Solvent-free synthesis, spectral correlations and antimicrobial activities of some aryl imines

Article abstract of DOI:10.1016/j.saa.2012.09.039

A series of aryl imines have been synthesized by Fly-ash: H ₂SO₄ catalyzed microwave assisted process under solvent-free conditions. The yields of the imines have been found to be more than 87%. The purity of all imines has been checked using their physical constants and spectral data as published earlier in literature. The UV λ_maxCN(nm), infrared νCN(cm^-1), NMR δ(ppm) of CH and CN spectral data have been correlated with Hammett substituent constants and F and R parameters using single and multi-linear regression analysis. From the results of statistical analysis, the effect of substituents on the above spectral data has been studied. The antimicrobial activities of All synthesised imines have been studied using Bauer-Kirby method.

Full text of DOI:10.1016/j.saa.2012.09.039

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 101 (2013) 239–248
Contents lists available at SciVerse ScienceDirect
Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
Solvent-free synthesis, spectral correlations and antimicrobial activities of some
aryl imines
R. Suresh a, D. Kamalakkannan a, K. Ranganathan a, R. Arulkumaran a, R. Sundararajan a, S.P. Sakthinathan a,
S. Vijayakumar a, K. Sathiyamoorthi a, V. Mala a, G. Vanangamudi a, K. Thirumurthy b, P. Mayavel b,
G. Thirunarayanan b,
⇑
a PG & Research Department of Chemistry, Government Arts College, C-Mutlur, Chidambaram 608 102, India
b Department of Chemistry, Annamalai University, Annamalainagar 608 002, India
g r a p h i c a l a b s t r a c t
h i g h l i g h t s
"
A series of aryl imines have been
synthesized using green oxidative
coupling of amines.
Better yields of the imines (more
than 85%) obtained.
O
N
Ar'
Fly-ash:H2SO4
Ar
C
NH2
Ar
H
MW, 460W
H
Ar'
(1-75)
"
"
These aryl imines were
characterized by their analytical and
spectral data.
"
"
Antimicrobial activities of these
imines have been studied.
Easy-workup procedure,
environmentally solvent-free
synthetic method was adopted.
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 7 August 2012
Received in revised form 11 September 2012
Accepted 20 September 2012
Available online 28 September 2012
A series of aryl imines have been synthesized by Fly-ash: H2SO4 catalyzed microwave assisted process
under solvent-free conditions. The yields of the imines have been found to be more than 87%. The purity
of all imines has been checked using their physical constants and spectral data as published earlier in lit-
erature. The UV kmaxC@N(nm), infrared m
C@N(cmꢀ1), NMR d(ppm) of CAH and C@N spectral data have
been correlated with Hammett substituent constants and F and R parameters using single and multi-lin-
ear regression analysis. From the results of statistical analysis, the effect of substituents on the above
spectral data has been studied. The antimicrobial activities of All synthesised imines have been studied
using Bauer–Kirby method.
Keywords:
Solvent-free synthesis
Fly-ash: H2SO4
Crown Copyright Ó 2012 Published by Elsevier B.V. All rights reserved.
Aryl imines
IR and NMR spectra
Spectral QSAR study
Antimicrobial activities
Introduction
ing the mechanism of transamination and racemization reactions
in biological systems [1,2]. Schiff’s bases of aliphatic aldehydes
Azomethines (AC@NA) are generally known as Schiff’s bases, to
honor Hugo Schiff, who had synthesized such compounds earlier.
Schiff’s bases, are bimolecular condensation products of primary
amines with carbonyl compounds. Schiff’s bases are characterized
by the AN@CHA (imine) group which finds importance in elucidat-
are relatively unstable which readily undergo polymerization
while those of aromatic aldehydes having an effective conjugation
system are found to be more stable. Schiff bases have been re-
ported to play very important role in many biological and chemical
reactions, due to the presence of the imine linkage. Schiff’s bases
are generally bi- or tri- dentate ligands capable of forming very sta-
ble complexes with transition metals [3,4]. Schiff’s bases, derived
from aromatic amines and aromatic aldehydes are reported to be
⇑
Corresponding author. Tel.: +91 4144 220015.
E-mail address: drgtnarayanan@gmail.com (G. Thirunarayanan).
1386-1425/$ - see front matter Crown Copyright Ó 2012 Published by Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.saa.2012.09.039

240
R. Suresh et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 101 (2013) 239–248
O
N
Ar'
Therefore the authors have taken efforts for the synthesis of chiral
imines from carbonyl compounds using fly-ash: H2SO4 catalyst
with microwave irradiation under solvent-free conditions. The var-
ious spectral data of these imines have been utilized for studying
the quantitative structure- activity-relationships through Ham-
mett correlations. The biological activities of these imine deriva-
tives have been studied using Bauer–Kirby [53] method.
Fly-ash:H2SO4
MW, 460W
Ar
C
NH2
Ar
H
H
Ar'
(1-75)
Scheme 1. Synthesis of aryl imines from aryl amines and aryl aldehydes in
presence of fly-ash: H2SO4 catalyst under microwave irradiation.
involved in the study of asymmetric catalysis [5], magnetic proper-
ties [6], photochromism [7], binding with DNA [8], construction of
supra molecular structures [9], the study of activity against Ehrli-
chascites carcinoma (EAC) [10], the field of dyes and pigments
[11], the development of corrosion inhibitors [12], anti-HIV [13]
and in the evaluation of physical properties in the crystalline state
[14]. Optically active imine derivatives possess multipronged bio-
logical activities such as antimicrobial [15], anticancer [16], anti-
plasmodic-antihypoxic [17], antitubularcular [18], nematicidal-
insecticidal [19], anti-inflammatory and lipoxygenase [20]. The
imine moieties are important intermediates and versatile starting
materials for the synthesis of chiral amines [21], pyrimidine deriv-
atives [22], phenylhydrazones [23], Mannich bases [24], indoles
Experimental
Materials and methods
All the chemicals involved in the present investigation, have
been procured from Sigma–Aldrich and E-Merck chemical compa-
nies. Fly ash has been collected from Thermal Power Plant-II, Neyv-
eli Lignite Corporation (NLC), Neyveli, Tamil Nadu, India. Melting
points of all imines have been determined in open glass capillaries
on SUNTEX melting point apparatus and are uncorrected. The UV
spectra of all the imines, synthesized, have been recorded with
ELICO-BL222 spectrophotometer (kmax nm) in spectral grade meth-
anol solvent. Infrared spectra (KBr, 4000–400 cmꢀ1) have been re-
corded on AVATAR-300 Fourier transform spectrophotometer.
Bruker AV400 NMR spectrometer operating at 400 MHz has been
utilized for recording 1H NMR spectra and 100 MHz for 13C spectra
in CDCl3 solvent using TMS as internal standard. Electron impact
(EI) (70 eV) and chemical ionization mode FAB + mass spectra have
been recorded with a JEOL JMS600H spectrometer.
[25], quinoxalines [26], imidazoles [27],
[28], amino triphenylmethanes [29], Michael adducts [30], allyl
products [31], optically active -alkyl aldehydes [32] by hydroge-
a-ethoxycarbamates
a
nation [33], nucleophilic addition with organometallics [34] and
cycloaddition reaction [35]. Many reagents have been used for
the synthesis of optically active imines such as Lewis acids [36],
molecular sieves in ionic liquids [37], solid super acids, K-10 mont-
morillonite [38], Tandeam catalysts [39], MnO2 [40], CaO [41],
ZnCl2 [21], MgSO4-PPTS [23], alumina [42], Ti(OR)4 [43], CuCl
[44], MCM-41-SO3nanocatalyst [45], P2O5ASiO2 [46] promoted by
microwave irradiation [40], Cinchona alkaloid-thiourea [47], Infra-
red [48] and ultrasound radiation [49]. These catalysts have been
applied for the synthesis of chiral amines by oxidative coupling
of amines [50,51], with carbonyl compounds [19,38,39] alcohols
[31] and acid chlorides [34,36]. The microwave assisted synthesis
has become popular in academic and pharmaceutical areas since
this involves a new enabling technology for developing new drugs.
Chemists and scientists [19,38,52] prefer solvent-free microwave
synthetic methods for synthesizing organic compounds, since they
involve shorter duration, operational simplicity, easy workup pro-
cedure, less hazardousness to humans and environment and better
yields. No report has been found in the literature regarding the
synthesis of imines with fly-ash: H2SO4 catalyst under microwave
assistance and spectral as well as biological activities of imines.
Preparation and characterization of catalyst
The fly-ash: H2SO4 catalyst has been prepared following the
procedure published in literature[54]. In a 50 mL Borosil beaker,
1 g of fly-ash and 0.8 mL (0.5 mol) of sulphuric acid have been ta-
ken and mixed thoroughly with glass rod. This mixture has been
heated on a hot air oven at 85 °C for 1 h, cooled to room tempera-
ture, stored in a borosil bottle and tightly capped.
synthesis of imines
An appropriate equi-molar quantities of aryl amines (2 mmol),
substituted benzaldehydes (2 mmol) and fly-ash: H2SO4 (0.5 g)
have been taken in ACE tube and tightly capped. The mixture has
been subjected to microwave irradiation for 6–12 min in a micro-
wave oven (Scheme 1) (LG Grill, Intellowave, Microwave Oven,
NH3
NH2
Acidic site of
Fly-ash:H2SO4
Acidic site of
Fly-ash:H2SO4
H3CO
H3CO
H
O
OH2
C
X
OH
C
X
NH3
N
H
N
H
H
H
H
H3CO
H3CO
H3CO
X
X
OH2
C
X
N
N
-H2O
C
H
Acidic site of
Fly-ash:H2SO4
Acidic site of
Fly-ash:H2SO4
H
H
H3CO
H3CO
(65; X=H)
Fig. 1. The proposed mechanism for the synthesis of benzylidene-4-methoxyanilines (entry 65–75) in presence of fly-ash: H2SO4 catalyst under microwave irradiation.

R. Suresh et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 101 (2013) 239–248
241
Table 1
Analytical and mass spectral data of aryl imines synthesized by aryl amines and substituted benzaldehydes reaction of the type Ar–NH2 + Ar0–CHO ? Ar–N = CH–Ar’.
Entry Ar
Ar0
Product
M.W. Yield
(%)
M.p. (°C)
Mass (m/z)
1
2
3
4
5
6
7
8
C6H5
C6H5
C6H5N@CHC6H5
181
271
211
195
226
231
265
261
60
80
57
64
56
42
49
77
52–53 (50–
52)[56]
129-130(128-
130)[59]
56–57 (54–
56)[56]
36–37 (35–
37)[56]
88–90 (87–
89)[56]
120–121 (118–
120)[56]
94–95 (90–
93)[56]
85–86 (84–
86)[56]
–
–
–
–
–
–
–
–
C6H5
C6H5
C6H5
C6H5
C10H7
C10H7
C10H7
C10H7
3,6-OCH3 CH3
4-CH3O-C6H4
4-CH3C6H4
4-NO2C6H4
C6H5
4-CH3
OC6H4N@CHC6H3(OCH3)2(3,4)
C6H5N@CHC6H4OCH3(4)
C6H5N@CHC6H4CH3(4)
C6H5N@CHC6H4NO2(4)
C10H7N@CHC6H5
4-ClC6H4
C10H7N@CHC6H4Cl(4)
C10H7N@CHC6H4OCH3(4)
4-CH3O-C6H4
9
10
4-CH3C6H4
4-NO2C6H4
C10H7N@CHC6H4CH3(4)
C10H7N@CHC6H4NO2(4)
245
276
68
69
71 (68–70)[56]
156–158 (153–
155)[56]
–
–
C10H7
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
C5H5N
C5H5N
C5H5N
C5H5N
C5H5N
C6H5
C5H4N@CHC6H5
182
216
221
196
227
75
64
51
50
86
80
85
78
89
90
70
70
80
75
80
75
65
60
60
70
90
94
94
92
84
83
96
94–96 (92–
95)[56]
85–86 (82–
85)[56]
158–159 (155–
157)[56]
122–124 (119–
122)[56]
123–124 (120–
123)[56]
80–81 (74–
80)[57]
83–84 (78–
82)[57]
79 (75–78)[57]
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
4-ClC6H4
C5H4N@CHC6H4Cl(4)
C5H4N@CHC6H4OCH3(4)
C5H4N@CHC6H4CH3(4)
C5H4N@CHC6H4NO2(4)
4-CH3O-C6H4
4-CH3C6H4
4-NO2C6H4
4-OHC6H4
4-Cl C6H4
2-Cl,4-
2-Cl,4-NO2C6H3N@CHC6H4OH(4) 277
NO2C6H3
2-Cl, 4-
NO2C6H3
2-Cl4-
NO2C6H3
2-Cl,4-
NO2C6H3
2-Cl, 4-NO2,
C6H4
2-Cl,4-NO2C6H3N@CHC6H4Cl(4)
295
304
306
291
260
295
277
4-N-(CH3)2 C6H4
3-NO2C6H4
4-CH3OCC6H4
C6H5
2-Cl,4-NO2C6H3N@CHC6H4 N-
(CH3)2(4)
2-Cl,4-
NO2C6H3N@CHC6H4NO2(3)
2-Cl,4-
NO2C6H3N@CHC6H5OCH3(4)
NO2SC6H6 N@CHC6H5
77–78 (72–
76)[57]
78–80 (77–
79)[57]
171–181 (170–
180)[58]
202–203 (200–
202)[58]
219–220 (218–
220)[58]
202–203 (200–
202)[58]
211–212 (210–
212)[58]
116–117 (114–
116)[58]
183–184 (180–
184)[58]
177 (175–
176)[58]
182–183 (180–
182)[58]
192–193 (190–
192)[58]
114–115 (112–
115)[59]
212–213 (210–
212)[59]
105–106 (102–
105)[59]
C6H6NO2S
C6H6NO2S
C6H6NO2S
C6H6NO2S
C6H6NO2S
C6H6NO2S
C6H6NO2S
C6H6NO2S
C6H6NO2S
C6H6NO2S
4-CH3C6H4
4-CH3C6H4
4-CH3C6H4
4-CH3C6H4
4-CH3C6H4
4-CH3C6H4
3-CH3OC6H4
2-Cl-C6H4
2-OH-C6H4
4-N(CH3)2-C6H4
4-CH3OC6H4
3,4,5-(OCH3)3C6H2
4-Cl-C6H4
3-OH-C6H4
3-NO2-C6H4
C4H4O
NO2S C6H6 N@CHC6H4Cl(2)
NO2S C6H6 N@CHC6H4OH(2)
NO2S C6H6 N@CHC6H4N(CH3)2(4) 320
NO2S C6H6 N@CH C6H4OCH3(4)
321
383
295
277
306
251
195
212
212
226
239
241
211
NO2SC6H6 N@CH C6H2(O
CH3)3(3,4, 5)
NO2S C6H6 N@CH C6H4Cl(4)
NO2SC6H6 N@CH C6H4OH(3)
NO2SC6H6 N@CH C6H4NO2(3)
NO2SC6H6N@CH C4H4O
C6H5
4-CH3C6H4N@CHC6H5
4-OH C6H4
2-OHC6H4
4-CH3OC6H4
4-N(CH3)2 C6H4
4-NO2C6H4
C6H5
4-CH3C6H4N@CHC6H4OH(4)
4-CH3C6H4N@CHC6H4OH(2)
4-CH3C6H4N@CHC6H4OCH3(4)
95–96 (93–
95)[59]
96 (92–95)[59]
4-CH3C6H4N@CHC6H4-
N(CH3)2(4)
4-CH3C6H4N@CHC6H4NO2(4)
125–126 (121–
125)[59]
156–157 (154–
157)[59]
3-CH3OC6H4N@CHC6H5
(continued on next page)

242
R. Suresh et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 101 (2013) 239–248
Table 1 (continued)
Entry Ar
Ar0
Product
M.W. Yield
(%)
M.p. (°C)
Mass (m/z)
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
4-CH3OC6H4
4-OH C6H4
2-OHC6H4
3,4-OCH3 C6H3
4-N(CH3)2 C6H4
4-NO2C6H4
C6H5
4-CH3OC6H4N@CHC6H4OH(4)
4-CH3OC6H4N@CHC6H4OH(2)
228
228
271
255
257
260
277
277
304
306
226
257
257
272
398
351
306
231
197
172
187
203
189
189
216
218
207
93
91
80
93
85
82
87
88
78
80
75
42
85
72
82
78
81
80
70
75
82
80
92
78
70
85
82
221–22 (215–
220)[59]
149–150 (145–
149)[59]
129-130(128-
130) [59]
135–136 (132–
135)[59]
71–72 (68–
72)[59]
62–63 (61–
62)[59]
170–171 (169–
170)[59]
177–178 (175–
177)[59]
192–193 (189–
192)[59]
175–176 (172–
175)[59]
140–141 (138–
140)[59]
160–161 (158–
161)[59]
162–163 (158–
162)[59]
182–183 (178–
182)[59]
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
4-CH3OC6H4
4-CH3OC6H4
4-CH3OC6H4
4-CH3OC6H4
4-BrC6H4
4-BrC6H4
4-BrC6H4
4-BrC6H4
4-BrC6H4
4-NO2C6H4
4-NO2C6H4
4-NO2C6H4
4-NO2-C6H4
4-IC6H4
4-CH3
OC6H4N@CHC6H3(OCH3)2(3,4)
4-CH3OC6H4N@CHC6H4-
N(CH3)2(4)
4-CH3OC6H4N@CHC6H4NO2(4)
4-BrC6H4N@CHC6H5
4-OH C6H4
2-OHC6H4
4-N(CH3)2C6H4
4-NO2C6H4
C6H5
4-BrC6H4N@CHC6H4OH(4)
4-BrC6H4N@CHC6H4OH(2)
4-BrC6H4N@CHC6H4-N(CH3)2(4)
4-BrC6H4N@CHC6H4NO2(4)
4-NO2C6H4N@CHC6H5
4-OH C6H4
2-OHC6H4
4-NO2C6H4
4-NO2C6H4N@CHC6H4OH(4)
4-NO2C6H4N@CHC6H4OH(2)
4-NO2C6H4N@CHC6H4NO2(4)
4-OH,3-OCH3, 5-
NO2C6H2
4-OH-3-OCH3-5-
NO2-C6H2
4-OH,3-CH3O-, 5-
NO2 C6H2
4-HOC6H4
4-IC6H4N@CHC6H2 4-OH,3-
OCH3,5-NO2
4-BrC6H4N@CHC6H2 4-OH, 3-
OCH3,5-NO2
4-ClC6H4N@CHC6H2 4-OH, 3-
OCH3,5-NO2
4-ClC6H4N@CHC6H4OH(4)
135–36 (135)[60]
4-BrC6H4
4-ClC6H4
4-ClC6H4
4-HOC6H4
C2H2N3
146–147
(145)[60]
143–44 (143)[60]
233–234
(232)[61]
197–198
(197)[61]
194–195
(194)[62]
191–192
(190)[62]
195–196
(195)[62]
178–179
(178)[62]
194–195
(193)[62]
235–236
(235)[62]
238–239
(238)[62]
205–206
(205)[62]
50–51
C6H5
4-HOC6H4N@CHC6H5
C6H5
C2H2N3N@CHC6H5
C2H2N3
4-CH3C6H4
4-CH3OC6H4
2-OHC6H4
4-OHC6H4
4-N(CH3)2 C6H4
4-NO2C6H4
4-ClC6H4
C2H2N3N@CHC6H4CH3(4)
C2H2N3N@CHC6H4OCH3(4)
C2H2N3N@CHC6H4OH(2)
C2H2N3N@CHC6H4OH(4)
C2H2N3N@CH2C6H4N(CH3)2(4)
C2H2N3N@CHC6H4NO2(4)
C2H2N3N@CHC6H4Cl(4)
C2H2N3
C2H2N3
C2H2N3
C2H2N3
C2H2N3
C2H2N3
65
66
C6H4OCH3(4) C6H5
C6H4OCH3(4) 2-BrC6H4
(4)H3COC6H4N@CHC6H5
(4)H3COC6H4N@CHC6H4Br(2)
177
276
90
86
177[M+],180, 134, 121,107, 104, 90, 77, 31
276[M+],278[M2+],257, 181, 167, 154,134,
121,107, 80, 77, 31
54–56
67
68
69
70
71
72
73
74
75
C6H4OCH3(4) 4-BrC6H4
C6H4OCH3(4) 2-ClC6H4
C6H4OCH3(4) 3-ClC6H4
C6H4OCH3(4) 4-ClC6H4
C6H4OCH3(4) 4-CH3C6H4
C6H4OCH3(4) 4-OCH3C6H4
C6H4OCH3(4) 2-NO2C6H4
C6H4OCH3(4) 3-NO2C6H4
C6H4OCH3(4) 4-NO2C6H4
(4)H3COC6H4N@CHC6H4Br(4)
(4)H3COC6H4N@CHC6H4Cl(2)
(4)H3COC6H4N = CHC6H4Cl(3)
(4)H3COC6H4N = CHC6H4Cl(4)
(4)H3COC6H4N = CHC6H4CH3(4)
276
231
231
231
211
92
85
90
86
90
85
92
86
80
138–139
51–53
276[M+],278[M2+],257, 181, 167, 154,
121,134, 107, 31
231[M+],233[M2+], 214, 138, 134,
124,121,111, 107, 31
231[M+],233[M2+],214, 138, 138,134,
124,121,111,107,31
231[M+], 233[M2+], 214,138, 134, 124,121,
116, 111, 107, 31
59–60
81–82 (79–
81)[63]
60–61
211[M+], 210,194,134,121,118, 107, 104, 91,
77, 15, 31
227[M+],210,134, 121,120, 107, 91, 77, 15, 31
(4)H3COC6H4N = CHC6H4OCH3(4) 227
140 (138–
139)[63]
64–65
(4)H3COC6H4N@CHC6H4NO2(2)
(4)H3COC6H4N@CHC6H4NO2(3)
(4)H3COC6H4N@CHC6H4NO2(4)
242
242
242
242[M+], 225,134,122,149, 135, 121, 107, 77,
31
242[M+],225,134,135, 149,122,121, 107, 93,
77, 31
242[M+],225,134,135, 149,122,121, 107, 93,
77, 31
59–60
125–126 (124–
125)[63]

R. Suresh et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 101 (2013) 239–248
243
O
X
NH2
N
C
H
H
Fly-ash:H2SO4
MW, 460W
H3CO
H3CO
X
(65-75)
X=H, 2-Br, 4-Br, 2-Cl, 3-Cl, 4-Cl, 4-Me, 4-OMe, 2-NO2, 3-NO2, 4-NO2
Scheme 2. Synthesis of substituted benzylidene-4-methoxyanilines (entries 65–75) in presence of fly-ash: H2SO4 catalyst under microwave irradiation.
160–800 W) and then cooled to room temperature. After separat-
ing the organic layer with dichloromethane the solid product has
been obtained on evaporation. The solid, on recrystallization from
benzene-hexane mixture afforded glittering product. The insoluble
catalyst has been recycled by washing with ethyl acetate (8 mL)
followed by drying in an oven at 100 °C for 1 h and reused for fur-
ther reactions.
lytic effect and the yield is shown in Fig. 2. The optimum quantity
of catalyst observed is found to be 0.5 g for the synthesis of imines.
The catalyst has been reused for the condensation reaction of
amines and benzaldehydes and it is given in Table 2. From the Ta-
ble 2, it is evident that the yield of the product is 87% in first two
runs. But the yield of the product is found to be 86.5%, 86.5% and
86% in third, fourth and fifth runs, respectively. It is observed that
there is no appreciable loss of catalytic activity up to fifth run.
Results and discussion
Spectral linearity
The waste air-pollutant fly-ash has many chemical species
[54,55] SiO2, Fe2O3, Al2O3, CaO, MgO and insoluble residues. The
waste fly-ash has been converted into useful catalyst fly-ash:
H2SO4 by mixing fly-ash and sulphuric acid. The sulphuric acid
and other chemical species present in the fly-ash have enhanced
the catalytic activity. During the course of the reactions these spe-
cies are found responsible for promoting condensation between
the aryl amines and aryl aldehydic groups leading to the formation
of imines. The proposed general reaction mechanism is shown in
Fig. 1. In these experiments the products have been isolated and
the catalyst has been washed with ethyl acetate, heated to
100 °C then reused for further five runs. There was no appreciable
change in the percentage of yield of imines. In this protocol the
reaction gave better yields of the imines during the condensation
without any environmental discharge. The analytical and mass
spectral data are presented in Table 1. The complete NMR spectral
data of unknown imines are presented in Supplementary Data (Ta-
bles S1and S2, See Supplementary Data). We have investigated the
catalytic effect of Fly-ash: H2SO4 on the synthezis of imines (Entry
65; Scheme 2) by varying the quantity of catalyst from 0.5 g to
1.5 g. As the quantity of catalyst increases from 0.5 g to 1.5 g, the
percentage of yield of the product is found to increase from 85%
to 87%. There is no significant increase of the percentage of the
product on further increase in the quantity of catalyst. This cata-
In the present study the spectral linearity of synthesized imines
has been studied by evaluating the substituent effects. The spectral
data observed for the imines, UV kmax (nm), infrared mC@N, the
proton chemical shifts d(ppm), of CAH and carbon chemical shifts
of C@N are correlated with various substituent constants.
UV–Vis spectral study
The measured absorption maxima (kmax nm) of synthesized imi-
nes (entries 65–75) are presented in Table 3. These values have
been correlated with Hammett substituent constants and F and R
parameters using single and multi-linear regression analysis [64–
66]. Hammett equation employed, for the correlation analysis,
involving the absorption maxima is as shown below in Eq. (1).
k ¼
q
r
þ ko
ð1Þ
where ko is the frequency for the parent member of the series. The
results of statistical analysis [64–66] of these values with Hammett
substituent constants are presented in Table 3.
Table 2
Reusability of catalyst on condensation of 4-methoxy aniline (2 mmol) and benzal-
dehydes (2 mmol) (entry 65) under microwave irradiation.
Run
1
2
3
4
5
Yield
87
87
86.5
86.5
86
Table 3
The UV, IR and NMR spectroscopic data of substituted benzylidene-4-methoxyani-
lines (entries 65–75).
Entry
X
kmax
(nm)
m
C@N
d1H (ppm)
CH
d
13C (ppm)
C@N
(cmꢀ1
)
65
66
67
68
69
70
71
72
73
74
75
H
2-Br
4-Br
2-Cl
3-Cl
4-Cl
4-Me
4-OCH3
2-NO2
3-NO2
4-NO2
329
338
340
341
335
335
330
326
350
351
377
1621.32
1617.80
1621.04
1620.27
1622.19
1629.40
1621.50
1615.88
1618.72
1619.65
1594.26
8.487
8.866
8.427
8.939
8.428
8.448
8.442
8.408
8.966
8.953
8.585
158.56
158.70
158.53
158.67
158.64
158.50
158.63
162.02
159.11
159.11
159.26
Fig. 2. Effect of catalyst loading.

244
R. Suresh et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 101 (2013) 239–248
Table 4
Results of statistical analysis of UV kmax (nm), IR m
C@N (cmꢀ1), NMR d1H (ppm)CH@N and d13C (ppm) C@N of substituted Benzylidene-4-methoxyanilines (entries 65–75)
with Hammett substituent constants
r
,
r+
,
rI,
r
R, F and R parameters.
Frequency
Constants
r
I
q
s
n
Correlated derivatives
kmax (nm)
r
0.972
0.872
0.769
0.754
0.864
0.931
332.31
335.38
324.07
346.51
322.71
347.32
32.58
21.50
42.35
52.44
46.41
47.21
8.57
11
11
11
11
11
11
H, 2-Br, 4-Br,2-Cl,3-Cl,4-Cl,4-Me,4-OMe,2-NO2,3-NO2,4-NO2
r+
rI
rR
F
10.30
10.56
10.78
10.11
10.82
R
m
C@N (cmꢀ1
)
r
0.763
0.826
0.832
0.952
0.856
0.931
1620.82
1619.67
1623.08
1616.69
1624.02
1616.68
ꢀ8.87
ꢀ4.73
8.52
8.83
8.68
8.51
8.51
8.68
11
11
11
11
11
11
H, 2-Br, 4-Br,2-Cl,3-Cl,4-Cl,4-Me,4-OMe,2-NO2,3-NO2,4-NO2
H, 2-Br, 4-Br,2-Cl,3-Cl,4-Cl,4-Me,4-OMe,2-NO2,3-NO2,4-NO2
H, 2-Br, 4-Br,2-Cl,3-Cl,4-Cl,4-Me,4-OMe,2-NO2,3-NO2,4-NO2
r+
rI
rR
F
ꢀ11.57
ꢀ16.87
ꢀ14.10
ꢀ13.21
R
d CH@N (ppm)
d C@N (ppm)
r
0.758
0.846
0.754
0.834
0.855
0.842
8.52
8.54
8.41
8.68
8.39
8.69
0.39
0.33
0.55
0.47
0.59
0.53
0.21
0.19
0.21
0.24
0.21
0.22
11
11
11
11
11
11
r+
rI
rR
F
R
r
0.830
0.953
0.802
0.836
0.701
0.742
159.30
159.39
159.11
158.87
159.03
158.76
ꢀ0.85
ꢀ1.19
ꢀ0.10
ꢀ1.94
0.08
1.02
0.88
1.07
0.99
1.07
0.93
11
11
11
11
11
11
r+
rI
rR
F
R
ꢀ2.41
r = correlation co-efficient;
q = slope; I = intercept; s = standard deviation; n = number of substituents.
m
¼
qr
þ
m0
ð4Þ
CH3
O
where mo is the frequency for the parent member of the series.
The observed
C@N stretching frequencies (cmꢀ1) are corre-
m
N
lated with various Hammett substituent constants and F and R
parameters through single and multi-regression analyses including
Swain–Lupoton’s parameters. The results of statistical analysis of
single parameter correlation are shown in Table 4. The correlation
C
H
H3CO
of
ent constants and R parameters is found to be satisfactory, with
negative value. The remaining constants fail to give a satisfactory
m
C@N (cmꢀ1) frequencies of imines with Hammett rR substitu-
Fig. 3. The resonance-conjugative structure.
q
correlation. This is due to the absence of polar, field and inductive
effects of the substituents and hence they are unable to predict the
reactivity on C@N stretches. This is associated with the conjugative
structure shown in Fig. 3. In short some of the single parameter
From Table 4, it is evident that Hammett
parameters give satisfactory correlations with kmax. The rI
and F values give poor correlations. All constants give positive
r constants and R
,
rR
q
correlations of m
C@N (cmꢀ1) frequencies with Hammett substitu-
values. This is due to the fact that the polar, resonance, filed and
inductive effects of the substituents are sufficiently weaker for pre-
dicting the reactivity on the absorption through conjugation. This
is attributed to the resonance conjugative structure shown in
Fig. 3. The multi regression analysis of these UV spectral data of
all imines with inductive, resonance and Swain–Lupton’s [67] con-
stants produce satisfactory correlations as shown in Eqs. (2) and
(3).
ent constants fail in correlation. So, the authors think that it is
worthwhile to seek the multi regression analysis which may pro-
duce a satisfactory correlation with Resonance, Field and Swain–
Lupoton’s [67] constants. This is shown in the following Eqs. (5)
and (6).
m
C@Nðcmꢀ1Þ¼1620:21ðꢁ6:39Þꢀ7:77ðꢁ2:45ÞrI þ13:34ðꢁ5:74ÞrR
ðR¼0:946;n¼11;P>90%Þ
ð5Þ
kmaxðnmÞ ¼ 332:26ðꢁ6:00Þþ31:51ðꢁ11:69ÞrI þ38:12ðꢁ14:78ÞrR
ðR ¼ 0:951;n ¼ 11;P > 90%Þ
ð2Þ
m
C@Nðcmꢀ1Þ ¼ 1621:70ðꢁ6:93Þꢀ10:95 ðꢁ3:49ÞF ꢀ8:32 ðꢁ1:63ÞR
ðR ¼ 0:945;n ¼ 11;P > 90%Þ
ð6Þ
kmaxðnmÞ ¼ 331:63 ðꢁ6:49Þ þ 34:31 ðꢁ12:62ÞF þ 31:92 ðꢁ13:69ÞR
ðR ¼ 0:985; n ¼ 11; P > 95%Þ
ð3Þ
1H NMR spectral study
IR Spectral study
The 1H NMR spectra of the imine derivatives under investiga-
tion have been recorded in deuteriochloroform solution employing
tetramethylsilane (TMS) as internal standard. The signals of the
imine protons have been assigned and are presented in Table 3.
In nuclear magnetic resonance spectra, the 1H or the 13C chem-
ical shifts (d, ppm) depend on the electronic environment of the
nuclei concerned. These chemical shifts have been correlated with
The infrared
m
C@N stretching frequencies (cmꢀ1
)
of the
synthesized imines (entries 65–72) have been recorded and
presented in Table 2. These data are correlated [64–66] with
Hammett substituent constants and Swain–Lupoton’s [67] param-
eters. In this correlation the structure parameter Hammett
equation employed is as shown in Eq. (4).

R. Suresh et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 101 (2013) 239–248
245
Table 5
Antibacterial activity of substituted benzylidene-4-methoxyanilines (entries 65–75).
Zone of Inhibition (mm)
Entry
X
Gram positive bacteria
Gram negative
bacteria
B.
subtilis
Micrococcus S.
aureus
E. coli Pseudomonas
65
66
67
68
69
70
71
72
73
74
75
H
2-Br
4-Br
2-Cl
3-Cl
4-Cl
4-Me
4-OMe
2-NO2
3-NO2
4-NO2
7
7
14
9
8
7
8
–
10
7
7
–
7
11
8
8
6
10
–
PLATE-1
PLATE-3
PLATE-2
PLATE-4
–
–
–
–
6
9
8
8
9
8
6
7
–
7
7
8
7
–
–
8
7
7
11
7
6
11
8
8
9
9
9
8
10
9
10
11
17
Standard Ampicillin 13
Control DMSO
19
14
13
ton’s [67] F and R parameters. This is shown in the following Eqs.
(8) and (9) for protons:
ðppmÞ
d
¼ 8:46 ðꢁ0:16Þ þ 1:47 ðꢁ0:31ÞrI þ 1:26 ðꢁ0:39ÞrR
ðR ¼ 0:954; n ¼ 11; P > 90%Þ
CH
ð8Þ
ð9Þ
ðppmÞ
PLATE-5
PLATE-7
PLATE-9
PLATE-6
PLATE-8
PLATE-10
d
¼ 8:49 ðꢁ0:17Þ ꢀ 1:47 ðꢁ0:33ÞF ꢀ 1:32 ðꢁ0:35ÞR
ðR ¼ 0:961; n ¼ 11; P > 95%Þ
CH
13C NMR spectral study
Organic chemists and researchers [64–66] have made extensive
study of 13C NMR spectra for a large number of ketones, styrenes,
styryl ketones and keto-epoxides. They have studied linear correla-
tion of the chemical shifts (ppm) of C , Cb and CO carbons with
a
Hammett
r constants in alkenes, alkynes, acid chlorides and sty-
renes. In the present study, the chemical shifts (ppm) of imine
C@N carbon, have been assigned and are presented in Table 3. At-
tempts have been made to correlate the dC@N chemical shifts
(ppm) with Hammett substituent constants, field and resonance
parameters, with the help of single and multi-regression analyses
to study the reactivity through the effect of substituents.
The chemical shifts (ppm) observed for the dC@N have been
correlated [64–66] with Hammett constants and the results of sta-
tistical analysis are presented in Table 4. The dC@N chemical shifts
(ppm) give satisfactory correlation with Hammett r+ constants.
Fig. 4. Antibacterial activity of substituted benzylidene-4-methoxyanilines (entries
The remaining Hammett
r, rI, rR, F and R parameters are found
to fail in correlation. This is due to the reason stated earlier with
resonance conjugative structure as shown in Fig. 3.
65–75).
reactivity parameters. Thus the Hammett equation has been used
in the form as shown in Eq. (7).
In view of inability of some of the
vidually satisfactory correlation, the authors think that it is worth-
while to seek multiple correlation involving all rI R, F and R
r constants to produce indi-
,
r
Logd ¼ Logd0 þ
qr
ð7Þ
parameters [67]. This is given in the following correlation Eqs.
where d0 is the chemical shift of the corresponding parent com-
pound. The assigned proton chemical shifts (ppm) of imines have
been correlated [64–66] with various Hammett sigma constants, F
and R parameters. The results of statistical analysis are presented
in Table 4. These proton chemical shifts (ppm) fail in correlation
with Hammett substituent constants and F and R parameters. All
(10) and (11).
dC@NðppmÞ ¼ 158:64 ðꢁ0:76Þ þ 0:52 ðꢁ1:48ÞrI ꢀ 2:18 ðꢁ1:87ÞrR
ðR ¼ 0:938; n ¼ 11; P > 90%Þ
ð10Þ
dC@NðppmÞ ¼ 158:21 ðꢁ0:74Þ þ 1:20 ðꢁ1:44ÞF ꢀ 2:94 ðꢁ1:56ÞR
ðR ¼ 0:985; n ¼ 11; P > 95%Þ ð11Þ
correlations give positive
q values. This shows that the normal sub-
stituent effect operates in all systems. The failure in correlation is
attributed to the conjugative structure shown in Fig. 3.
In view of the inability of the Hammett
r
constants to produce
Microbial activities
individually satisfactory correlations with the imine proton chem-
ical shifts, the authors think that, it is worthwhile to seek multiple
correlations involving either rI and rR constants or Swain–Lup-
Imine derivatives possess a wide range of biological activities
[1,2,13,15–20]. These multipronged activities are associated with

246
R. Suresh et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 101 (2013) 239–248
Fig. 5. Antibacterial activity of substituted benzylidene-4-methoxyaniline (entries 65–75).
different substituents and the unsaturation of C@N moiety in be-
tween the aryl rings. Hence, it is intended to examine their antimi-
crobial activities against their respective microbes-bacterial and
fungal strains.
synthesized imines have been studied against three gram positive
pathogenic strains Micrococcus luteus, Bacillus subtilis Staphylococ-
cus aureus and two gram negative strains Escherichia coli and Kleb-
siella Pneumoniae species. The disc diffusion technique was
followed using the Kirby–Bauer [53]method, at a concentration
of 250 lg/mL with Ampicillin used as the standard drug. The zone
Antibacterial sensitivity assay
of inhibition is compared using Table 5 and the corresponding
Clustered column Chart is shown in Fig. 5. A very good antibacte-
rial activity has been possessed by all substituents on the microor-
ganisms in general. All the compounds showed excellent activities
on Micrococcus luteus and E.coli species. The substituents H, 4-Cl
and 3-NO2 have very good activity against Pseudomonas. The sub-
stituents 4-Cl and 2-NO2 have improved antibacterial activity
against Bacillus subtilis than other species.
Antibacterial assay has been performed by using Kirby-Bauer
[53]disc diffusion technique. In each Petri plate about 0.5 mL of
the test bacterial sample has been spread uniformly over the solid-
ified Mueller Hinton agar using sterile glass spreader. Then the
discs with 5 mm diameter made up of Whatmann No. 1 filter pa-
per, impregnated with the solution of the compound have been
placed on the medium using sterile forceps. The plates have been
incubated for 24 h at 37 °C by keeping the plates upside down to
prevent the collection of water droplets over the medium. After
24 h, the plates have been visually examined and the diameter val-
ues of the zone of inhibition were measured. Triplicate results have
been recorded by repeating the same procedure.
Antifungal assay
Antifungal assay has been performed using Kirby–Bauer [53]
disc diffusion technique. PDA medium was prepared and sterilized
as above. It has been poured (ear bearing heating condition) in the
Petri-plate which has been already filled with 1 mL of the fungal
species. The plates have been rotated clockwise and counter
clock-wise for uniform spreading of the species. The discs have
been impregnated with the test solution. The test solution has been
prepared by dissolving 15 mg of the Schiff ‘s bases in 1 mL of DMSO
solvent. The medium has been allowed to solidify and kept for
24 h. Then the plates have been visually examined and the diame-
The antibacterial screening effect of synthesized Schiff’s bases is
shown in Fig. 4 (Plates 1–10). The antibacterial activities of all the
Table 6
Antifungal activities of substituted benzylidene-4-methoxyanilines (entries 65–75).
Entry
X
Zone of Inhibition (mm)
PLATE-1
PLATE-2
A. niger
Penicilium scup
65
66
67
78
H
2-Br
4-Br
2-Cl
6
7
7
6
8
7
8
8
6
6
69
3-Cl
70
4–Cl
7
–
71
72
73
75
75
Standard
Control
4-Me
4-OMe
2-NO2
3-NO2
4-NO2
Miconazole
DMSO
–
–
6
–
8
6
7
7
8
8
PLATE-3
PLATE-4
14
15
Fig. 6. Antifungal activity of substituted benzylidene-4-methoxyanilines (entries-
65–75).

R. Suresh et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 101 (2013) 239–248
247
Fig. 7. Antifungal activity of substituted benzylidene-4-methoxyanilines (entries 65–75) – clustered column chart.
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results have been recorded by repeating the same procedure.
The antifungal activities of substituted imines have been stud-
ied and are shown in Fig. 6 for Plates (1–4) and the zone of
inhibition values of the effect is given in Table 6. The clustered
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pounds have moderate antifungal activity against both the fungal
species namely A.niger and Penicilium scups. The Schiff’s bases
having substituents 3-Cl, 3NO2 and 4-NO2 have shown good anti-
fungal activity against A.niger alone. The substituents 2-Br, 3-Br
and 2-NO2 have shown good antifungal activity against Penicilium
scup alone.
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A series of aryl imines have been synthesized by oxidative cou-
pling of aryl amines and substituted benzaldehydes using micro-
wave irradiation in the presence of fly-ash: H2SO4 under solvent-
free conditions. This reaction protocol offers a simple, eco-friendly,
non-hazardous, easier work-up procedure and high yields. These
imines were characterized by their physical constants, spectral
data. The UV, IR, NMR spectral data of these imines have been cor-
related with Hammett substituent constants, F and R parameters.
From the results of statistical analyses, the effects of substituents
on the spectral data have been studied. The antimicrobial activities
of all synthesized imines have been studied using Bauer-Kirby
method.
Acknowledgement
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The authors thank DST NMR Facility Unit, Department of Chem-
istry, Annamalai University, Annamalainagar-608002 for recording
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