Synthesis, crystal structure and spectroscopic study of para substituted 2-hydroxy-3-methoxybenzalideneanilines

Article abstract of DOI:10.1016/S0022-2860(03)00453-8

Eight Schiff bases derived from 2-hydroxy-3-methoxybenzaldehyde and different para substituted anilines have been synthesized and their structures were elucidated by physical measurements and FT-IR. NMR assignments were made using ¹H, ¹³

Full text of DOI:10.1016/S0022-2860(03)00453-8

Journal of Molecular Structure 658 (2003) 87–99
www.elsevier.com/locate/molstruc
Synthesis, crystal structure and spectroscopic study of para
substituted 2-hydroxy-3-methoxybenzalideneanilines
a,
a
b
b
*
Guan-Yeow Yeap , Sie-Tiong Ha , Nobuo Ishizawa , Katsumi Suda ,
Peng-Lim Boeya, Wan Ahmad Kamil Mahmooda
aSchool of Chemical Sciences, Universiti Sains Malaysia, Minden 11800, Penang, Malaysia
bMaterials and Structures Laboratory, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226-8503, Japan
Received 8 May 2003; revised 24 June 2003; accepted 3 July 2003
Abstract
Eight Schiff bases derived from 2-hydroxy-3-methoxybenzaldehyde and different para substituted anilines have been
synthesized and their structures were elucidated by physical measurements and FT-IR. NMR assignments were made using 1H,
13C NMR and aided by 2D COSY homonuclear, HMQC and HMBC heteronuclear correlation techniques. IR spectral analysis
of the model compounds was found useful in understanding the degree of stabilization upon this series of enol-imino tautomers,
which possess different substituents in the aniline fragment. In order to rationalize the stabilization of tautomer in solid state, the
crystallography data of 1-{(4-methylphenylimino)methyl}- and 1-{(4-chlorophenylimino)methyl}-2-hydroxy-3-methoxyphe-
nol were adopted wherein the 4-chloro (4-Cl) and 4-methyl (4-CH3) containing compounds crystallized into orthorhombic
lattice with a non-centrosymmetric space group P212121: The relationship between the stabilization of bonding involved in
heteronuclear six-membered ring of the tautomer and the conformation of the molecules in crystal phase was reported.
q 2003 Elsevier B.V. All rights reserved.
Keywords: 2-Hydroxy-3-methoxybenzaldehyde; Schiff bases; FT-IR; NMR; Single crystal X-ray diffraction analysis
1. Introduction
are prompted to generate the derivatives by introdu-
cing different substituents into the existing skeleton of
the molecule. The presence of ortho hydroxyl group,
for instance, has been regarded as one of the
importance elements which favours for the existence
of intramolecular hydrogen bonding (O–H· · ·N and
O· · ·H–N) and also the tautomerism which accounts
for the formation of either enol-imino or keto-amino
tautomer [3].
It has well been documented that Schiff bases are
important in diverse fields of chemistry and biochem-
istry owing to their biological activities [1,2]. Apart
from the biological activities, photochromism is
another characteristic of these materials leading to
its application in various areas such as the control and
measurement of radiation intensity, display systems
and optical computers. In view of the importance and
also the usefulness of these compounds, the chemists
In the field of coordination chemistry, this type of
ortho hydroxylated Schiff bases has received an
overwhelming attention particularly on the study of
complex formation [4–7]. Recently, the liquid
crystals researchers have also made a significant
*
Corresponding author. Fax: þ60-4657-4854.
E-mail address: gyyeap@usm.my (G.-Y. Yeap).
0022-2860/03/$ - see front matter q 2003 Elsevier B.V. All rights reserved.
doi:10.1016/S0022-2860(03)00453-8

88
G.-Y. Yeap et al. / Journal of Molecular Structure 658 (2003) 87–99
revelation that the introduction of lateral polar
hydroxy group will enhance the molecular polariz-
ability as well as to stabilize the liquid crystalline
compounds [8]. One of the typical examples is the
study on the effect of lateral hydroxyl group on
mesomorphism of azobenzene derivatives [9].
spectrophotometer in the frequency range 4000–
400 cm21. All samples were prepared in KBr discs.
2.3. NMR measurements
NMR spectra were recorded in CDCl3 at 298 K on
a Bruker 400 MHz Ultrashield Spectrometer equipped
with a 5 mm BBI inverse gradient probe. Chemicals
shifts were referenced to internal TMS. The concen-
tration of solute molecules was 150 mg in 1.0 ml
CDCl3. Standard Bruker pulse programs [10] were
used throughout the entire experiment. The spectro-
scopic details of NMR were summarized in Table 1.
In this paper, we report a series of Schiff bases
whereinthesubstituentsof2-hydroxyand3-methoxyin
the aldehyde ring were remained unaltered. However,
the aniline used in the reaction with this aldehyde was
either non-substituted or substituted by F, Cl, Br, NO2,
CH3, OCH3 and CN at para position. The effects
attributed to the emergence of the tautomeric con-
formers in crystal and liquid phases were studied by
spectroscopic methods and X-ray diffraction analysis.
2.4. X-ray data collection, structure solution and
refinement for 1-{(4-methylphenyl-imino)methyl}-2-
hydroxy-3-methoxyphenol (3) and 1-{(4-
methylphenylimino)-methyl}-2-hydroxy-3-
methoxyphenol (6)
2. Experimental section
4-Bromoaniline, 4-chloroaniline and 4-amino-
phenol were obtained from Merck (Germany). 4-
Nitroaniline and 4-methylaniline were purchased
from Riedel-de Haen (Germany). Aniline, 4-fluor-
oaniline and 4-aminobenzonitrile and 4-methoxya-
niline were obtained from BDH (England), Acros
Organics (USA) and Fluka Chemie (Switzerland),
respectively. 2-Hydroxy-3-methoxybenzaldehyde (o-
vanilin) was purchased from TCI Chemical Com-
pany (Japan).
Crystal data are given in Table 2. Whist
the final coordinates and equivalent isotropic
displacement parameters are given in Table 3, the
selected bond lengths, angles and torsion angles
are given in Table 4. The molecular structures with
2.1. Synthesis of Schiff bases
The Schiff bases were synthesized by mixing
equimolar amounts of o-vanilin with the appropriate
aniline, both dissolved in absolute ethanol. The
reaction mixture was refluxed for 5 h and the
precipitate thus formed upon cooling to room
temperature (300 K) was filtered. The solid residue
was crystallized from absolute ethanol or chloroform
and dried under reduced pressure. The representation
of Schiff bases with numbering scheme is shown in
Scheme 1.
2.2. FT-IR measurements
The FT-IR spectra of Schiff bases of o-vanilin were
recorded by using a Perkin Elmer 2000-FT-IR
Scheme 1.

G.-Y. Yeap et al. / Journal of Molecular Structure 658 (2003) 87–99
89
Table 1
Acquisition parameters used in the NMR measurements
Parameters* Experiment
1H NMR
13C NMR
2D COSY
2D HMQC
2D HMBC
SF
400.1 MHz
20 ppm
100.6 MHz
250 ppm
400.1 MHz
18 ppm
F1 ¼ 100:6 MHz
F2 ¼ 400:1 MHz
F1 ¼ 200 ppm
F2 ¼ 20 ppm
F1 ¼ 100:61 MHz
F2 ¼ 400:1 MHz
F1 ¼ 200 ppm
F2 ¼ 20 ppm
SW
PW
AQ
D1
8.3 ms (308 flip angle) 20.0 ms (908 flip angle) 8.3 ms (908 flip angle) 8.3 ms (908 flip angle) 8.3 ms (908 flip angle)
4.0 s
1.0 s
16
1.3 s
2.0 s
1879
65 K
0.3 s
0.09 s
0.4 s
2.0 s
1.0 s
1.0 s
NS
TD
8
20
18
66 K
F1 ¼ 256
F1 ¼ 512
F1 ¼ 512
F2 ¼ 2048
F2 ¼ 1024
F2¼4096
*Abbreviations: F1, 13C channel; F2; 1H channel (except 2D COSY where F1 and F2 are 1H channels); SF, spectrometer frequency; SW,
spectral width; PW, pulse width; AQ, acquisition time; D1, relaxation delay, NS, number of scan; TD, number of data points.
the atom-numbering scheme for 3 and 6 are shown in
Fig. 1(a) and (b), respectively.
Rint ¼ 0:103 from 13,827 measured ones for 6, and
used for structure determination and the refinement
procedure.
Intensity data were collected at 293 K using the
imaging-plate diffractometer Rigaku R-axis Rapid
with Mo Ka radiation monochromated by a graphite
plate. The cylindrically shaped imaging plate covers
the two-theta angular range between 260 and 1408
with a crystal-film distance of 127.4 mm. The
diffraction pattern was recorded on an imaging plate
by oscillating the crystal around the v axis by 38 at a
speed of 2408 per second for 3, and by 28 at a speed of
1208 per second for 6. In all, 173 images for 3 were
taken successfully by varying v with three sets of
different x and w values, and 178 images with two sets
for 6. Overlaps of 0.28 in v were taken into
consideration at both ends of each scan.
The structures were solved by the direct method
using SHELXS [11] and SIR2002 [12]. The positional
and atomic displacement parameters (ADPs) were
refined using SHELXL [13]. All the hydrogen atom
positions were found from the difference Fourier map.
The ranges of the C–H and O–H bond distances
˚
found for compounds 3 and 6 were 0.83(7)–1.18(9) A
˚
and 0.81(4)–1.09(5) A, respectively. The positional
and isotropic atomic displacement parameters of
hydrogen atoms were refined together with other
structural parameters by the full-matrix least-squares
procedure based on the squared value of the structure
factors. The weighting scheme v ¼ 1=½s2ðFo2Þ þ
2
ð0:1226PÞ þ 0:5279PŠ where P ¼ ðFo2 þ 2Fc2Þ=3 was
Cell dimensions were refined together with other
offset parameters of the diffractometer using all
reflections with I . 2sðI0Þ measured in the 2u range
between 260 and 1408. Since there are virtually no
significant reflections at 2u angles higher than 558, all
data in this region was eliminated. Equivalent
reflections were merged using the Rapid-auto pro-
gram package. Friedel pairs were considered as
equivalent for the merge.
2
used for
3
and v ¼ 1=½s2ðFo2Þ þ ð0:0254PÞ þ
0:0000PŠ where P ¼ ðFo2 þ 2Fc2Þ=3 were used for 6.
The final RðFÞ and vRðF2Þ values became 0.0841 and
0.2579 for 3, and 0.0453 and 0.1056 for 6.
3. Results and discussion
In all, 2749 independent reflections with Rint
0:099 were retrieved from 13,021 measured reflec-
tions for 3, and 1721 independent reflections with
¼
All title compounds formed solid with sharp
melting points. The structural formula of these
compounds is shown in Scheme 1.

90
G.-Y. Yeap et al. / Journal of Molecular Structure 658 (2003) 87–99
Table 2
Table 3a
Summary of crystal data, intensity data collection and structure
Fractional atomic coordinates and isotropic temperature factors
2
(A ), with standard deviations in the least significant digits in
˚
refinement for compounds 3 and 6 at 298 K
parentheses for compound 3. For anisotropic atoms, the equivalent
isotropic temperature factors are shown
Compound
3
6
2
U (A )
˚
Atom
x=a
y=b
z=c
Formula
C14H12ClNO
298
C15H15NO2
298
Data collection
temperature (K)
Mr
CL1
N1
0.2704(3)
0.5657(1)
0.6020(3)
0.7241(3)
0.7436(3)
0.5892(4)
0.4906(4)
0.4849(4)
0.5746(4)
0.6731(4)
0.6782(4)
0.5511(5)
0.4531(5)
0.4443(5)
0.5341(4)
0.6350(4)
0.6420(4)
0.5225(4)
0.7549(6)
0.0051(1)
0.2164(2)
0.2958(2)
0.3903(2)
0.1664(3)
0.1476(3)
0.0986(3)
0.0671(2)
0.0848(3)
0.1342(3)
0.3907(3)
0.3655(3)
0.3171(3)
0.2924(3)
0.3182(3)
0.3676(3)
0.2414(3)
0.4364(3)
0.086
0.067
0.076
0.082
0.066
0.068
0.072
0.068
0.078
0.076
0.075
0.082
0.080
0.066
0.065
0.069
0.070
0.085
261.7
241.28
20.5636(9)
20.8442(9)
21.2178(9)
20.3675(10)
20.2561(12)
20.0626(12)
0.0225(10)
Crystal colour
Orange,
transparent
Orange,
transparent
O1
O2
Crystal
0.25 £ 0.38 £ 0.99 0.19 £ 0.29 £ 0.35
C1
dimension
Crystal system,
space group
C2
P212121
P212121
C3
C4
C5
20.0781(12)
20.2695(13)
21.2938(12)
21.2277(13)
21.0367(12)
20.9001(11)
20.9653(10)
21.1669(10)
20.6989(11)
21.4337(13)
Unit cell parameters
˚
a (A)
˚
b (A)
C6
4.9076(2)
12.5700(4)
20.1739(8)
1244.50(8)
4
5.9190(2)
9.2428(3)
23.3431(6)
1277.06(7)
4
C7
C8
˚
c (A)
3
˚
C9
V (A )
C10
C11
C12
C13
C14
Z
Dc (g cm23
)
1.397
1.255
m (mm21
F(000)
)
0.299
0.083
544
512
2u range for
data collection
(degrees)
54.90
54.96
h, k, 1 range
26 # h # 6;
216 # k # 16;
226 # l # 26
13,021
27 # h # 7;
212 # k # 12;
230 # l # 30
13,827
Table 3b
No. of measured
reflections
No. of independent
reflections
No. of refined
parameters
R
Fractional atomic coordinates and isotropic temperature factors
2
(A ), with standard deviations in the least significant digits in
˚
2749
211
1721
223
parentheses for compound 6. For anisotropic atoms, the equivalent
isotropic temperature factors are shown
2
U (A )
˚
x=a
y=b
z=c
0.0149(1)
0.084
0.258
1.048
0.045
0.1056
0.754
Rw
O2
0.0940(5)
0.5743(3)
0.5271(3)
0.3773(3)
0.3599(4)
0.4248(4)
0.2971(4)
0.2799(4)
0.4461(4)
0.3410(4)
0.2125(5)
0.3408(5)
0.6115(6)
0.1925(4)
0.4351(4)
0.4233(5)
0.2777(5)
0.3273(8)
0.2698(5)
0.066
0.060
0.053
0.050
0.047
0.051
0.057
0.051
0.057
0.065
0.061
0.074
0.065
0.059
0.062
0.062
0.081
0.063
Goodness of
fit on S
O1
0.4204(5)
0.5707(6)
0.7119(7)
0.2626(7)
0.2641(7)
0.4284(9)
0.0851(8)
1.0063(7)
20.0736(9)
20.0756(9)
20.1015(11)
0.0952(10)
0.9125(8)
1.0603(8)
0.6551(8)
1.1636(11)
0.8004(8)
0.0888(1)
0.1763(1)
0.2250(1)
0.0835(1)
0.1171(1)
0.1630(1)
0.0437(1)
0.3194(2)
0.0686(2)
0.0366(2)
20.0186(2)
0.1081(2)
0.2258(2)
0.2721(2)
0.2726(2)
0.3695(2)
0.3191(2)
N1
Max. residual
0.283
0.148
C1
23
23
˚
electron density (e A
Min. residual
˚
electron density (e A
)
C11
C10
C13
C12
C4
20.292
20.165
)
3.1. IR spectroscopy
C8
C7
IR frequencies of various diagnostic bands in the
solid state (KBr) are collected in Table 5. The IR
spectra of 1, 4, 5, 6 and 8 are shown in Fig. 2 as
representative illustration. The exact positions of the
characteristic bands discussed in the later part of this
section vary from one compound to another since the
substituent in the aniline fragment is different.
C14
C9
C6
C5
C2
C15
C3

G.-Y. Yeap et al. / Journal of Molecular Structure 658 (2003) 87–99
91
Table 4a
˚
Bond lengths (A), bond angles (degrees) and torsion angles
Table 4b
˚
Bond lengths (A), bond angles (degrees) and torsion angles
(degrees) for compound 3
(degrees) for compound 6
CL1–C4
1.749(5) N1–C1
1.404(7)
1.346(6)
1.417(8)
1.381(8)
1.359(8)
1.372(9)
1.382(8)
1.404(8)
1.434(8)
O2–C12
1.364(4) O1–C11
1.419(4) N1–C13
1.376(6) C1–C2
1.335(4)
1.272(5)
1.387(5)
1.416(5)
1.406(6)
1.377(6)
1.385(5)
1.371(6)
1.388(6)
N1–C13
1.301(7) O1–C11
1.379(6) O2–C14
1.407(7) C1–C6
N1–C1
O2–C12
C1–C6
C1–C2
C11–C10
1.417(5) C11–C12
1.456(5) C10–C9
1.371(6) C4–C5
C2–C3
1.372(8) C3–C4
C10–C13
C4–C5
1.380(8) C5–C6
C12–C7
C7–C8
1.372(8) C7–C12
1.358(9) C9–C10
1.408(7) C10–C13
1.408(7)
C4–C15
1.502(6) C4–C3
C8–C9
C8–C7
1.402(5) C8–C9
C10–C11
C6–C5
1.395(5) C2–C3
C11–C12
C14–O2
1.438(6)
C1–N1–C13
N1–C1–C2
C2–C1–C6
C2–C3–C4
CL1–C4–C5
C4–C5–C6
C8–C7–C12
C8–C9–C10
C9–C10–C13
O1–C11–C10
C10–C11–C12
O2–C12–C11
N1–C13–C10
C13–N1–C1–C2
C1–N1–C13–C10
122.7(5) C12–O2–C14
124.2(5) N1–C1–C6
116.9(5) C1–C2–C3
120.4(5) CL1–C4–C3
119.4(4) C3–C4–C5
118.3(5) C1–C6–C5
120.8(5) C7–C8–C9
121.3(6) C9–C10–C11
120.2(5) C11–C10–C13
121.7(4) O1–C11–C12
118.6(5) O2–C12–C7
114.9(4) C7–C12–C11
123.5(5)
116.5(5)
118.9(5)
120.6(5)
119.7(4)
120.9(5)
122.9(6)
120.1(6)
119.0(5)
120.9(5)
119.7(5)
124.9(5)
120.2(5)
C1–N1–C13
N1–C1–C2
O1–C11–C10
C10–C11–C12
C11–C10–C9
N1–C13–C10
O2–C12–C7
C5–C4–C15
C15–C4–C3
C12–C7–C8
C1–C6–C5
C1–C2–C3
C12–O2–C14
C1–N1–C13–C10
C13–N1–C1–C2
N1–C1–C2–C3
C6–C1–C2–C3
O1–C11–C10–C9
O1–C11–C12–C7
120.3(3) N1–C1–C6
124.1(4) C6–C1–C2
122.4(3) O1–C11–C12
118.9(3) C11–C10–C13
119.0(3) C13–C10–C9
123.0(3) O2–C12–C11
125.7(4) C11–C12–C7
121.8(5) C5–C4–C3
120.6(4) C7–C8–C9
122.0(4) C10–C9–C8
120.9(4) C4–C5–C6
120.4(4) C4–C3–C2
116.4(4)
117.5(3)
118.3(4)
118.7(3)
120.2(4)
120.6(3)
114.6(4)
119.7(4)
117.6(4)
118.6(4)
121.7(4)
121.2(4)
121.6(4)
213.6 C13–N1–C1–C6
2179.9 C14–O2–C12–C7
168.3
25.9
2179.2
2180.0
20.6
1.7
173.9
C13–N1–C1–C6
N1–C1–C6–C5
156.0
2179.9
3.1
227.2
C14–O2–C12–C11 175.3
N1–C1–C2–C3
N1–C1–C6–C5
C1–C2–C3–C4
C2–C3–C4–C5
2178.1 C2–C1–C6–C5
21.4
2178.4 O1–C11–C12–O2 21.0
178.4 C10–C11–C12–O2 177.8
C12–C11–C10–C13 2172.7 C10–C11–C12–C7 22.8
C6–C1–C2–C3
C2–C1–C6–C5
C2–C3–C4–CL1
CL1–C4–C5–C6
C4–C5–C6–C1
C8–C7–C12–O2
C7–C8–C9–C10
C8–C9–C10–C13
C9–C10–C11–C12
21.0
1.7
O1–C11–C10–C13 6.0
179.7
2179.1 C3–C4–C5–C6
20.8 C12–C7–C8–C9
2179.4 C8–C7–C12–C11
21.0
20.3
20.6
20.6
2179.0
C12–C11–C10–C9
C11–C10–C9–C8
C13–C10–C9–C8
C11–C12–C7–C8
C3–C4–C5–C6
2.9
C11–C10–C13–N1 2.2
21.5
174.0
1.5
C9–C10–C13–N1
O2–C12–C7–C8
C15–C4–C5–C6
C5–C4–C3–C2
2173.2
0.9
C8–C9–C10–C11
C9–C10–C11–O1
2179.2
180.0
1.5
179.9
20.3
C13–C10–C11–O1 0.5
0.3
C13–C10–C11–C12 179.2
C11–C10–C13–N1 0.8
C9–C10–C13–N1
2179.6
C15–C4–C3–C2
C9–C8–C7–C12
C1–C2–C3–C4
2178.2 C7–C8–C9–C10
20.1
21.0
0.1
O1–C11–C12–O2 21.5
C10–C11–C12–O2 179.8
C1–C6–C5–C4
22.7
O1–C11–C12–C7
C10–C11–C12–C7
179.6
0.9
a tendency to migrate to azomethine N atom via the
O–H· · ·N intramolecular hydrogen bonding as that
observed for meta substituted 5-methoxysalicylaldi-
mine isomer [(Sal-m-OH-ph)H] in the solid state [16].
Although the probable structures for compounds
1–8 have been postulated as enol-imino tautomer
resembling (Sal-m-OH-ph)H, but further inspection
upon the IR spectra have revealed that the stabiliz-
ation of the enol-imino tautomer may vary depending
on the type of para substituent. This conformational
The absorption bands assignable to the stretching of
CyN bond for compounds 1–8 were observed at
frequencies range of 1613–1619 cm21 and these
values conform with that reported in the IR spectrum
for unsubstituted aromatic Schiff base which pos-
sesses the formulation of C6H5CHyNC6H5 [14,15]. A
band assignable to the stretching of O–H bond is
found to be broadened within the frequency range of
3000–2500 cm21. This observation implies that the H
atom from the O–H group in compounds 1–8 has

92
G.-Y. Yeap et al. / Journal of Molecular Structure 658 (2003) 87–99
Fig. 1. ORTEP drawings with atom-numbering scheme for (a) Schiff base 3, and (b) Schiff base 6.
study was based on the comparison of IR spectra of all
Schiff bases 1–8 at different vibrational regions
similar to the experimental observation carried out
earlier on salicylidineaniline and naphthylidinequino-
lineamine [15,17].
the highest CyN stretching frequency. The discre-
pancy can be ascribed to the stabilization of CyN
bond resulting from the resonance effect whereby
the lone-pair electrons from the F atom were
delocalized via p-bonding in the conjugated skel-
eton comprising aniline fragment and the exocyclic
CyN. The presence of very strong intensity of n(O–
H) also suggests that the azomethine CyN is not
affected by H atom through intramolecular hydrogen
A first comparison was made among compounds
2–4 wherein the compound was substituted by F, Cl
or Br atom, respectively. The presence of the most
electronegative F atom in compound 2 gave rise to

G.-Y. Yeap et al. / Journal of Molecular Structure 658 (2003) 87–99
93
Table 5
Infrared frequencies (cm21) of compounds 1–8 in the solid state
(KBr)
substituent caused the intensities of n(CyN) band
increased in comparison with compound 5 which
possesses electron withdrawing NO2 group. Another
feature is that there are bands corresponding to the
stretching of phenolic C–O bond in different intensity
at 1283–1270 cm21 resemble that reported for ortho
hydroxyl group [16]. Again, the comparison among
compounds 2–4 substantiates that the compound 2
with strong band at 1278 cm21 possesses highest
percentage of enol-imino tautomer due to the
stabilization of phenolic C–O bond.
Compound
n (CyN)
n (CyC)arom
n (C–O)
1
2
3
4
5
6
7
8
1615 vs
1618 vs
1615 vs
1614 s
1589 s
1270 s
1278 s
1273 m
1274 m
1273 w
1275 s
1276 s
1283 m
1600 m
1572 m
1570 w
1572 m
1595 s
1613 m
1617 s
1619 vs
1619 s
1580 m
1592 vs
Abbreviations: s, strong; m, medium; w, weak; v, very
3.2. NMR spectroscopy
1
The H and 13C NMR data recorded in CDCl3
bonding. The information with respect to the
intensity of the band assigned for n(CyC) within
the aromatic skeleton in compound 2 also supports
that the percentage of enol-imine is comparatively
higher than 3 and 4 [17].
The stabilization of CyN bond which is accoun-
table partially for the existence of enol-imino
tautomer can also be inferred from the IR spectra of
6–8 in which the s-electron donors of CH3, OCH3
and the p-electron density from cynide (CN)
solutions are shown in Tables 6 and 7, respectively.
The assignments with respect to the different
resonances in Tables 6 and 7 are obtained by means
of 1H–1H and 13C–1H correlation spectroscopic
measurements.
For compound 1, two signals each appeared as a
singlet at d ¼ 8:60 and 13.75 ppm can be ascribed to
CHyN and OH, respectively. The appearance of both
signals indicated that the tautomeric equilibrium
Fig. 2. FT-IR spectra of compounds 1, 4, 5, 6 and 8 in the solid state (KBr).

Table 6
1H NMR chemical shifts (ppm) of Schiff bases 1–8 in CDCl3
Compounds
Chemical shifts (ppm)
H2, H6
H3, H5
H7
H8
H9
H13
H14
OH
R
1
2
3
4
5
6
7
8
7.28–7.29 (m)
7.20–7.23 (m)
7.15–7.17 (m)
7.12–7.14 (m)
7.37–7.40 (m)
7.22 (d)b
7.40–7.44 (m)
7.04–7.08 (m)
7.31–7.33 (m)
7.49–7.51 (m)
8.30–8.33 (m)
7.22(d)b
6.99–7.00 (dd)a
6.96 (d)a
6.85–6.89 (t)
6.82–6.86 (t)
6.82–6.86 (t)
6.85–6.89 (t)
6.93–6.97 (t)
6.86–6.90 (t)
6.82–6.86 (t)
6.86–6.90 (t)
6.97–7.02 (dd)a
6.94–6.98 (dd)a
6.94–6.97 (dd)a
6.97–7.00 (dd)a
7.06–7.09 (dd)a
6.97–7.02 (dd)a
6.93–6.98 (dd)a
7.01–7.04 (m)a
8.60 (s)
8.53 (s)
8.54 (s)
8.56 (s)
8.66 (s)
8.61 (s)
8.55 (s)
8.59 (s)
3.92 (s)
3.89 (s)
3.89 (s)
3.91 (s)
3.96 (s)
3.94 (s)
3.91 (s)
3.90 (s)
13.75 (s)
13.51 (s)
13.41 (s)
13.40 (s)
12.90 (s)
13.88 (s)
13.90 (s)
12.74 (s)
7.26–7.27 (t)
–
6.95–6.96 (dd)a
6.98–6.99 (dd)a
7.07–7.08 (dd)a
6.99–7.00 (dd)a
6.95–6.96 (t)a
7.01–7.04 (m)a
–
–
–
2.38 (s)
3.79 (s)
–
7.22–7.26 (m)
7.28–7.30 (m)
6.89–6.92 (m)
7.64–7.66 (m)
S ¼ singlet, d ¼ doublet, dd ¼ double doublets, t ¼ triplet, m ¼ multiplet. TMS used as internal standard.
The peaks attributed to H2, H3, H5 and H6 atoms overlapped with each other giving rise to a doublet.
The peak for H7 atom overlapped with the peak for H9 atom.
a
b
Table 7
13C NMR chemical shifts (ppm) of Schiff bases 1–8 in CDCl3
Compounds
Chemical shifts (ppm)
C1
C2
C3
C4
C5
C6
C7
C8
C9
C10
C11
C12
C13
C14
R
1
2
3
4
5
6
7
8
151.87
151.58
151.67
151.71
151.90
151.87
151.65
151.77
121.59a
122.97a
122.85a
123.23a
122.33a
121.40a
122.69a
122.47a
129.83b
116.67b
129.87b
132.87b
125.63b
130.43b
114.99b
133.89b
127.42
160.81
132.88
120.85
148.95c
137.42
159.30
110.45
129.83b
116.67b
129.87b
132.87b
125.63b
130.43b
114.99b
133.89b
121.59a
122.97a
122.85a
123.23a
122.33a
121.40a
122.69a
122.47a
115.18
115.19
115.38
115.45
116.24
114.97
114.80
116.11
118.97
119.03
119.10
119.14
119.08
118.86
118.83
119.08
124.25
124.20
124.30
124.32
124.69
124.11
123.98
124.69
119.58
119.39
119.33
119.35
119.55
119.58
119.67
119.48
148.47
148.79
148.79
148.83
148.95c
148.87
148.79
148.81
145.30
163.03
162.78
163.31
163.40
165.86
162.00
160.67
165.50
56.57
56.50
56.53
56.58
56.65
56.57
56.53
56.58
–
–
–
–
–
144.61
146.97
147.52
146.55
145.85
141.27
146.21
21.47
55.87
114.77
TMS used as internal standard.
C20 and C60 are equivalent.
C30 and C50 are equivalent.
Overlapped.
a
b
c

G.-Y. Yeap et al. / Journal of Molecular Structure 658 (2003) 87–99
95
favors the enol-imino form in the solution (CDCl3) as
that reported by Nazir and his coworkers [18]. This
phenomenon has further been supported by NMR
technique as that reported by Alarcon et al. [19]
wherein the ortho hydroxy group attached to the
aldehyde ring in HOC6H5CHyNC6H5 exists as an
enol-imino tautomer when left in the solution
(CDCl3). A singlet assignable to the chemically and
magnetically equivalent protons in methoxy group
was observed at d ¼ 3:92 ppm [20]. Compounds 2–8
show similar characteristic as those discussed in
compound 1.
experiment and the relationship between different
protons in 2 are shown in Fig. 3.
Assignment of protonated carbons was made by
two 2D 13C–1H HMQC experiment using delay
1
values which corresponds to J(C,H) [21,22]. From
HMQC, CH3 and CH carbons were assigned
unambiguously (Table 7). Heteronuclear multiple-
bond correlation (1H–13C HMBC), a modified
version of HMQC was applied on 7 in order to
determine the long range 1H–13C connectivities
1
[21,22]. The long range H–13C correlations for 7
(Table 9) and its two dimensional spectra are
shown in Fig. 4.
The signal attributed to H7 atom in 1–8 appeared
in different patterns (Table 6). It appeared as double
doublets in compounds 1, 3–6, doublet in 2, triplet in
7 and as indisguishable multiplet in compound 8.
Actually the signals assignable to H7 atom in
compounds 1–7 occurred between a pair of doublets
which resulted from the resonance of H9 atom. The
complexity of this region is well confirmed through
dimensional 2D heteronuclear-correlated spectro-
scopic technique (2D 13C–1H HMQC) with the aid
of 2D 1H–1H COSY experiment. The couplings
between the different protons in 1–8 as inferred from
the 2D COSY correlation studies are tabulated in
Table 8. The representative spectra of 2D COSY
Inspection from COSY and HMBC for 7 can be
summarized as follows:
(a) By 2D COSY experiment, hydroxyl proton (O–
H) was found to be not correlated with any other
protons.
(b) However, observation from HMBC experiment
showed that the hydroxyl proton (O–H) corre-
lated with C1, C10 and C11 atoms.
(c) The hydroxyl proton (O–H) was located
nearer to C1 rather than H13 owing to the
relationship observed for hydroxyl proton with
1
C1 atom via H–13C correlation.
Table 8
1H–1H correlations from 2D COSY for Schiff bases 1–8
Atom H
1H–1H COSY correlations
1
2
3
4
5
6
7
8
2 or 6
3 or 5, 7, 9, 14,
Y
3 or 5, 7, 9
3 or 5, 7
3 or 5, 7, 9
3 or 5
7, 9, 13, 14, Y
3 or 5, 13, Y
3 or 5
3 or 5
7
2 or 6, 7, 13, Y
2 or 6, 8, 9
2 or 6, 7, 9
2 or 6, 3 or 5,
8, 9, 13, 14
7, 9
2 or 6
2 or 6, 7, 9
2 or 6
8, 9
7, 9, 13, 14, Y
2 or 3 or 5 or
6, 8, 9, 14
7, 9
2 or 6, 13, Y
8, 9, 14
2 or 6
8, 9
2 or 6, 8, 9, 14
2 or 6, 8, 9, 14
8
9
7, 9
7, 9
7, 9
7, 9
7, 8
7, 9
7, 8
7, 9
2 or 6, 7, 8, 13
2 or 6, 3 or 5,
7, 8, 13, 14
7, 9, 14
7, 8, 13
2 or 6, 7, 8, 13
2 or 3 or 5 or
6, 7, 8, 13
2 or 3 or 5 or
6, 9
7, 8, 13
13
14
3 or 5, 9
7
9
7
9
7
–
–
2 or 6, 3 or 5
7
9
–
7
2 or 3 or 5 or
6, 7, Y
OH
Y
–
–
–
–
–
–
–
–
–
–
–
–
–
2 or 6, 3 or 5
2 or 3 or 5 or
6, 14
2 or 6, 3 or 5
Y ¼ proton in the substituent R (1: R ¼ H; 6: R ¼ CH3; 7: R ¼ OCH3).

96
G.-Y. Yeap et al. / Journal of Molecular Structure 658 (2003) 87–99
Fig. 3. (a) Homonuclear 1H–1H 2D COSY spectrum, and (b) 1H–1H relationships as inferred from 2D COSY and supported by HMQC spectra
of compound 2.

G.-Y. Yeap et al. / Journal of Molecular Structure 658 (2003) 87–99
97
Table 9
2D 1H–13C HMQC and HMBC correlations for 7
The summary from (a)–(c) suggest that the
probable conformation for compound 7 in CDCl3
solution can be depicted in Fig. 5.
Atom
HMQC
HMBC [J(C, H)]
The molecular structure as postulated for com-
pound 7 shows that the azomethine proton was located
at the other side opposing to the OH group. As such,
the hydroxyl proton (O–H) lies nearer to the aromatic
C1 atom, which is adjacent to the azomethine N imine
atom.
1J
2J
3J
4J
intraJ
Y*
H14
H8
H3
H5
H9
H7
H2
H6
H13
OH
Z*
C14
C8
C3
C5
C9
C7
C2
C6
C13
–
–
C4
–
–
–
–
C11
C11
–
–
–
C10
–
C4
C4
–
–
C12, C13
C12, C13
C10
–
–
C11
–
3.3. X-ray crystal structures
–
–
–
C13
–
–
C4
C4
C12
–
C12
From Fig. 1(a) and (b), the compounds can be
described as comprising of central part of CyN
bridging 3-methoxy-2-hydroxybenzaldehye and para
substituted aniline fragments. The figures show that
the azomethine proton of H4 and H2 in compounds 3
and 6 are lying on the other side opposite to
–
–
C12
C10
C11
C1, C9, C11
C10
–
C1
*Atoms in the R substituent whereby Y ¼ hydrogen and
Z ¼ carbon intra ¼ intramolecular interaction.
Fig. 4. The HMBC spectrum resulted from long range 1H–13C connectivities on Schiff base 7 (Y ¼ proton in the substituent, in this case the
methoxy group OCH3).

98
G.-Y. Yeap et al. / Journal of Molecular Structure 658 (2003) 87–99
˚
its analogue 6 [1.95(3) A], both compounds still exist
as predominant form of enol-imines in solid state. One
of the factors could be due to the position of phenolic
H atom from azomethine N atom in compounds 3 and
6 resulting from the strain incurred by heteronuclear
six-membered ring in which the bond angles O1–
C11–C10 were found to be 121.7(4)8 and 122.4(3)8,
respectively. As such, both phenolic H atom in 3 and 6
were positioned at the distance not favourable for the
formation of keto-imino tautomer. This result was
also evident from the N· · ·O atomic distances in
Fig. 5. The stereoisomer structure of compound 7 at ambient
temperature in CDCl3.
˚
˚
compounds 3 [2.611(6) A] and 6 [2.624(4) A] which
favoured the formation of enol-imines. The dom-
inance of enol-imino tautomer in the presence
compounds can also be exemplified by the range
the hydroxyl protons H4 and H2, respectively. This
result conforms with the postulated structure (Fig. 5)
as inferred from the NMR spectra whereby the
compound dissolved in CDCl3. Data in Table 3 have
clearly illustrated that the bond distance of CyN in
˚
of C11–O1 bond length [1.335(5)–1.346(6) A]
(Table 4) which is comparable or higher than the
˚
compound 3 [1.301(7) A] is found to be longer than
˚
single bond values in enols [1.333(7) A] [23]. Another
˚
that in its analogue 6 [1.272(6) A]. This piece of
evidence could be deduced from the dihedral angles
[C13–N1–C1–C2] of 3 (213.6)] and 6 [227.2]
which indicate that the present molecules are not
coplanar. As a result, the transfer of the hydrogen in
the ground state with small energy requirement is not
likely to occur [24,25].
information suggests that the bond strength associated
with the effective charge of CyN bond in compound 6
is comparatively higher in comparison with that in
compound 3. As such, the stretching frequency of
CyN in compound 6 (1617 cm21) was found lying at
higher frequency than compound 3 (1615 cm21). The
elongation of CyN bond in compound 3 can also be
substantiated from the presence of six-membered
pseudoaromatic chelate rings which possess OH–
CyC–CyN (enol-imino tautomer) heterodienic moi-
eties. The rings are formed by an intramolecular
hydrogen bond whereby the phenolic H atom (H12 in
compound 3) appeared to be directed to the azo-
methine N atom, which resulted in a decrease in
electron density at N1 atom. As a consequence, the
bond strength of CyN become lesser than that in
compound 6.
4. Supplementary material
Crystallographic data for the structures 3 and 6 in
this paper have been deposited at Cambridge Crystal-
lographic Data Centre, 12 Union Road, Cambridge
CB2 1EZ, UK and can be obtained upon request.
Structure factors table is available from the authors.
Acknowledgements
Fig. 1(a) and (b) also show that the phenolic H
atoms in molecules 3 and 6 are located at phenolic O
atom. This observation can be rationalized by the
stabilization effects experienced by enol-imino tauto-
mers as that discussed in IR section wherein the
characteristic of CyN bonds in all compounds 1–8
were remained unchanged even though the strength of
this bond was found to be altered from one compound
to another owing to the presence of different
substituents R in the aniline fragment. Although the
bond distance of N1· · ·H12(O1) in compound 3
This work was financed by the Ministry of Science,
Technology and Environment of Malaysia and
Universiti Sains Malaysia through IRPA Grant No.
305/PKIMIA/610947 and FRGS Grant No. 304/PKI-
MIA/670032.
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