Chitosan supported Zn(II) mixed ligand complexes as heterogeneous catalysts for one-pot synthesis of amides from ketones via Beckmann rearrangement

Article abstract of DOI:10.1016/j.molstruc.2016.10.053

Chitosan supported Zn(II) mixed ligand complexes have been synthesized and characterized by FT-IR, UV–Vis, TGA, XRD, FESEM, EDX, AAS and Elemental Analysis. These complexes have been found to be efficient and recyclable heterogeneous catalysts for the one-pot synthesis of amides via Beckmann rearrangement. All three complexes can be easily filtered out from the reaction medium and reused up to five times without significant loss of catalytic activity. The reported protocol is economical and novel in the sense that amides can be easily synthesized in only one-step. All products were obtained as white to off-white crystalline solids and fully characterized by ¹H NMR, FT-IR and Mass Spectra.

Full text of DOI:10.1016/j.molstruc.2016.10.053

Journal of Molecular Structure 1130 (2017) 368e373
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Journal of Molecular Structure
journal homepage: http://www.elsevier.com/locate/molstruc
Chitosan supported Zn(II) mixed ligand complexes as heterogeneous
catalysts for one-pot synthesis of amides from ketones via Beckmann
rearrangement
*
Anuradha, Shweta Kumari, Samaresh Layek, Devendra D. Pathak
Department of Applied Chemistry, Indian Institute of Technology (ISM), Dhanbad, 826004, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 12 September 2016
Received in revised form
Chitosan supported Zn(II) mixed ligand complexes have been synthesized and characterized by FT-IR, UV
eVis, TGA, XRD, FESEM, EDX, AAS and Elemental Analysis. These complexes have been found to be
efficient and recyclable heterogeneous catalysts for the one-pot synthesis of amides via Beckmann
rearrangement. All three complexes can be easily filtered out from the reaction medium and reused up to
five times without significant loss of catalytic activity. The reported protocol is economical and novel in
the sense that amides can be easily synthesized in only one-step. All products were obtained as white to
off-white crystalline solids and fully characterized by 1H NMR, FT-IR and Mass Spectra.
16 October 2016
Accepted 16 October 2016
Available online 17 October 2016
Keywords:
Chitosan
©
2016 Elsevier B.V. All rights reserved.
Ketones
Amides
Beckmann rearrangement
1
. Introduction
additives, multistep and tedious synthesis, high temperature con-
dition, use of hazardous solvents [9] etc. In the literature, only a few
methods have been addressed for one-pot synthesis of amides from
ketones with hydroxylamine hydrochloride using catalyst such as,
The amide bond is one of the most important functional groups
in organic chemistry [1]. It is an integral part of proteins which are
the building blocks of living being [2]. It is found in numerous
natural products, pharmaceutical molecules [3], and key structural
motif in many agrochemicals, polymers and fine chemicals [4]. The
synthesis of amides has attracted considerable interest over several
years and a number of methods have been devised [5]. Conven-
tionally, amides are prepared by the reaction of an amine with a
carboxylic acid derivative [6]. These methods suffer from poor
atom-efficiency and the use of highly hazardous reagents [7]. In a
quest for atom economical and cost effective protocols, Beckmann
rearrangement has emerged as an attractive alternative for the
synthesis of amides from ketones [8]. In this context, various types
of catalysts/reagents have been reported in the literature such as,
3 2
anhydrous oxalic acid [11], FeCl $6H O [12], alumina supported
sodium hydrogen phosphate [13] activated fly ash [14], etc.
Therefore, there is a need to find new methodologies which are
simple, inexpensive, efficient, eco-friendly and sustainable. Herein,
our interest lies in exploring new route for one-pot synthesis of
amides from ketones via Beckmann rearrangement.
In recent years, there has been a growing interest in Chitosan
(CS) as a novel support for anchoring transition metal complexes
due to its insolubility in common organic solvents. Chitosan finds
extensive applications in several fields such as, medicines, fuel cells,
ferroelectric materials and catalysis [15e20]. A spurt in Chitosan
chemistry emanated mainly due to the presence of free amino and
hydroxyl groups, which can easily undergo a variety of chemical
modifications [21]. Chitosan supported Schiff base complexes have
been widely used as biochemical, antimicrobial and catalytic re-
agents [22] etc. Prompted by these reports, we herein, describe the
synthesis and development of CS supported Zn(II) mixed ligand
complexes as heterogeneous catalysts for one-pot synthesis of
amides from ketones.
5 3 3 2
sulphuric acid, triphosphazene, PCl , [C SO Hmim][Cl]-ZnCl [9] etc
in various medium such as, vapour phase, liquid phase, ionic liq-
uids, and super critical water [10] etc. These above catalysts/re-
agents suffer from several drawbacks such as, use of strong acids,
*
Corresponding author.
E-mail address: ddpathak@yahoo.com (D.D. Pathak).
http://dx.doi.org/10.1016/j.molstruc.2016.10.053
022-2860/© 2016 Elsevier B.V. All rights reserved.
0

Anuradha et al. / Journal of Molecular Structure 1130 (2017) 368e373
369
2
. Experimental
OH
O
OH
O
CHO
X
OH
O
HO
*
*
2.1. Synthesis of Schiff base ligand (L)
O
HO
*
n
*
+
Ethanol, 26-30 h
reflux
n
HC
N
NH
2
OH
The ligand (L) was synthesized by the literature procedure [23].
A mixture of salicyaldehyde (0.1221g, 1 mmol) and 3-nitroaniline
0.1381g, 1 mmol) in ethanol under reflux condition. After cool-
ing, the yellow precipitate was collected by filtration and recrys-
(
X
X= H
X= 5-Br
X= 3-OMe
tallized from ethanol (Scheme 1).
Scheme 2. Synthesis of CS supported Schiff base ligands (CS-Sal, CS-SalBr, and CS-
SalOMe).
2
.2. Synthesis of CS supported Schiff base ligands (CS-Sal, CS-SalBr,
and CS-SalOMe)
An ethanolic solution of CS (0.1882g, 1 mmol) was stirred for 5 h.
To this suspension, an ethanolic solution of substituted sali-
cyaldehyde (1 mmol) was added drop wise. The resultant reaction
mixture was heated under reflux for 26e30 h. After cooling, the
SalOMe-Zn-L are shown in Fig. 1. The FT-IR spectrum of L
ꢂ1
(
1
Fig. 1a), exhibited strong characteristics bands at 1622 cm and
ꢂ1
527 cm , attributed to
n
C]N bond and aromatic
n
NO
2
group,
respectively. These characteristic bands confirm the successful
formation of L [23]. On comparing the FT-IR spectra of CS (Fig. 1b)
and CS-SalBr (Fig. 1c), two new additional peaks appeared in CS-
yellow solid was filtered, washed with ethanol (3 ꢀ 2 mL) and ether
ꢁ
(
2 ꢀ 2 mL) and then dried under vacuum at 50 C (Scheme 2).
ꢂ1
ꢂ1
SalBr at 1630 cm and 1220 cm , respectively, due to
and phenolic CeO groups, respectively. The formation of C]N
bond clearly indicates the condensation reaction between eCHO
group of substituted salicyaldehyde and eNH group of CS had
n C]N
2.3. Synthesis of CS supported Zn(II) mixed ligand complexes (CS-
n
Sal-Zn-L, CS-SalBr-Zn-L, and CS-SalOMe-Zn-L)
2
CS supported Schiff base ligands (CS-Sal, CS-SalBr, and CS-
SalOMe) (1 mmol) in ethanol was stirred for 5 h at room temper-
taken place. In the FT-IR spectrum of CS-SalBr-Zn-L (Fig. 1e), v C]N
frequencies were slightly shifted to lower frequencies
ature. To this suspension, clear solution of Zn(OAc)
mmol) in ethanol and ethanolic solution of Schiff base ligand (L)
0.2420g, 1 mmol) were added and reflux for 12e15 h. The resulting
product, after the filtration, was washed with ether (3 ꢀ 10 mL) and
2
.2H
2
O (0.2195g,
ꢂ1
ꢂ1
(D
v ¼ 26 cm ) at 1604 cm , compared with CS-SalBr. The shifting
1
(
of these absorption bands to lower wave numbers confirmed the
coordination of Zn(II) metal ion with azomethine nitrogen atom of
the Schiff base [24]. A new peak was observed in complex CS-SalBr-
ꢁ
then dried under vacuum in 50 C (Scheme 3).
ꢂ1
Zn-L at 1525 cm due to NO
2
group, indicates L coordinated with
Zn(II) metal ion. Some new sharp peaks at 533, 551, 479 and
2
.4. Synthesis of amides
ꢂ1
4
65 cm was observed in the complex CS-SalBr-Zn-L due to v
0
0
ZneN, v ZneN , v ZneO and v ZneO bonds, respectively. These
observations confirm the coordination of zinc with N, N O and O
Ketones (1 mmol) and hydroxylamine hydrochloride (0.0694g,
mmol) were dissolved in CH CN (10 mL) and stirred for
3
0
0
1
sites of L and CS-SalBr [25]. The FT-IR spectrum of residual catalyst
10e15 min. The complex (CS-SalBr-Zn-L) (10 mol%) were added to
after the catalytic reaction exhibited all characteristics peaks of
the reaction flask. The reaction mixture was heated under reflux for
specific time (3e7 h). After completion, the reaction mixture was
cooled to room temperature and the catalyst was removed by
filtration. The filtrate was treated with ethyl acetate (3 ꢀ 10 mL).
The combined organic layers were treated with saturated brine
solution and dried over anhydrous sodium sulphate. The removal of
solvent yields crude product, which after purification by column
chromatography over Silica gel (100e200 mesh), afforded the
desired products.
0
eC]N, NO
2
, ZneN, ZneN , ZneO, and ZneO' (Fig. S1) and spectrum
was found to be indistinguishable to the spectrum of fresh catalyst
CS-SalBr-Zn-L (Fig. 1e). After five cycles of catalytic reactions,
noticeable changes in intensities of characteristics peaks (C]N,
0 0
2
, ZneN, ZneN , ZneO and ZneO ) were observed as shown in
NO
Fig. S1.
The UVeVisible spectra of the L, CS, CS-SalBr, and CS-Sal-Zn-L,
CS-SalBr-Zn-L, CS-SalOMe-L are depicted in Fig. 2. The electronic
spectra of L (Fig. 2a) and CS-SalBr (Fig. 2c) show two significant
bands below 400 nm. These bands might be due to n-p* and p-p*
3
. Results and discussion
transitions of azomethine C]N group and aromatic C]C group of
benzene ring, respectively, [26]. The electronic spectra of com-
plexes observed slight shift in the position and intensity of these
bands. This might be due to coordination of metal ion with ligand.
The spectra show an absorption band in the visible region
The chemical and structural features of L, CS, and CS supported
Schiff base ligands (CS-Sal, CS-SalBr, and CS-SalOMe) and com-
plexes (CS-Sal-Zn-L, CS-SalBr-Zn-L, and CS-SalOMe-Zn-L) were
studied by FT-IR, UVeVis, TGA, XRD, FESEM, EDX, AAS and
Elemental Analysis.
The FT-IR data of Schiff base Ligand L, CS, CS supported Schiff
base ligands (CS-Sal, CS-SalBr, CS-SalOMe), and complexes (CS-Sal-
Zn-L, CS-SalBr-Zn-L, CS-SalOMe-Zn-L) are given in Table S1. The FT-
IR spectra of L, CS, CS-SalBr, CS-Sal-Zn-L, CS-SalBr-Zn-L, and CS-
412e462 nm and this might be due to metal to ligand charge
transfer [27]. The magnetic property of complex CS-SalBr-Zn-L
10
revealed the diamagnetic nature as expected for d configuration
[28]. All these findings as expected form a square planar geometry
of the complex.
The TGA analysis is used to explain the thermal stability and
mode of decomposition of ligand and complexes. The TGA ther-
mogram of CS-SalBr, CS-Sal-Zn-L, CS-SalBr-Zn-L, and CS-SalOMe-
Zn-L are depicted in Fig. 3. The TGA thermogram of CS-SalBr
(Fig. 3a) shows two mass loss stages. The initial mass loss (~15%)
CHO
NH2
OH
C
H
N
Ethanol, 5-6 h
reflux
+
ꢁ
up to 100 C is attributed to the loss of physically adsorbed water
OH
NO2
NO
2
molecules present in the CS unit. The maximum and significant
second mass loss (~50%) occurs at higher temperature
Scheme 1. Synthesis of Schiff base ligand (L).

370
Anuradha et al. / Journal of Molecular Structure 1130 (2017) 368e373
OH
O
OH
O
O
HO
*
O
HO
*
*
*
n
n
C
H
N
+
Zn(OAc) .2H O
+
Ethanol, 12-15 h
reflux
HC
N
2
2
HC
N
O
O
OH
OH
NO2
Zn
N
CH
X
X
X = H
X = 5-Br
X = O-Me
NO2
Scheme 3. Synthesis of CS supported Zn(II) mixed ligand complexes (CS-Sal-Zn-L, CS-SalBr-Zn-L, and CS-SalOMe-Zn-L).
ꢁ
(
320e360 C), attributed to the decrease in the number of amino
groups after Schiff base formation [29]. Similarly, all the complexes
show two mass loss stages. The initial first mass loss (~10%), (~5%)
ꢁ
and (~14%), respectively, up to ~100 C is due to physically absorbed
water. The second mass loss (~32%), (~25%) and (~35%), respec-
ꢁ
ꢁ
tively, occur in the range of (200e240 C), (300e320 C) and
(
ꢁ
280e300 C), respectively. This may be due to the introduction of a
metal ion on polymer matrix, which obstructs the chain packing
and cause loosening of the packed structure. Thus, it may be
concluded that CS-SalBr decomposes at higher temperature and
thermally more stable than the complexes.
The XRD analysis of CS, CS-SalBr and CS-SalBr-Zn-L are shown in
Fig. 4. In the XRD spectrum of CS (Fig. 4a), the weak diffraction peak
ꢁ
observed at diffraction angle 2
q
¼ 9.18 and sharp diffraction peak
ꢁ
at 2
q
¼ 20.47 are indicative of high degree of crystallinity present
in CS [30]. In the XRD spectrum of CS-SalBr (Fig. 4b), peak observed
ꢁ
at 2q
¼ 22.22 due to reduction in the number of free amino groups
on CS after the condensation with salicyaldehyde derivative [31].
This modification leads to the deformation of hydrogen bonds and
lowering the crystallinity of CS. XRD spectrum of CS-SalBr-Zn-L
(
Fig. 4c) showed that the intensity of all peaks decreased. Thus, it
Fig. 1. FT-IR spectra of (a) L, (b) CS, (c) CS-SalBr (d) CS-Sal-Zn-L, (e) CS-SalBr-Zn-L, and
f) CS-SalOMe-Zn-L.
may be concluded that coordination of Zn(II) ion with the ligand
had taken place [32].
(
Fig. 2. UVeVisible spectra of (a) L, (b) CS, (c) CS-SalBr, and (d) CS-Sal-Zn-L, (e) CS-SalBr-Zn-L, and (f) CS-SalOMe-Zn-L.

Anuradha et al. / Journal of Molecular Structure 1130 (2017) 368e373
371
SalBr-Zn-L) (Fig. S5), indicated the uniform distribution of C, N, O, Br
and Zn on CS surface.
The Elemental Analysis data of CS, CS-SalBr, CS-Sal-Zn-L, CS-
SalBr-Zn-L, and CS-SalOMe-Zn-L are given in Table S2. The Degree
of Substitution (DS) of CS-SalBr to eNH
as 0.65 using the equation given below, proposed by Inukai et al.
34].
2
group on CS was calculated
[
aðC=NÞm ꢂ ðC=NÞo
DS ¼
n
C/N ratio of CS and its Schiff base CS-SalBr are calculated as 5.22
and 13.39, respectively. The increase in C/N value indicated the
chemical modification on CS polymeric chain. Further, with the
help of AAS analysis, the zinc content are found to be (19 mg),
(25 mg) and (20 mg) per litre for (CS-Sal-Zn-L, CS-SalBr-Zn-L, and
CS-SalOMe-Zn-L) complexes, respectively, against the calculated
value of (23 mg), (29 mg) and (25 mg), respectively.
The catalytic activities of the complexes were examined for one-
pot synthesis of amides from ketones. To optimize the reaction
conditions, various parameters (time, temperature, solvents and
catalyst loadings) were studied. The reaction of acetophenone
Fig. 3. TGA thermogram of (a) CS-SalBr, (b) CS-Sal-Zn-L, (c) CS-SalBr-Zn-L, (d) CS-
SalOMe-Zn-L.
(1 mmol) and hydroxylamine hydrochloride (1 mmol) was chosen
as a model reactant. When a blank reaction was carried out at room
temperature for 36 h (Table 1, entry 1), and under reflux (Table 1,
entry 2), no product was isolated. When the reaction was per-
formed in the presence of CS (10 mol%) in acetonitrile at room
temperature and under reflux respectively, for 30 h, no product was
observed (Table 1, entries 3e4). The same reaction was carried out
in CS (25 mol%) under reflux, no product was isolated (Table 1, entry
5). It clearly indicates that CS itself does not catalyze the reaction.
When the reaction was performed in the presence of all three
complexes (CS-Sal-Zn-L, CS-SalBr-Zn-L, and CS-SalOMe-Zn-L) at
room temperature (Table 1, entry 6e8), no product was isolated.
Subsequently, the reaction was performed in presence of 10 mol% of
the complexes (CS-Sal-Zn-L, CS-SalBr-Zn-L, and CS-SalOMe-Zn-L)
respectively, in acetonitrile under reflux, afforded the desired
products in 57%, 89% and 69% yield, respectively, (Table 1, entries
9e11). Among the three complexes, CS-SalBr-Zn-L afforded good
yield under the reflux. No significant improvement in yield of the
product was noticed when the catalyst loading was increased from
10 mol% to 20 mol% (Table 1, entry 12). Among various solvents
Fig. 4. XRD analysis of (a) CS, (b) CS-SalBr, and (c) CS-SalBr-Zn-L.
Table 1
Optimization of reaction conditions for the synthesis of amides from acetophenone
a
The FESEM images of CS, CS-SalBr, CS-Sal-Zn-L, CS-SalBr-Zn-L,
and CS-SalOMe-Zn-L are depicted in Fig. S2. FESEM micrograph of
CS (Fig. S2a) displays flaky nature, even and smooth surface. In
Fig. S2b, the roughness observed on the surface of CS-SalBr,
demonstrated the chemical modification of CS surface through
Schiff base formation. On the other hand, FESEM micrograph of
complexes CS-Sal-Zn-L, CS-SalBr-Zn-L, and CS-SalOMe-Zn-L
and hydroxylamine hydrochloride.
ꢁ
Yield (%)b
Entry Catalyst (mol%)
Time (h) Temp. ( C) Solvents
1
2
3
4
5
6
7
8
9
10
e
36
36
30
30
30
36
36
r.t.
reflux
r.t.
reflux
reflux
r.t.
CH
CH
CH
3
3
3
CN
CN
CN
e
e
e
CS (10)
CS (10)
CS (25)
CS-Sal-Zn-L (10)
CS-SalBr-Zn-L (10)
e
CH CN
3
e
CH
CH
CH
CH
CH
CH
CH
CH
e
3
CN
3
CN
3
CN
3
CN
3
CN
3
CN
3
CN
3
CN
e
e
(Fig. S2c, d and e) exhibits different morphologies. The roughness
r.t.
r.t.
e
of the surface of the complexes increases and this might be due to
the imprinting of metal ion on the modified CS [33]. The EDX
provides the detailed chemical analysis of CS, CS-SalBr, CS-Sal-Zn-L,
CS-SalBr-Zn-L, and CS-SalOMe-Zn-L is given in Fig. S3. The EDX
analysis of complexes revealed the presence of zinc in 5.97%, 7.70%,
and 6.97% in weight percent, respectively. Morphology and EDX
analysis of the residual CS-SalBr-Zn-L catalyst (recovered after the
reaction) (Fig. S4), shows no change in the surface morphology of
CS-SalBr-Zn-L and revealed the amount of zinc in 6.79% in weight
percent after 5th cycle. Further, elemental mapping was done to
understand the distribution of C, N, O, Br and Zn in complex (CS-
CS-SalOMe-Zn-L (10) 36
e
CS-Sal-Zn-L (10)
CS-SalBr-Zn-L (10)
15
03
reflux
reflux
reflux
reflux
50
reflux
reflux
reflux
reflux
57
89
69
82
e
1
1
1
1
15
16
1
2
3
4
CS-SalOMe-Zn-L (10) 11
CS-SalBr-Zn-L (20)
CS-SalBr-Zn-L (10)
CS-SalBr-Zn-L (10)
CS-SalBr-Zn-L (10)
CS-SalBr-Zn-L (10)
CS-SalBr-Zn-L (10)
12
24
24
20
15
24
H
2
O
e
DCE
PhMe
CH OH
3
51
53
Trace
17
a
Reaction conditions: acetophenone (1 mmol), hydroxylamine hydrochloride
(
1 mmol).
Isolated yield after work up.
b

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Anuradha et al. / Journal of Molecular Structure 1130 (2017) 368e373
used, such as, H
e17), CH CN was found to be the best solvent (Table 1, entry 10)
and H O was found to be the worst solvent (Table 1, entry 14). No
2
O, CH
3
CN, DCE, PhMe and CH
3
OH, (Table 1, entries
2
group migrated in preference to 4-Cl and 4-NO counterpart.
1
3
Cyclohexanone gave product in good yield (Fig. 5, entry 3j). The
1
2
isolated products were fully characterized by H NMR, FT-IR and
product was isolated, when the reaction was performed without
solvent at 50 C (Table 1, entry 13).
Mass Spectra as given in ESI.
ꢁ
After optimization of the reaction conditions, the scope of the
reaction was further extended for the synthesis of amides by the
reaction of various aromatic ketones bearing either electron-
donating group (EDG) or electron-withdrawing group (EWG). The
results obtained, are summarized in (Fig. 5, entry 1e13). Aceto-
4
. Conclusion
In summary, Schiff base ligand L, CS supported Schiff base li-
gands (CS-Sal, CS-SalBr, and CS-SalOMe) and the resultant CS
supported Zn(II) mixed ligand complexes (CS-Sal-Zn-L, CS-SalBr-
Zn-L, and CS-SalOMe-Zn-L) have been synthesized and character-
ized by various spectroscopic and instrumental techniques. All the
three complexes were found to be good heterogeneous catalyst for
one-pot synthesis of amides from ketones. The best results were
obtained with complex (CS-SalBr-Zn-L). It contains an electron
phenone bearing EWG such as -Cl, eBr and eNO
sponding amides in high yields (Fig. 5, entries 3b-3d). However,
acetophenone bearing eOCH , eOH and eNH afforded corre-
2
, afforded corre-
3
2
sponding amides in low yields (Fig. 5, entry 3e-3i). In all cases of
alkylarylketones (Fig. 5 enteries 3a-3i), it is generally the aryl group
migrates preferentially over the alkyl group. Ortho-hydrox-
yacetophenone was found to be less reactive than para-hydrox-
yacetophenone, presumably due to steric hindrance and afforded
product in low yield (Fig. 5, entry 3h). Benzophenone and its de-
2
withdrawing Br and NO group, which presumably enhances the
Lewis acidity of the complex. The reported methodology is novel in
the sense that it yields amides from ketones in one-step and the
catalysts can be easily filtered out and reused up to five times with
no significant loss of the catalytic activity.
2
rivatives such as, p-NO and p-Cl afforded corresponding product in
moderate yields (Fig. 5, entry 3k-3m). In case of 3l and 3 m, phenyl
H
O
N
R1
C
O
C
CS-SalBr-Zn-L (10 mol%)
R1
+
NH OH HCl
.
R
2
R
CH
3
CN, reflux
1
2
3
H
N
H
H
N
CH3
CH3
N
CH3
C
C
C
O
O
O
Cl
Br
3a, 89%
3b, 89%
3c, 88%
H
H
N
H
N
N
CH3
CH3
CH3
C
O
C
O
C
O
O N
H CO
H C
2
3
3
3
d, 87%
3e, 71%
3f, 70%
H
N
H
N
C
H
N
CH3
CH3
CH3
C
O
C
O
O
HO
OH
NH
2
3g, 72%
3i, 60%
3h, 66%
O
O
C
H
N
NH
C
N
H
O
Cl
3j, 88%
3k, 79%
3l, 78%
O
C
N
H
O N
2
3m, 77%
Fig. 5. The reaction of various aromatic ketones with hydroxylamine hydrochloride under optimized reaction conditions.

Anuradha et al. / Journal of Molecular Structure 1130 (2017) 368e373
373
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[
We are thankful to the CRF ISM, and SAIF Panjab University,
Chandigarh for providing help in the analysis of the samples.
Anuradha and SL acknowledge the receipt of ISM fellowship.
Appendix A. Supplementary data
[
Supplementary data related to this article can be found at http://
dx.doi.org/10.1016/j.molstruc.2016.10.053.
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