
support or even as an active component [13-14].
In the present communication, we report the formation of Mg-NHC on the surface of nano
MgO (N-MgO) by reacting with imidazolium salts (Scheme 1) and subsequent application in
catalysis under heterogeneous conditions.
Scheme 1: Synthesis of Mg-NHC complex on the surface of nano MgO
2

ACCEPTED MANUSCRIPT
2
. Experimental:
Materials:
The nano MgO was prepared as follows:
2.1.
The metal acetate tetrahydrate (0.05 mol) was initially dissolved in 150 ml of absolute
ethanol under constant stirring. The pH of the solution was then adjusted to 5 using 1 M
oxalic acid. The mixture was continuously stirred until a thick white gel was formed. The gel
formed was left overnight for further gelation process before being dried in an oven at 100°C
for 12 h. The dried materials were grounded using mortar and pestle to produce fine powder
precursors. Subsequently, the precursors were annealed at 800° C for 24 h to form MgO.
2.2. Synthesis of Mg-NHC complex:
In a typical experiment, nano MgO (0.5 mmol) was added into the round bottom flask
containing toluene (3.0 mL) and imidazolium salt (0.05 m.mol). The reaction was stirred at
80 C for 6 h followed by filtration and dried at 80 C for 12 h.
2.3.
Characterization of Mg-NHC complex:
Mg-NHC complex was characterized by using X-ray Diffraction (XRD), Scanning Electron
Microscope (SEM), Transmission Electron Microscope (TEM), X-Ray Photo Electron
Spectroscopy (XPS) and Raman Spectroscopic techniques.
2.4. Catalytic Reaction:
20.0 mg of Mg-NHC was taken into the round bottom flask containing toluene (2.0 mL) and
subsequently aldehyde (0.5 mmol) was added. To the resultant mixture nitromethane (2.5
mmol) was added and stirring continued at 60 C for 12 h. After completion of the reaction,
the reaction mixture was filtered to separate the catalyst and purified the product using
column chromatography (using Hexane: EtOAc, 9:1)
An oven dried flask was charged with Mg-NHC (0.020 g), glucose (0.180 g). To the flask
DMF (3.0 mL) was added and heated to 100 C with magnetic stirring. After completion of
the reaction, the reaction mixture was cooled to room temperature, and catalyst was
3

ACCEPTED MANUSCRIPT
removed and washed several times with methyl butyl ether and kept at -10C for 4h to get
the crystals of HMF. The reaction mixture was analyzed by using UV-Visible spectroscopy.
Final reaction mixture was analyzed by using HPLC using water and methanol: DMF
(
9:0.5:0.5) using C-18 column.
3
. Results and Discussion:
Mg-NHC complexes were prepared using various imidazolium salts on the surface of N-MgO
L1, L2, L3 and L4] as shown in the scheme 2. Initially, the L1-MgO catalyst was
[
characterized by XRD. No significant change was observed in the plane positions after
modifying the N-MgO with imidazolium salts (Fig. S1, Supporting Information). However, a
small decrease in particle size was observed. The crystallite size was calculated to be 19.6
nm for N-MgO and 16.4 nm for Mg-NHC using Debye-Scherer equation. In the Raman
-1
spectroscopy, significant band was observed at 522 cm , is the indication for M-O-M or M=O
bands [15]. It remains same after surface modification of MgO with 5 mol% of the ligand (L
1) whereas the peak intensity decreases as the concentration of the ligand increases (Fig.
-1
1
). No peak corresponding to 522 cm was observed using 20 mol% of the L1. This may be
due to the coverage of active sites of N-MgO with ligand. At higher concentrations of the
ligand, fluorescence can be seen (Fig. S2).
4

ACCEPTED MANUSCRIPT
Scheme 2. Various imidazolium salts used for the synthesis of N-Mg-NHC complexes
Fig. 1. Raman Spectra a) nano-MgO; b) MgO-L1 (5 mol %); c) MgO- L2 (5mol %); d)
MgO-L1 (10 mol %); e) MgO-L3 (5 mol%); f) MgO-L1 (20 mol%)
In the SEM of the as synthesized MgO, plate like structures stacked on each other were
observed whereas the images of ligand coated samples shows the decoration of particles
on the sheets (Fig. S3). Moreover elemental mapping indicates that 2% of nitrogen was
present in the sample. The transmission electron microscopy (TEM) of Mg-NHC complex
shows MgO particles are of 10-20 nm in diameter (Fig. S4). Further the formation of Mg-
NHC complex was confirmed by XPS technique. The N1s emission at 403 e.V of Mg-NHC
complex confirms the presence of Mg-NHC ( Fig. S5 and S6) whereas the Mg 2p binding
energy was observed at 198.6 eV (Fig.S7)
5

ACCEPTED MANUSCRIPT
The catalytic properties of the surface modified N-MgO were initially evaluated in the
nitroaldol addition reaction between 4-chlorobenzaldehyde and nitromethane, which is an
important C-C bond formation reaction (Table 1) [16-17]. The rate of the reaction is very fast
using Mg-NHC compared with only nano MgO (< 5%) (Table 1, entry 1). The Mg-NHC L1
acts as a versatile nano heterogeneous catalyst with the yield of up to 55% in toluene at
60C (Table 1, entry 3). The heterogeneous nature of this catalyst system was demonstrated
by recycling of the catalyst for three times. After completion of the first cycle, the catalyst
was removed, washed sequentially with toluene, and dried for 30 min. A new reaction was
then performed with fresh reactants under the same conditions. To the best of our
knowledge, our work is the first report of using N-MgO and NHCs as surface modifiers [10].
As the ligand concentration increases, the rate of reaction increases at the beginning (43%
with 5mol% to 55% with 10 mol %) and later on decreases (20 mol %) (Table 1, entry 2, 3, 4
&
5). Further increase in the concentration leads to drastic decrease in the reaction rate and
notably there was no reaction with 50 and 100 mol% of L1. This may be due to the surface
of N-MgO is completely covered with the imidazolium salts and difficult for activation of
carbonyl group of an aldehyde by interaction with the active sites of surface. Effect of
temperature has a significant effect on the rate of reaction. Notably, no reaction was
observed at room temperature using Mg-NHC complex in the nitroaldol reaction between 4-
chlorobenzaldehyde and nitromethane whereas at 80C, there was no significant change in
the yield. Clearly, the success of the reaction will strongly depend on the nature of surface
modifier. A screening of various alternative ligands showed pronounced effect on the activity
(
Scheme 2). Surprisingly, no reaction was observed with L3, L4 and L6 on the surface
modified N-MgO whereas with L2, 50% yield was obtained (Table 1). In general, NHC
ligands possessing bulky groups show more activity than normal NHCs in catalysis.
However, in the present case, the reactivity of linear NHC on the surface of N-MgO is higher
than bulk NHCs. This may be due to linear NHCs accommodate properly on the surface of
nanomaterials (due to high surface area) and effectively participate in the nitroaldol reaction
6

ACCEPTED MANUSCRIPT
and hence more reactivity of linear NHCs (Table 1). This clearly indicates, the role of surface
chemistry expands into different dimensions towards not only catalysis, but also into the
mode of interaction [18] (Fig. S8).
The N-MgO has single crystallite polyhedral structure, which is having the presence of high
surface concentrations of edge/corner and various exposed crystal planes (such as 002,
001, 111, 220), leads to inherently high surface reactivity per unit area [19,20]. These active
sites may accelerate the nitroaldol reaction in the presence of imidazolium salts with the
formation of Mg-NHC complex. In order to test the effectiveness of the Mg-NHC complex in
the nitroaldol reaction, various aldehydes containing electron donating and with drawing
groups have been examined. Notably, with 4-methoxybenzaldehyde gave the desired
product in 52% yield where as benzaldehyde gave 74% yield. Remarkably, when the
reaction was performed with 1-naphthaldehyde, reaction went on well with 78% yield in 36 h
(
Scheme 3). When the reaction was performed with nitroethane, corresponding nitroalcohol
product was isolated in 68% yield.
7

ACCEPTED MANUSCRIPT
Table 1
Nitroaldol reaction between 4-chlorobenzaldehyde with nitromethane at 60 C using N-MgO-
NHC complex
Entry Catalyst
NHC salt Yield (%)
(
-
mol %)
1
2
3
N-MgO
< 5%
43
N-MgO-L1
N-MgO-L1
5
b
c
d
e
10
55, 52 , 53 , 55 , N.R
4
5
6
7
8
9
N-MgO-L1
N-MgO-L1
N-MgO-L1
N-MgO-L2
N-MgO-L2
N-MgO-L2
N-MgO-L2
N-MgO-L2
N-MgO-L3
N-MgO-L4
N-MgO-L5
N-MgO-L6
15
20
100
5
36
10
N.R
50
10
20
50
100
5
38
34
1
0
1
2
3
4
5
N.R
N.R
N.R
N.R
43
1
1
1
1
1
5
5
5
N.R
Reaction conditions:
a
aldehyde (0.5 mmol), nitromethane (2.5 mmol), toluene (2.0 mL), catalyst (20.0 mg)
IInd cycle
IIIrd cycle
at 80C
at room temperature
b
c
d
e
8

ACCEPTED MANUSCRIPT
Scheme 3 Nitroaldol reaction between different aldehydes with nitromethane at 60 C using
Mg-NHC complex
Later, the catalytic properties of Mg-NHC complex was utilized in the glucose
dehydration, which leads to the formation of 5-hydroxymethylfurfural (HMF) (Scheme
4
). This reaction has emerged as an important transformation, as the resultant
compounds represent valuable intermediates for the biofuel [21-23]. The dehydration
of glucose has been reported by using various acid and base catalysts under
homogeneous conditions as well as under heterogeneous conditions [24]. Therefore
the dehydration reaction was performed in different solvents like water and DMF
which are commonly, used solvents. Though full conversion was observed in water,
however selectivity towards HMF was very less (23%). However, in DMF higher
selectivity was observed upto 89% with a conversion of 99.5% at 100
However, neither MgO alone nor ligands catalyze the reaction.
C in 2 h.
9

ACCEPTED MANUSCRIPT
Scheme 4 Synthesis of MHF using Mg-NHC complex
4. Conclusion
In conclusion, we have reported the formation of an Mg-NHC on the surface of nano
crystalline MgO. This novel catalyst with its new ensemble of catalytically active entities
catalyzed CC bond formation and dehydration reaction. The NHC ligands containing
both linear and bulk groups may give the good reactivity. Further work will try to elucidate
the underlying active principle of this new class of heterogeneous catalysts for surface
catalysis.
5. Acknowledgements
KVSR thanks to DST (file number: SR/NM/NS-1169-2012) for providing financial
support.
1
0

ACCEPTED MANUSCRIPT
6. References
[
1] (a) M. N. Hopkinson, C. Richter, M. Schedler, F. Glorius, Nature 510 (2014) 485-496.
(
b) D. J. Nelson, S. P. Nolan, Chem. Soc. Rev. 42 (2013) 6723-6753.
[
2] (a) D. Enders, T. Balensiefer, Acc. Chem. Res. 37 (2004) 534-541.
(
b) D. Enders, O. Niemeier, A. Henseler, Chem. Rev. 107 (2007) 5606-5655
[
3] (a) N. Marion, S. Diez-Donzalez, S. P. Nolan, Angew. Chem. Int. Ed. 46 (2007) 2988-
000.
3
[
[
[
4] Q. Liu, T. Rovis, J. Am. Chem. Soc. 128 (2006) 2552-2554.
5] C. E. Willians, Organomet. Chem 36 (2010) 1-28.
6] A. J. Arduengo, H. V. R. Dias, F. Davidson R. L. Harlow, J. Organomet. Chem 462 (1993)
13-18.
[
[
7] A. J. Arduengo, F. Davidson, R. Krafczyk, W. J. Marshall, M. Tamm, Organometallics. 17
1998) 3375-3382;
(
8] H. Schumann, J. Gottfriedsen, M. Glanz, S. Dechert, J. Demtschuk, J. Organomet.
Chem. 617 (2001) 588-600
[
[
9] D. Zhang, H. Kawaguchi, Organometallics 25 (2006) 5506-5509.
10] K. V.S. Ranganath, S. Onitsuka, A. Kiran, J. Inanaga, Cat. Sci. Technol 3 (2013) 2161-
2181.
[
[
[
11] I. E. Watchs, K. Rautray, ACS Catal 2 (2012) 1235-1246.
12] Q. Kuang, X. Wang, Z. Jiang, Z. Xie, L. Zheng, Acc. Chem. Res. 47 (2014) 308-318.
13] K. V. S. Ranganath, F. Glorius, Catal. Sci. Technol. 1 (2011) 13-22.
1
1

ACCEPTED MANUSCRIPT
[
14] K. V. S. Ranganath, A. Schfaer, F. Glorius, Angew. Chem. Int. Ed. 49 (2010) 7786-
7789.
[
[
15] C. Hess, J. H. Lunsford, J. Phys. Chem B. 107 (2003) 1982-1987.
16] (a) T. Yasukawa, H. Miyamura, S. Kobayashi, Chem. Soc. Rev. 43 (2014) 1450-1461.
(
b) C. M. R. Volla, I. Atodiresei, M. Rueping, Chem. Rev. 114 (2014) 2390-2431.
[
17] (a) M. Sutradhar, M. F. C. Guedes da Silva, A. J. L. Pombeiro, Catal. Commun. 57
(
(
2014) 103-106
b) A. Karmakar, M. F. C. Guedes da Silva, A. J. L. Pombeiro, Dalton Trans. 43 (2014)
7795-7810.
(
c) A. Karmakar, M. F. C. Guedes da Silva, S. Hazra, A. J. L. Pombeiro, New. J.
Chem. 39 (2015) 3004-3014.
[
18] C. Richter, K. Schaepe, F. Glorius and B. J. Ravoo, Chem. Commun. 50 (2014) 3204-
3207.
[
[
19] Y. Jiang, S. Decker, C. Mohs, K. J. Klabunde, J. Catal. 180 (1998) 24-35
20] R. Richards, W. Li, S. Decker, C. Davidson, O. Koper, V. Zaikovski, A. Volodin, T.
Rieker, K. J. Klabunde J. Am. Chem. Soc. 122 (2000) 4921-4925.
[
21] Recent Review on HMF synthesis and properties: A. A. Rosatella, S. P. Simeonov, R.
F. M. Frade, C. A. M. Afonso, Green Chem. 13 (2011) 754-793.
[
[
22] (a) H. Zhao, J. E. Holladay, H. Brown, Z. C. Zhang, Science 316 (2007) 1597-1600;
(
b) G. Yong, Y. Zhang, J. Y. Ying, Angew. Chem. Int. Ed. 47 (2008) 9345-9348.
23] (a) E. Nikolla, Y.R.-Leshkov, M. Moliner, M. E. Davis, ACS Catal. 1 (2011) 408-410
(
(
b) F. S. Asghari, H. Yoshida, Carbohydrate Research 341(2006) 2379-2381.
c) J. B. Binder and R. T. Raines, J. Am. Chem. Soc.131 (2009) 1979-1985.
1
2

ACCEPTED MANUSCRIPT
[
24] (a) H. Xu, Z. Miao, H. Zhao, J. Yang, J. Zhao, H. Song, N. Liang, L. Chou, Fuel 145
(
(
2015) 234-240.
b) S. De, S. Dutta, A. K. Patra, A. Bhaumik, B. Saha, J. Mater. Chem. 21 (2011) 17505-
17510.
(
c) K. Nakajima, Y. Baba, R. Noma, M. Kitano, J. N. Kondo, S. Hayashi, M. Hara, J. Am.
Chem. Soc. 133 (2011) 4224-4227.
(
d) K.V.S. Ranganath, M. Sahu, M. Shaikh, P.K. Gavel, S. Khilari, A. Kiran Kumar and P.
Das, New. J. Chem 40 (2016) 4468-4471.
1
3

ACCEPTED MANUSCRIPT
Graphical Abstract
1
4

ACCEPTED MANUSCRIPT
Highlights

Mg-NHC complex is synthesized on the surface of nano-MgO using simple
protocol.


Mg-NHC complex is used as a heterogeneous catalyst.
Linear NHCs performed better activity than bulk NHCs.
1
5