Structural investigation on phenyl- and pyridin-2-ylamino(methylene) naphthalen- 2(3H)-one. Substituent effects on the NMR chemical shifts

Article abstract of DOI:10.1002/mrc.2626

Schiff bases bearing phenyl and pyridyl groups were synthesized by condensation of appropriate amines with 2- hydroxynaphthaldehyde. These Schiff bases were obtained as colored crystalline solids. The proton NMR spectra of these compounds showed a doublet

Full text of DOI:10.1002/mrc.2626

Research Article
Received: 21 February 2010
Revised: 5 May 2010
Accepted: 7 May 2010
Published online in Wiley Interscience: 15 June 2010
(www.interscience.com) DOI 10.1002/mrc.2626
Structural investigation on phenyl-
and pyridin-2-ylamino(methylene)naphthalen-
2(3H)-one. Substituent effects on the NMR
chemical shifts
Taracad K. Venkatachalam,a∗ Gregory K. Pierens,a Marc R. Campitellib
and David C. Reutensa
Schiff bases bearing phenyl and pyridyl groups were synthesized by condensation of appropriate amines with 2-
hydroxynaphthaldehyde. These Schiff bases were obtained as colored crystalline solids. The proton NMR spectra of these
compounds showed a doublet for the NH protons indicating a keto tautomer for these Schiff bases. The pyridyl-substituted
Schiff bases containing hydroxyl moiety were found to show the most downfield shift for the NH protons in DMSO solvent, and
this was rationalized due to the formation of a six- and five-membered ring using hydrogen bonds for these two compounds.
Correspondingly, the olefinic proton of the Schiff bases is also found to be a doublet due to coupling to the amine proton. These
Schiff bases exhibited thermochromic properties. Detailed NMR spectral analysis for both the phenyl- and pyridyl-substituted
c
Schiff bases is presented. Copyright ꢀ 2010 John Wiley & Sons, Ltd.
Keywords: phenyl; pyridyl; substituent effects; NMR; chemical shifts
Introduction
converts the keto tautomer into an enol tautomer and vice versa.
This conversion involves a zwitterionic compound. We carefully
examinedthephysicochemicalcharacteristicsoftheseSchiffbases
in order to elucidate their structure.
Tautomeric equilibrium reflects the transfer of a proton from a
nitrogen atom to an oxygen atom in Schiff bases. This class of
compounds has important biological applications,[1–5] interesting
electronic properties,[6–8] and may exhibit photochromic and
thermochromic characteristics.[9–45] There have been several
reports in the literature providing experimental evidence for
the formation of enol–keto tautomers in solution and in solid
state of Schiff bases. Additionally, recent papers describe the
isolation and preparation of some of the keto tautomers.[28,41–46]
In addition, single crystal X-ray crystallography has been used
to establish the solid-state structure of keto or enol tautomers
of Schiff bases. It is known that some Schiff bases derived from
2-hydroxynaphthaldehyde form the keto tautomer preferentially,
whereas salicylaldehyde-derived Schiff bases form both enol and
keto tautomers.[46] We describe a complete NMR analysis of the
structure of naphthylidene Schiff bases and correlate the chemical
shifts with substituents present in the molecular frame.
Inthisregard,wenotethatpreviousworkershavereportedsome
of the compounds in the present study. For example, compound
4 was reported by Ranganathan et al.[36,37] However, the authors
assigned the peak at δ = 15.40 ppm to an OH group, which is in
contrast to the results obtained in the present study. Compounds
6, 10 and 12 were reported to exist as a keto tautomer from X-ray
crystallographic studies, a finding consistent with our solution
data.[38,42–44] Issa et al.[39] reported compound 1 which possesses
a pyridyl moiety and hydroxy functionality in its structure. The
authorsassignedthepeakatδ = 8.50 ppmtoCH N. Twosinglets
at δ = 9.70 and 9.75 ppm were ascribed to the free OH group
in the pyridyl moiety and the hydrogen-bonded OH group of
naphthalene ring, respectively. Our data are contrary to the above
values (Table 1). We obtained a doublet at δ = 15.14 ppm for the
NHprotonsfollowedbyabroadsingletatδ = 10.93corresponding
to the OH group and another doublet at δ = 9.72 indicative of
CH N protons coupled to the NH proton. We believe that Issa
et al. had an enol tautomer. Our claim is further substantiated by
Results and Discussion
The synthesis of Schiff bases containing a napthyl ring has been
achieved starting from 2-hydroxylnaphthaldehyde. Both pyridyl-
and phenyl-substituted aromatic amines were chosen to elucidate
the role played by the heterocyclic structure. The structures of the
Schiff bases prepared in the present study are shown in Fig. 1, and
the structural features were examined using proton and carbon
NMR spectroscopy. Generally, Schiff bases are proposed to exist
in two tautomeric forms as shown in Scheme 1. In this scheme, it
is evident that the equilibrium involves a proton transfer, which
∗
Correspondence to: Taracad K. Venkatachalam, Centre for Advanced Imaging,
The University of Queensland, Brisbane, Queensland 4072, Australia.
E-mail: t.venkatachalam@uq.edu.au
a
Centre for Advanced Imaging, The University of Queensland, Brisbane,
Queensland 4072, Australia
b
Eskitis Institute for Cell and Molecular Therapies, Griffith University, Brisbane,
Queensland 4111, Australia
c
Magn. Reson. Chem. 2010, 48, 585–592
Copyright ꢀ 2010 John Wiley & Sons, Ltd.

T. K. Venkatachalam et al.
CH3
N
H
H
N
N
H
N
O
O
O
O
O
OH
OH
O
OH
(2)
(1)
(3)
CH3
N
H
N
N
H
N
Cl
CF3
I
H
N
N
Br
O
O
(4)
(5)
(6)
N
H
N
H
H
N
Cl
N
Br
O
O
Cl
(8)
(9)
(7)
Cl
CF3
H
N
H
H
N
OCH3
N
O
O
CF3
(10)
(11)
(12)
20
20
15
16
14
N
14
13
15
16
12
12
13
N
N
11
9
11
18
18
8
8
1
2
1
2
17
9
17
O
O
7
6
7
6
19
19
3
3
10
10
4
5
5
4
Figure 1. Structures of Schiff bases containing both pyridyl and phenyl rings with substituents and example of the numbering system used.
the X-ray crystal structure reported by Ozek et al.[40] Sosa et al.[34]
reported that compound 8 exists in solution as a keto tautomer
and as an enol tautomer in solid state. Our results indicate that
both in solid (X-ray crystal data, unpublished) and solution state,
only the keto tautomeric structure is prevalent. The contradictory
published results motivated our examination of the structure of a
complete set of these Schiff bases.
Table 1 shows the observed proton chemical shifts for pyridyl-
substituted Schiff bases. We examined two electron-donating
groups (OH and Me) and three electron-withdrawing groups
namely, Cl, Br and CF3 in the pyridyl ring of these Schiff bases.
The numbering of the protons and carbons for each compound
is shown in Fig. 1. In all the pyridyl compounds (1,4–7), the NH
protonshowedadoubletintheNMRspectrumdemonstratingthat
the proton resides on the nitrogen atom of the compound rather
than on the oxygen atom. The coupling constant for this doublet
was found to vary considerably with the structure of the Schiff
bases. The coupling constants varied from 10.1 to 1.3 Hz. We found
that the higher coupling constant value was obtained for pyridyl-
substitutedSchiffbasescomparedtothephenyl-substitutedSchiff
bases. Table 2 shows the observed chemical shift for phenyl-
substituted Schiff bases containing various substituted groups.
The NH protons appeared as a doublet in all these compounds.
It is interesting to point out that the chemical shifts for the NH
protons with Cl-, Br- and I-substituted anils were in the range
of δ = 15.24–15.27, whereas the chloro-methoxy compound
11 shifted downfield (δ = 15.34 ppm). Interestingly, the di-
trifluoromethyl-substituted Schiff base 12 showed a chemical
shift value of δ = 14.62 ppm. Normally, the OH protons appear
below 10 ppm unless they are hydrogen bonded. Therefore, we
can ascribe the high-frequency shift of the proton signals to
hydrogen bonding with the oxygen atom forming a stable six-
membered ring transition state as shown below. However, caution
should be exercised in representing the open structure of the keto
form, given the strength of the intramolecular hydrogen bond
(Scheme 2).
This strong intramolecular hydrogen bonding ability of the
proton is well documented in the literature[8,20,30] and is consistent
withtheobserveddeshielding.However,westillseeaconsiderable
high-frequency shift for the hydroxyl-substituted Schiff bases
compared to halogen-substituted compounds. There are two
possible explanations for this observation. One relates to the use
of the solvent dimethyl sulfoxide (DMSO) which is comparatively
polar. The second is the additional capability of this proton
c
www.interscience.wiley.com/journal/mrc
Copyright ꢀ 2010 John Wiley & Sons, Ltd.
Magn. Reson. Chem. 2010, 48, 585–592

Substituent effects on the NMR chemical shifts
+
N
N
-O
H
N
N
N
N
H
HO
O
H+ (acidic solvent)
H+
H
N
+
N
N
+
N
O
HO
H
H
Scheme 1. Keto–enol tautomeric equilibrium depicting various species..
to form hydrogen bonds with another electronegative atom
in the vicinity. We propose that the NH proton also forms a
hydrogen bond with the 2-substituted OH group in the structure
as shown below forming both a six- and a five-member rings.
Similar bifurcated intramolecular hydrogen bond formation was
described by Rozwadowski[47] and Dziembowska et al.[48] This
intramolecular hydrogen bond was present in Schiff bases derived
from 2-hydroxylnaphthaldehyde and 8-aminoquinoline. Abbas
et al.[49] also provided spectroscopic evidence in favor of a
bifurcated intramolecular hydrogen bond consistent with our
notion of a bifurcated structure for the compound (Scheme 3).
Structure (i) shows only one intramolecular hydrogen bond
between the NH proton and the oxygen atom of the C O group.
In structures (ii) and (iii), we invoke an additional five-member ring
in the structure. The strong hydrogen-bonding characteristic of
the OH group may be responsible for further deshielding of NH
protons compared to the other compounds. Additionally, the free
hydroxyl group proton for this compound was found to be around
δ = 10.63 ppm, a chemical shift value close to an aldehyde proton
signal, perhaps due to the formation of this hydrogen-bonded
structure. Caution should be exercised here because compound
1 which possesses a hydroxyl group in its structure showed a
high-frequency shift of δ = 15.41 ppm. This value is higher than
that observed in the phenyl-substituted Schiff bases raising the
possibility of an influence from the presence of a nitrogen atom in
the phenyl ring.
confirming the presence of keto tautomer structure. The 15N
chemical shifts for the compounds 4–7 were in the range of
δ = 112–136 ppm. The sweep width used for obtaining the
spectra was 45 ppm and the reference frequency was centered
at δ = 125.0 ppm. No internal or external standard was used to
reference the 15N chemical shifts. Apart from the above data, we
have X-ray crystal structure data (unpublished) which provides
further support for the keto tautomeric structure of these Schiff
bases.
We next examined the chemical shift observed for the olefinic
protons ( CH) in the pyridyl-substituted compounds. The olefinic
protons in the pyridyl-substituted compound showed chemical
shift values between δ = 9.71 and 9.93 ppm. The hydroxyl-
substituted pyridine compounds showed the lowest values
compared to the other compounds. This is consistent with the
finding that NH protons of compounds 4,5 and 6 showed the
highest deshieldings. We expect that the pyridyl-substituted
compounds would show a difference in the olefinic proton
chemical shift. Values for the olefinic proton chemical shift in
the phenyl-substituted Schiff bases (Table 2) ranged between
δ = 9.33 and 9.49 ppm, considerably lower than those observed
for the pyridyl-substituted compounds. The halogen-substituted
compounds showed almost an identical chemical shift value
for this proton, which is expected because of the electron-
withdrawing nature of the halogens. The free hydroxyl group
protons appeared at δ = 10.35 and 10.06 ppm in compounds 2
and 3, respectively. This value is lower than that observed for the
pyridyl-substituted Schiff base (δ = 10.93 ppm). It is plausible that
the nitrogen atom in the pyridyl ring plays a role in shifting these
hydroxyl group protons downfield compared to the substituted
phenyl compounds.
The15NNMRHSQCexperimentswereconductedforafewofthe
derivatives to evaluate the nature of the bonding arrangement.
We chose only four compounds for the study to make our
claim regarding the keto tautomeric structure of the compound.
Compounds 4–7 showed a single peak in the 15N HSQC spectrum
c
Magn. Reson. Chem. 2010, 48, 585–592
Copyright ꢀ 2010 John Wiley & Sons, Ltd.
www.interscience.wiley.com/journal/mrc

T. K. Venkatachalam et al.
values (3JNH−H) between the NH proton and the olefinic proton.
Table 5 shows the coupling constants observed for individual
compounds. Compounds 1–3 showed the largest values in the
range of δ = 10.1–9.4 Hz indicating that these compounds exist
as a keto tautomer. We note that these compounds were run in
DMSO-d6 due to their poor solubility in chloroform and caution
that the solvent could have contributed to the higher coupling
constants shown by these compounds. Focusing on the coupling
constant values for compounds 4–7, we found the J-values to be
between 6.1 and 7.1 Hz. Interestingly, these compounds have a
pyridyl group in their structure compared to compounds 2 and
3. The phenyl-substituted compounds showed smaller coupling
constants ranging from 3.2 to 1.2 Hz. From the literature, a J-value
of 12.85 Hz has been calculated for the pure keto form. Based on
this value, the proton transfer equilibrium lies more toward the
keto form in the case of compounds 1–8, whereas the enol form
is favored in case of phenyl-substituted Schiff bases. X-ray crystal
structuredataforsomeof thecompoundsdiscussedinthepresent
study (unpublished results) showed the proton to be residing
solely on the nitrogen atom and not on the oxygen atom despite
these small coupling constants. X-ray crystal structure studies
did not show a change in this equilibrium even as temperature
was lowered. Our findings indicate that it is difficult to predict
the direction of equilibrium on the basis of coupling constants.
Furthermore, the solvent used may also shift the equilibrium[46]
leading to a different equilibrium in the solution compared to the
solid state.
In the subsequent sections, we focused on the carbon chemical
shifts observed for these Schiff bases. Table 3 shows the carbon
chemical shifts observed for the pyridyl-substituted Schiff bases.
The carbon atom C-1 in all these compounds appeared around
δ = 108–110 ppm depending on the substituent present in
the structure. The carbonyl carbon chemical shift is of primary
importance in these anils because it forms a strong intramolecular
hydrogen bond with the NH group and is therefore expected to
have a much greater effect in determining the direction of the
keto–enol equilibrium. Compound 1 showed the most downfield
shift of δ = 181.1 ppm implying a keto tautomeric structure for
thiscompoundastheC Ogroupgenerallyfallsinthisregion. This
is consistent with the highest coupling constant [3JNH−H] value
observedforthiscompound(Table 5).Compounds2and3showed
a value of approximately δ = 178 ppm, slightly lower than that
observed for compound 1 indicating a possible slight shift in the
proton transfer equilibrium, once again correlating with the result
obtained for the coupling constants (J = 9.6 Hz). Compounds4–6
showed values of δ = 176.4–179.6 ppm, which are slightly lower
than previously discussed compounds. Interestingly, compound
7 showed a value of δ = 180.3 ppm, which is close to compound
1 (δ = 181.1 ppm); however, it showed only a 3.0-Hz coupling
constant. The phenyl-substituted Schiff bases (compounds 8–12)
showedchemicalshiftvaluesintherangeof δ = 165–169 ppmfor
C-2 which is consistent with the 3JNH−H coupling constant. Based
on these results, we conclude that phenyl-substituted compounds
tend to favor an enol tautomer, whereas the pyridyl-substituted
compounds tend to form the keto tautomer. It is also possible that
the phenyl compounds exist as a zwitterionic form based on the
shieldings observed in NMR spectra.
Table 1. Proton chemical shifts for pyridyl-substituted anilsa
Compounds
1
4
5
6
7
Solvent
3
DMSO
CDCl3
CDCl3
CDCl3
CDCl3
6.72
(d, 9.3)
7.82
6.95
(d, 9.2)
7.76
(d, 8.3)
7.66
(d, 7.7)
7.33
(t, 7.3)
7.52
(t, 7.6)
8.14
(d, 8.3)
–
6.94
(d, 9.2)
7.76
(d, 9.2)
7.63
(t, 7.6)
7.33
(t, 7.2)
7.52
(t, 7.6)
8.13
(d, 8.2)
–
6.89
6.87
(d, 9.1) (d, 9.3)
7.71 7.73
(d, 9.4) (d, 9.3)
7.59 7.58
(d, 7.6) (d, 7.7)
7.29 7.33
(t, 7.3) (t, 7.4)
7.51 7.52
(t, 7.5) (t, 7.6)
8.11 8.08
(d, 8.2) (d, 8.1)
4
5
6
7
8
(d, 9.3)
7.67
(bd, 7.7)
7.28
(t, 7.3)
7.50
(dt, 7.6)
8.09
(d, 8.0)
–
9
–
–
–
–
10
11
–
–
–
9.71
9.93
(d, 6.1)
15.38
(d, 6.1)
–
9.92
(d, 6.3)
15.36
(d, 6.2)
–
9.91
9.81
(d, 10.1)
15.15
(d, 10.1)
–
(d, 7.2) (d, 7.1)
15.41 15.41
(d, 7.2) (d, 7.1)
12
13
14
15
–
–
–
–
–
–
–
–
7.98
8.42
(d, 2.5)
–
8.51
(d, 2.0)
–
8.57
(bs, 1.7)
–
(dd, 1.2, 0.6)
7.15
16
17
18
19
6.99
(d, 7.3)
7.61
(dd, 7.8, 4.6)
7.35
7.70
7.83
7.97
(dd, 7.9, 1.2) (dd, 8.4, 2.5) (dd, 8.4, 2.0) (t, 7.7) (d, 1.7)
–
7.12
(d, 8.4)
–
7.07
(d, 8.3)
–
6.94
(d, 7.7)
2.60
–
10.94
(OH)
–
(Me)
a Proton chemical shifts for pyridyl-substituted anils (Note: NH protons
are represented as H-12 in all compounds and showed a doublet
corresponding to a keto tautomer. Similarly, proton labeled as H-11 is
the olefinic proton which also showed a doublet in all the compounds
demonstrating a keto tautomer. Note: The multiplicity and coupling
constants in Hz are shown in parenthesis.
Comparing H-17 in the pyridyl-substituted Schiff bases, we
observe that compound 1 showed the shielding (δ = 7.35 ppm)
compared to the rest of the compounds (Table 1). The other
compounds showed a chemical shift value of approximately
δ = 8.0 ppm. This shift is attributed to the electron-withdrawing
character of the halogens in the molecule. Compound 7, which
has a trifluoromethyl group, showed the highest deshielding
(δ = 8.57 ppm) for H-15 due to the strong electron-withdrawing
characteristics compared to halogen atoms. In the phenyl-
substituted Schiff bases, H-17 showed a systematic increase in
the chemical shift going from chloro to iodo (Table 3). The iodo
compound had a 0.32-ppm high-frequency shift compared to
the chloro compound. Compound 11 having a methoxy group
in the structure shifted H-17 to lower frequency (δ = 7.00 ppm)
consistent with the electron-donating nature of the methoxy
group.
The other carbons on the naphthyl ring were found to be
in similar ranges for each of the compounds. Considering the
chemical shift of C-11, except for compound 1, values were
between δ = 149 and 151 ppm. The chemical shift for C-
11 in the phenyl-substituted compounds (Table 4) was around
Further insight into the position of the proton transfer in
the equilibrium was gained by evaluating the coupling constant
c
www.interscience.wiley.com/journal/mrc
Copyright ꢀ 2010 John Wiley & Sons, Ltd.
Magn. Reson. Chem. 2010, 48, 585–592

Substituent effects on the NMR chemical shifts
Table 2. Proton chemical shifts for phenyl-substituted anilsa
Compounds
2
3
8
9
10
11
12
Solvent
3
DMSO
DMSO
CDCl3
CDCl3
CDCl3
CDCl3
CDCl3
6.79
(d, 9.3)
7.79
6.77
(d, 9.2)
7.78
(m)
7.12
(d, 9.1)
7.82
7.12
(d, 9.0)
7.83
7.11
(d, 9.0)
7.82
7.14
(d, 9.0)
7.82
7.20
(d, 9.0)
7.91
(d, 9.0)
7.80
(bs)
4
5
6
7
8
(d, 9.3)
7.67
(d, 8.9)
7.73
(d, 9.0)
7.74
(d, 9.0)
7.73
(d, 9.0)
7.75
7.66
(d, 7.8)
7.26
(t, 7.3)
7.48
(t, 7.6)
8.39
(d, 8.3)
–
(d, 7.9)
7.26
(d, 7.9)
7.36
(d, 7.8)
7.37
(d, 7.8)
7.36
(d, 7.8)
7.36
7.42
(t, 7.5)
7.60
(t, 7.6)
8.19
(d, 8.3)
–
(t, 7.4)
7.48
(t, 7.4)
7.54
(t, 7.4)
7.54
(t, 7.3)
7.54
(t, 7.3)
7.55
(t, 7.5)
8.38
(t, 7.6)
8.10
(t, 7.6)
8.11
(t, 7.8)
8.10
(t, 7.5)
8.14
(d, 8.0)
–
(d, 8.3)
–
(d, 8.4)
–
(d, 8.4)
–
(d, 8.3)
–
9
10
11
–
–
–
–
–
–
–
9.50
9.47
(d, 9.3)
15.69
(d, 9.3)
–
9.33
9.35
9.33
9.35
9.49
(d, 1.4)
14.62
(d, 1.4)
–
(d, 9.3)
15.72
(d, 9.3)
–
(d, 3.1)
15.27
(d, 3.1)
–
(d, 3.2)
15.25
(d, 3.2)
–
(d, 3.0)
15.24
(d, 3.0)
–
(d, 2.4)
15.34
(d, 2.4)
–
12
13
14
7.93
7.79
(m)
7.29
7.24
7.11
7.45
7.76
(bs)
(dd, 8, 1.0)
6.95
(d, 8.6)
7.42
(d, 8.6)
7.57
(d, 7.8)
7.76
(d, 2.5)
–
15
16
17
18
–
–
(t, 7.7)
7.11
(d, 8.7)
–
(d, 8.6)
–
(d, 8.3)
–
6.90
(td, 8.1)
6.88
–
7.81
(bs)
–
(t, 7.6)
7.01
7.42
(d, 8.7)
7.29
(d, 8.6)
–
7.57
(d, 8.6)
7.24
(d, 8.6)
–
7.76
(d, 8.3)
7.11
(d, 7.8)
–
7.00
(d, 8.7)
7.26
(dd, 7.9, 1.0)
–
(d, 8.0)
–
7.76
(bs)
–
(dd, 8.6, 2.5)
–
19
20
10.35
–
10.06
2.31
–
–
–
3.95
–
(CH3)
(OCH3)
a The multiplicity and coupling constants are given in the parenthesis below the chemical shifts.
R
H
N
R
R
N
N
O
H
O
H
O
Scheme 2. Hydrogen bonding structures for the Schiff bases..
HO
H
O
N
O
N
N
H
O
N
H
N
H
N
O
O
H
(iii)
(ii)
(i)
Scheme 3. Bifurcated structural features for Schiff bases containing hydroxyl group in their structures..
δ = 149–160 ppm, and there was only a slight variation in
chemical shift values. However, C-16 showed an interesting
trend in halogen-substituted phenyl Schiff bases. For example,
the chloro compound 8 showed a chemical shift of δ = 132.0 ppm
followedbybromoandiodocompounds.Ashieldingofthiscarbon
was observed in the iodo-substituted derivative perhaps owing
c
Magn. Reson. Chem. 2010, 48, 585–592
Copyright ꢀ 2010 John Wiley & Sons, Ltd.
www.interscience.wiley.com/journal/mrc

T. K. Venkatachalam et al.
Table 3. Carbon chemical shifts for pyridyl-substituted anils
Table 5. Observed coupling constants between NH proton and
olefinic proton in Schiff bases
Compounds
Coupling constants
1
4
5
6
7
Compound
[3JNH−H in Hz]
Solvent
DMSO
CDCl3
CDCl3
CDCl3
CDCl3
1
10.1
9.6
9.4
6.1
6.3
7.1
7.1
3.2
3.2
3.0
2.4
1.2
1
107.9
181.1
125.9
139.7
129.3
123.2
128.8
119.1
134.0
126.1
145.3
–
109.0
176.4
123.7
139.0
129.3
124.0
128.5
119.4
133.8
127.2
151.7
–
109.1
176.7
123.9
139.2
129.4
124.1
128.6
119.4
133.8
127.2
151.4
–
108.5
179.6
125.2
139.3
129.3
123.7
128.4
119.2
134.2
126.8
149.3
–
110.3
180.3
124.7
140.9
129.5
124.9
129.0
119.6
133.6
127.3
148.5
–
2
2
3
3
4
4
5
5
6
6
7
7
8
8
9
9
10
11
12
10
11
12
13
14
15
16
17
18
19
140.5
–
151.9
–
152.2
–
151.5
–
152.3
–
138.7
121.9
123.8
143.4
–
147.7
128.7
138.2
117.2
–
149.9
116.9
141.0
117.6
–
158.4
120.5
138.7
112.3
24.4 (Me)
143.8
122.2
135.4
121.9
123.6 (CF3)
We further extended our studies to determine if introduction
of the groups on the pyridyl or phenyl ring affects the naphthyl
protons/carbons and the following paragraph summarizes these
results. To investigate the effect of changing the substituents on
the pyridine/phenyl ring, the chemical shifts were monitored and
compared. Tables 1 and 2 show the proton chemical shifts for
compounds 1,2 and3 in DMSO-d6 solvent. Significant chemical
shift change was observed for the NH protons for this set
of compounds. The pyridyl-substituted compounds showed a
shielding of approximately 0.5 ppm compared to the phenyl-
substituted compounds perhaps due to the influence of the
nitrogen atom in the pyridyl ring. In the case of H-11, an opposite
trend was obtained. In other words, the pyridyl-substituted
compound showed a deshielding by 0.2 ppm compared to the
to the electron-withdrawing capacity of this group. The other
compounds showed a value closer to δ = 116–117 ppm. C-17 also
behavedsimilarlywithchlorineoriodinesubstitutioninthephenyl
ring. In the case of C-18, compounds 2 and 3 showed the highest
deshielding owing to the presence of hydroxyl functionality in the
structure of the compound.
Table 4. Carbon chemical shifts for phenyl-substituted anils
Compounds
2
3
8
9
10
11
12
Solvent
DMSO
DMSO
CDCl3
CDCl3
CDCl3
CDCl3
CDCl3
1
107.7
177.7
125.1
138.0
129.0
123.0
128.1
119.7
133.9
125.8
149.4
–
107.6
178.0
125.3
137.9
129.0
123.0
128.1
119.7
134.0
125.8
148.9
–
108.9
168.4
121.4
136.5
129.4
123.7
128.1
118.9
133.0
127.4
155.8
–
109.0
168.4
121.5
136.6
129.4
123.7
128.1
119.0
133.0
127.5
155.9
–
109.0
168.8
121.6
136.7
129.4
123.7
128.1
118.9
133.0
127.4
155.5
–
109.1
166.9
121.0
135.9
129.3
123.5
128.0
119.1
132.9
127.5
155.6
–
109.2
165.7
121.2
136.9
129.5
124.1
128.4
119.2
132.8
127.8
159.8
–
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
128.6
117.6
119.8
126.8
116.0
148.4
–
128.8
117.6
127.9
127.2
115.8
146.1
–
144.7
121.7
129.7
132.0
129.7
121.7
–
145.2
122.1
132.7
119.9
132.7
122.1
–
145.7
122.4
138.6
90.8
140.2
122.0
123.4
153.5
112.6
120.6
–
149.4
120.2
133.1
119.7
133.1
120.2
–
138.6
122.4
–
–
20.4(Me)
–
–
–
56.4(OMe)
123.0(CF3)
c
www.interscience.wiley.com/journal/mrc
Copyright ꢀ 2010 John Wiley & Sons, Ltd.
Magn. Reson. Chem. 2010, 48, 585–592

Substituent effects on the NMR chemical shifts
(a)
16
15
14
13
12
11
10
9
(b) 190.00
180.00
170.00
160.00
150.00
140.00
130.00
120.00
110.00
100.00
C1
C2
C3
C4
C11
H3
H4
H11
NH
8
7
6
4
5
6
7
8
9
10 11 12
4
5
6
7
8
9
10 11 12
compounds
compounds
Figure 2. Comparison of proton (a) and carbon (b) chemical shifts for compounds soluble in CDCl3 (4–12).
phenyl-substituted analogs. This trend is expected because the
electron density at the NH proton is altered as described earlier.
We next examined the chemical shift of H-4 and found that a
marginal change was observed for the pyridyl-substituted analog
(shielding). No significant changes were observed for H-3 in these
three compounds.
Tables 3 and 4 show the 13C chemical shift analysis for
compounds 1–3 in DMSO-d6. It is evident from the figure that the
carbonyl chemical shifts (C O) for the pyridyl compound were
higher (δ = 181.1 ppm) compared to the phenyl compounds
(δ = 178.2 ppm). Considering C-11 chemical shifts (olefinic
carbon), once again the pyridyl compound gave an opposite
trend with a shift value of δ = 145.3 ppm compared to the phenyl
analog (δ = 149.0). This is similar to the trend observed for the
change in proton chemical shift values. The only other carbon
which showed a slight variation was C-4. The pyridyl compound
showed a 2-ppm deshielding compared to the phenyl derivative.
We next focused our attention on the changes in chemical
shifts for various protons in the pyridyl and phenyl series of
Schiff bases. Figure 2a illustrates the comparative analysis of
chemical shifts for this set of compounds. Considering first the
NH protons, the pyridyl-substituted compounds showed a high-
frequency shift compared to the phenyl-substituted compounds
forallsimilarsubstituentsexceptforcompound12.Compound12,
containing two CF3 groups in meta-position, behaved differently
from all others in the series. In the case of H-11, the pyridyl-
substituted compound showed a deshielding of 0.06 ppm. The
above trend in the pyridyl/phenyl-substituted compounds differs
from that observed in the hydroxyl-substituted compounds 1–3.
It is plausible that DMSO plays a considerable role in changing the
chemical shift values. The H-4 protons were affected marginally,
whereas H-3 protons were shielded by 0.2 ppm in the pyridyl
series.
Then, we analyzed the 13C chemical shift characteristics of
the Schiff bases (Fig. 2b). The carbonyl C-2 in the pyridyl series
was deshielded by 12 ppm compared to the phenyl compounds.
Similarly, C-11 was found to show a 5-ppm shielding in the
pyridyl series. Once again, the CF3-substituted compound showed
a maximum high-frequency shift of approximately δ = 160 ppm.
C-4 also showed a 5-ppm deshielding for the pyridyl series. Only
marginal changes were observed for C-3 (2 ppm). It is interesting
to point out that the chemical shift value for C-1 did not change in
all the compounds.
Figure 3. Minimum energy conformation of compound 8 predicted from
molecular modeling.
molecularmodelingstudies, theproximityoftheoxygenatomand
the NH suggests intramolecular hydrogen bonding which results
in the formation of a six-membered ring, affecting the chemical
shift values.
Having compiled the spectral analytical data, we focused our
attention on the structure of the compounds. Based on our
X-ray crystal data (unpublished results), we believe that these
compounds exist as keto or zwitterionic compounds. Most of
the compounds showed a planar structure owing to the strong
intramolecular hydrogen bonding between the oxygen atom
and the NH group which provided a stable six-membered ring
configuration. This facilitates delocalization of electrons resulting
in the intense color exhibited by these Schiff bases. We also
conductedmolecularmodelingstudiesonafewofthecompounds
and found that the compounds exist in a planar configuration
(Fig. 3). These Schiff bases possess thermochromic properties,
changing their color with temperature.
In summary, we have synthesized Schiff bases and compared
their proton and carbon NMR chemical shifts and found that
pyridyl-substituted compounds showed the highest deshieldings
in the series. Further work is in progress to gain insight into the
solid-state structure for these Schiff bases.
The chemical shift changes observed for various substituents in
bothpyridylandphenylseriesarerationalizedduetotheinductive
and resonance effects. It is possible that there are electronic
interactions between the ortho-substituted electronegative atom
(O in the case of OH group or Cl) and the amine proton. From the
Experimental
All the chemicals were obtained from Sigma–Aldrich and were
used without further purification. NMR spectra were acquired in
CDCl3 and DMSO-d6. The chemical shifts are reported relative to
c
Magn. Reson. Chem. 2010, 48, 585–592
Copyright ꢀ 2010 John Wiley & Sons, Ltd.
www.interscience.wiley.com/journal/mrc

T. K. Venkatachalam et al.
CDCl3 peak at δ = 7.27 ppm and DMSO-d6 at δ = 2.54 ppm.
NMR spectra were acquired using both chloroform and DMSO-
d6 as solvents. DMSO-d6 was used for some of the compounds
(especially those with the polar hydroxyl group) because of the
poorsolubilityofthesecompoundsinchloroform.ForNMRstudies,
10 mg of the Schiff bases were used in 0.5 ml of the NMR solvent.
The NMR data were acquired on a Bruker 900 MHz NMR
spectrometerequippedwithacryoprobe. Theprotonspectrawere
acquired with a sweep width of 18 ppm centered at δ = 7 ppm.
The carbon spectra were acquired with a sweep width of 220 ppm
centered at δ = 110 ppm. The DQCOSY and TOCSY experiments
were also acquired with a sweep width of 18 ppm using a 90◦ pulse
[12] M. D. Cohen, S. Flavian, L. Leiserowitz, J. Chem. Soc. B 1967, 329.
[13] H. Unver, D. M. Zengin, K. Guven, J. Chem. Crystallogr. 2000, 30, 359.
[14] L. Antonov, M. F. Fabian, D. Nedeltcheva, F. S. Kamounah, J. Chem.
Soc. Perkin Trans. 2000, 2, 1173.
[15] G. O. Dudek, E. P. Dudek, J. Am. Chem. Soc. 1964, 86, 4283.
[16] G. O. Dudek, R. H. Holm, J. Am. Chem. Soc. 1962, 84, 2691.
[17] G. O. Dudek, J. Am. Chem. Soc. 1963, 85, 694.
[18] G. O. Dudek, G. Volpp, J. Am. Chem. Soc. 1963, 85, 2697.
[19] A. M. Aisri, K. O. Badahdah, Molecules 2007, 12, 1796.
[20] E. Hadjoudis, S. D. Chatziefthimiou, I. M. Mavridis, Curr. Org. Chem.
2009, 13, 269.
[21] S. R. Salman, J. C. Lindon, R. D. Farrrant, T. A. Carpenter, Magn.
Reson. Chem. 1993, 31, 991.
[22] W. Schilf, P. Cmoch, A. S. Chelmieniecka, E. Grech, J. Mol. Struct.
2009, 921, 34.
[23] W. Schilf, J. Mol. Struct. 2004, 689, 245.
[24] W. Schilf, B. Kazie, Z. Rozwadowski, B. Bleg, T. Dziembowoska, J.Mol.
Struct. 2004, 700, 61.
[25] J. Schmidt, A. Hoffmann, H. W. Spiess, D. Sebastiani, J. Phys. Chem. B
2006, 110, 23204.
[26] P. Fita, E. Luzina, T. Dziembowska, D. Kopec, P. Piatkowski,
C. Z. Radzewicz, A. Grabowaska, Chem. Phys. Lett. 2005, 416, 305.
[27] D. Rohrbach, W. R. Heineman, E. Deutsch, Inorg. Chem. 1979, 18,
2536.
[28] R. K. Kholi, P. K. Bhattacharaya, Bull.Chem.Soc.Japan1976, 49, 2872.
[29] R. J. Abraham, M. Mobli, R. J. Smith, Magn. Reson. Chem. 2003, 41,
26.
1
of 9 µs with 256 and 350 increments, respectively. The {15N} H
HSQCspectrumwasacquiredwithsweepwidthsof 18and45 ppm
for proton and nitrogen, respectively, and the nitrogen centered
at δ = 118 ppm. There was no attempt to reference the nitrogen
chemical shift. It was used to identify the proton directly attached
to the nitrogen. The raw data were usually multiplied by an
exponential or shifted sine squared function before performing
the Fourier transform. Assignments of the protons and carbons
were made using the data generated from COSY, TOCSY, DQCOSY
NMR techniques.
General procedure for the synthesis of anil derivatives: appro-
priate amine compound (0.05 mol), 2-hydroxynaphthaladehyde
(0.05 mol) and 50 ml of absolute ethanol were added to a 100-ml
round bottom flask, the contents stirred and the resulting solution
refluxed over an oil bath for 5 h. The precipitated anils were filtered
under vacuum and washed with copious amount of ethanol and
dried in vacuum. Further purification was achieved by crystalliza-
tion using chloroform or methanol. Condensation of the above
aldehyde with appropriately substituted amines in ethanol gave
the Schiff bases in good yields.
[30] P. V. Rao, C. P. Rao, E. K. Wegelius, K. Rissanen, J. Chem. Cryst. 2003,
33, 139.
[31] G. J. M. Fernandez, F. Rio-Portilla, B. Quiroz-Garcia, R. A. Toscano,
R. Salcedo, J. Mol. Struct. 2001, 561, 197.
[32] R. Herzfeld, P. Nagy, Curr. Org. Chem. 2001, 5, 373.
[33] M. D. Cohen, G. M. J. Schmidt, S. Flavian, J. Chem. Soc. 1964, 2041.
[34] J. M. Sosa, M. Vinkovic, D. V. Topic, Croat. Chem. Acta 2006, 79, 489.
[35] S. I. Gusev, I. A. Kozhevnikova, Z. Neorganischeskoi Khimii 1964, 9,
2403.
[36] H. Ranganathan, D. Ramaswamy, T. Ramaswami, M. Santappa, Ind.
J. Chem. Sec. A 1986, 25A, 127.
[37] H. Ranganathan, D. Ramaswamy, T. Ramaswami, M. Santappa,
Chem. Lett. 1979, 10, 1201.
[38] H. Unver, M. Yildiz, A. Kiraz, N. O. Iskeleli, A. Erdonmez, B. Dulger,
T. N. Durlu, J. Chem. Cryst. 2006, 36, 229.
Acknowledgement
[39] R. M. Issa, A. M. Khedr, H. Rizk, J. Chin. Chem. Soc. 2008, 55, 875.
[40] A. Ozek, S. Yuce, C. Albayrak, M. Odabasoglu, O. Buyukgungor, Acta
Cryst. 2004, E60, o356.
We thank the Queensland NMR Network (QNN) for granting access
to the Bruker 900 MHz spectrometer.
[41] T. J. Meng, X. Q. Qin, W. X. Zhao, X. Q. Huang, G. D. Wei, Acta Cryst.
2008, E64, o1520.
[42] T. Hokelek, M. Isilklan, Z. Kilic, Anal. Sci. 2000, 16, 99.
[43] Y. L. Kai, S. Z. Min, Q. Y. Qing, Z. D. Xia, W. Yue, W. R. Shun, R. Ai. Min,
F. Ji. Kang, Gaodeng Xuexiao Huaxue Xuebao 2006, 27, 711.
[44] M. Gunduz, S. Bilgic, O. Bilgic, D. Ozogut. ARKIVOC 2008, 13, 115.
[45] H. Unver, M. Kabak, D. M. Zengin, T. N. Durlu. Z. Naturforsch 2001,
56b, 1003.
References
[1] D. Chen, A. E. Martel, Inorg. Chem. 1987, 23, 1026.
[2] J. W. Pyrz, A. L. Roe, L. J. Stern, L. Que Jr, J. Am. Chem. Soc. 1985, 107,
614.
[3] J. Costamagna, J. Vargas, R. Latorre, A. Alvarado, G. Mena, Coord.
[46] T. Dziembowska, M. Szafran, A. Katrusiak, Z. Rozwadowski, J. Mol.
Chem. Rev. 1992, 119, 67.
[4] N. E. Dixon, C. Gazzalo, J. J. Watters, R. L. Blackely, B. Zerner, J. Am.
Chem. Soc. 1975, 97, 4130.
Struct. 2009, 929, 32.
[47] Z. Rozwadowski, W. Schilf, B. Kamienski, Magn. Reson. Chem. 2005,
43, 573.
[48] T. Dziembowska, Z. Rozwadowski, A. Filarowski, P. E. Hansen, Magn.
Reson. Chem. 2001, 39, S67.
[49] K. A. Abbas, S. R. Salman, S. M. Kanan, Z. A. Fataftah, J. Appl.
Spectrosc. 1996, 41, 119.
[5] C. T. Walsh, W. H. Orme Johnson, Biochemistry 1987, 26, 4901.
[6] Y. Nishida, K. Kino, S. Kida, J. Chem. Soc. Dalton Trans. 1987, 1157.
[7] J. W. Ledbetter Jr, J. Phys. Chem. 1977, 81, 54.
[8] S. R. Salman, S. R. Farrant, J. C. Lindon, Spect. Lett. 1991, 24, 1071.
[9] J. W. Ledbetter, J. Phys. Chem. 1966, 70, 2245.
[10] P. Nagy, R. Harzfield, Spect. Lett. 1998, 31, 221.
[11] M. Rospenk, I. Krol-Starzomska, A. Filarwoski, A. Koll, Chem. Phys.
2003, 287, 113.
c
www.interscience.wiley.com/journal/mrc
Copyright ꢀ 2010 John Wiley & Sons, Ltd.
Magn. Reson. Chem. 2010, 48, 585–592