Synthesis and characterization of anils exhibiting thermochromism

Article abstract of DOI:10.1071/CH09579

Several Schiff bases containing a hydroxy naphthyl moiety and substituted pyridyl groups were synthesized. The pyridyl substituted Schiff bases were isolated as a single stable tautomer at room temperature. High-resolution proton and carbon NMR spectrosco

Full text of DOI:10.1071/CH09579

Full Paper
CSIRO PUBLISHING
Aust. J. Chem. 2010, 63, 1272–1282
www.publish.csiro.au/journals/ajc
Synthesis and Characterization of Anils Exhibiting
Thermochromism
T. K. Venkatachalam,A,C Gregory K. Pierens,B and David ReutensA
ACentre for Advanced Imaging, The University of Queensland, Brisbane, Qld 4072, Australia.
BCentre for Magnetic Resonance, The University of Queensland, Brisbane, Qld 4072, Australia.
CCorresponding author. Email: t.venkatachalam@uq.edu.au
Several Schiff bases containing a hydroxy naphthyl moiety and substituted pyridyl groups were synthesized. The pyridyl
substituted Schiff bases were isolated as a single stable tautomer at room temperature. High-resolution proton and carbon
NMR spectroscopy showed that these compounds exist as keto tautomers. The Schiff bases showed extraordinary stability
and did not convert to the enol tautomer, even at high temperatures. Most of the compounds exhibited thermochromic
properties.
Manuscript received: 5 November 2009.
Manuscript accepted: 18 March 2010.
Introduction
Recently, Asiri et al.[11] described the synthesis and charac-
terization of new anils possessing a heterocyclic core. Ultraviolet
and infrared spectroscopic studies of keto-enol formation of
these pyridine based naphthyl anils, suggested the existence of
both keto and enol forms in the solution state. Salaman et al.[13]
used solution and solid-state 13C NMR spectroscopic meth-
ods to demonstrate the tautomeric equilibrium of 2-hydroxy
naphthaldehyde derived anils. Schilf et al.[14–16] studied the
effects of deprotonation of Schiff bases using proton sponges and
other strong nitrogen containing bases on tautomerism. 1H and
13C NMR spectroscopy was used to monitor the proton transfer,
which only occurred under specific conditions with very strong
bases (pKa >18.18).
Thermochromic compounds have found wide use and
application in recent years. For example, thermochromic liq-
uid crystals are routinely used in thermometers. Thermo-
chromic dyes have been used in thermometric applications
such as household battery state indicators as well as in
inks, paints, papers, semiconductor devices, and in other tem-
perature sensitive devices. Due to the wide application of
thermochromic compounds, we attempted to synthesize and
characterize certain anils possessing both a naphthyl and a
pyridyl ring in their structures, and subsequently examine
their thermochromic properties. In these compounds, substitu-
tion of the pyridyl ring with various electron-withdrawing or
-donating groups resulted in the exclusive formation of the
keto-tautomers.
Schiff bases containing hydroxyl groups have attracted attention
because of their photochromic and thermochromic proper-
ties, which have been attributed to keto-enol tautomerism.[1–26]
Photochromismisareversibleprocessinwhichasinglechemical
species exhibits two different absorption spectra in the presence
of light, depending on its electronic state. Compounds exhibit-
ing photochromism are present in an equilibrium state. Since
the 1950s, it has been known that Schiff bases derived from sal-
icylaldehyde and aniline derivatives (anils) are thermochromic,
changing their colours from yellow to orange at different tem-
peratures in solid state. The reversible thermochromic property
associated with these anils has been attributed to keto-enol
tautomerism, with proton transfer, as shown in Scheme 1.
The tautomers are in a temperature dependent equilibrium
with proton transfer resulting either in the excited state or the
ground state of the molecule. It has been postulated that in
planar molecules the amine basicity is higher for the nitrogen
atom in the molecule resulting in the lone electron pair not
being involved with either ring. On the contrary, in non-planar
molecules this overlap occurs with the ring. In other words, the
OH· · ·N bond is stronger in the ground state in planar molecules.
An increase in temperature results in a high proportion of
the keto form, thus resulting in a deepening of the colour of
the crystal. Cohen et al.[27] have shown that the difference in the
energy associated with such a thermochromic process is minimal
(1.76 kcal mol−1).
H
R
N
R
R
N
N
O
H
O
H
O
Scheme 1.
© CSIRO 2010
10.1071/CH09579
0004-9425/10/081272

Synthesis and Characterization of Anils Exhibiting Thermochromism
1273
HO
N
CHO
OH
OH
ϩ
EtOH/reflux
5–6 h
N
N
R
NH2
OH
R
CHO
N
EtOH/reflux
5–6 h
OH
ϩ
ϩ
OH
NH2
OH
CHO
EtOH/reflux
5–6 h
N
N
H2N
NH2
OH
HO
Scheme 2. Synthesis of various anils.
Results and Discussion
band at 3500 cm−1, thus indicating the absence of the OH group.
Based on the above result we tentatively assigned the structure of
the compound as a keto tautomer. In the previous reports,[13–24]
the presence of hydrogen bonding in the enol tautomer has been
deduced from the chemical shift of the OH protons with a broad
singlet at ∼14 ppm. The chemical shift of the hydroxyl group
in our compound is 1.42 ppm downfield of 14 ppm, leading us
to conclude that it is due to the NH group of the keto tautomer.
It is also interesting to note that Dudek et al.[7–10] postulated in
1963 that the proton on the NH in naphthalene derivatives might
split into doublets, although they were not able to observe this
phenomenon perhaps due to the low field of their spectrometers.
Compound 2 showed doublets at 15.27and 9.34 ppm in the
1H NMR spectrum, corresponding to the NH and CH=N pro-
tons, respectively (Fig. 1). These values are consistent with the
data obtained by Sosa et al.[28] Similarly, the 13C NMR spectrum
showed a signal at 168.4 ppm in CDCl3, which was assigned
to the C=O carbon in the keto tautomer (Fig. 2). This value
was also similar to that obtained by Sosa et al. (169.4 ppm in
DMSO-d6), who suggested that this Schiff base exists as the keto
tautomer in solution. These authors reported that the infrared
spectral data gave evidence for an enol tautomer in solid state
due to the absence of a carbonyl stretching band. In contrast, our
infrared spectral data provided the carbonyl-stretching band at
1631 cm−1, suggesting that it is likely that the keto tautomer
exists in the solid state. This is further substantiated by the
X-ray crystal structure data (unpublished results) for compound
2 which showed a keto tautomeric structure in the solid state.
Examination of the proton NMR spectrum of compound 3
revealed similar findings with doublets for the NH and CH=N
protons, corroborating the existence of a keto tautomeric form
forthiscompound. Our13CNMRspectraldataalsoprovidedevi-
dence for a keto tautomeric form of 3, with a signal at 176.7 ppm
for the C=O carbon atom. Gusev et al.[29] have prepared the lead
complex of this compound using lead nitrate, although no NMR
data were published.
The synthesis of anils was accomplished using a general protocol
shown in Scheme 2. In brief, appropriate amino compounds were
condensed with 2-hydroxy naphthaldehyde, which was chosen
specifically to retain the hydroxy moiety. This hydroxyl group
can then be conveniently functionalized to obtain other substi-
tuted anils. The anils were obtained as crystalline materials with
brilliant colours. Table 1 shows the structures of anils prepared
in the present study and the colour of each compound at room
temperature, which varied from bright orange to pale yellow.
The UV spectra of these anils in both methanol and chloroform,
showed an intense peak at ∼400–460 nm. Additional absorption
peaks were also observed at around 350 nm, in keeping with
previous reports,[11] suggesting the existence of both enol and
keto forms. This was irrespective of the substituents on the pyri-
dine ring, which did not result in preferential formation of either
tautomer.
The FT-IR spectra were similar for most of the anils, with
the presence of a broad band between 3500 and 3200 cm−1, and
peaks between 1700 and 1600 cm−1. In certain anils, the C=O
stretching band was much stronger compared with other anils.
Nonetheless, these data were consistent with the presence of both
enol and keto tautomeric forms.
The NMR spectra were carefully analyzed for each of the
compounds in the present study to thoroughly understand the
structure of the compounds. Although there is an abundance
of published literature on the structural determination of these
anils, there are inconsistencies in the interpretation of the physi-
cal data. To overcome these discrepancies we have undertaken a
detailed study to synthesize these anils and examine their NMR
spectra using various techniques and to compare our data with
published values. The proton NMR of compound 1 showed a
doublet at 15.32 ppm with a coupling constant of ∼2 Hz, which
was assigned to the NH proton. We rationalize that the formation
of this doublet is attributed to the coupling between the NH pro-
ton and the neighbouring CH proton.This is further substantiated
by the fact that the CH=N proton also resonated as a doublet,
with similar coupling constant (∼2 Hz).The 13C NMR spectrum
showed a signal at 166.9 ppm, which was assigned as the C=O
carbon, even though the signal is shifted upfield in comparison
to typical C=O groups. However, the IR spectrum did not show a
Ranganathan et al.[30,31] reported that the proton NMR spec-
trum of compound 4 showed a doublet at 15.40 ppm, which was
assigned to a hydroxyl group in the structure. These authors also
observed a doublet for the CH=N proton. Our findings were
similar, however we assigned these doublets to the coupling of

1274
T. K. Venkatachalam, G. K. Pierens, and D. Reutens
Table 1. Characteristics of compounds
Structure
Colour
Thermochromic
Cl
H
N
OMe
Orange yellow crystals
Changes to bright yellow at −40◦C and deep orange at 50◦C
O
O
O
1
H
N
Cl
Greenish yellow crystals
Orange yellow crystals
Changes colour to crimson green at −40◦C
2
H
N
Br
N
N
Converts to yellow at −40◦C. Becomes red at 50◦C
3
H
N
Cl
Orange yellow
Crimson yellow
Converts to deep yellowish green at −40◦C. Becomes orange at 50◦C
O
4
CF3
CF3
H
N
Crimson green at −40◦C. Higher temperatures gave slight orange tinge
O
5
Cl
O
H
N
CF3
Changes to bright yellow at −40◦C and light orange at 50◦C
Converts to yellow powder at −40◦C. Becomes dark orange at 50◦C
Changes to yellow at −40◦C. At higher temperatures it remains orange
N
N
Pale yellow crystals
6
HO
H
N
Orange powder
O
7
OH
HO
N
N
Deep orange crystals
8
H
N
N
Yellow crystals
Pale yellow
Does not change colour at −40◦C. When the temperature is increased to 50◦C,
O
Me
a slight orange colour is observed
9
N
N
Does not change colour at −40◦C. Changes to darker yellow at 50◦C
Changes to light yellow green on −40◦C. Remains olive green at 50◦C
OH
HO
10
H
N
I
O
Olive green crystals
11

Synthesis and Characterization of Anils Exhibiting Thermochromism
1275
15.36
15.32
15.28
15.24
15.20
f1 [ppm]
9.40 9.38 9.36 9.34 9.32 9.30 9.28
f1 [ppm]
15.5
15.0
14.5
14.0
13.5
13.0
12.5
12.0
11.5
11.0
10.5
10.0
9.5
9.0
8.5
8.0
7.5
7.0
f1 [ppm]
Fig. 1. 1H NMR spectrum of compound 2 in CDCl3 at room temperature (only the NH doublet and the CH doublet are shown in the spectrum).
172
168
164
160
156
152
148
144
140
136
132
128
124
120
116
112
108
f1 [ppm]
Fig. 2. 13C NMR spectrum of compound 2 in CDCl3 at room temperature (note the peak at 169 ppm corresponding to the C=O of the keto-enamine
tautomer).

1276
T. K. Venkatachalam, G. K. Pierens, and D. Reutens
15.55
15.50
15.45
15.40
15.35
15.30
f1 [ppm]
9.87
9.84
9.81
f1 [ppm]
9.78
15.5
15.0
14.5
14.0
13.5
13.0
12.5
12.0
11.5
11.0
f1 [ppm]
10.5
10.0
9.5
9.0
8.5
8.0
7.5
7.0
6.5
Fig. 3. 1H NMR spectrum of compound 6 in CDCl3 at room temperature (only the doublets are shown for NH and CH protons).
the NH proton with the CH=N proton. The 13C NMR spec-
trum showed a signal at 176.4 ppm corresponding to the carbon
of the C=O group. Additional evidence for the keto tautomeric
structure was derived from the IR spectral data, which lacked
the 3500 cm−1 band typical of the OH functional group. Ran-
ganathan et al.[30,31] also suggested that electron-withdrawing
groups on the pyridyl moiety would tend to favour a keto tau-
tomer, which is consistent with our notion for a keto tautomeric
structure for this compound. Our UV spectral data provide a
strong absorption peak at 466 nm, which is consistent with the
quinanoid structure of compound 4.
the percentage of keto tautomer is 35%, while in acidic CDCl3
it was 64%.
The trifluoromethyl-chloro substituted pyridyl Schiff base
(compound 6) showed a doublet at 15.42 ppm with a coupling
constant of ∼6 Hz (Fig. 3). This doublet is assigned to the
NH proton coupling with CH. We observed another doublet
at 9.83 ppm corresponding to the CH=N proton. The chemi-
cal shifts of all other aromatic protons were consistent with the
assigned structure of the compound. This is further substanti-
ated by the 13C NMR spectral data in which the C=O resonance
appeared at 180.3 ppm (Fig. 4). Further confirmation of the keto
tautomericstructureforthiscompoundisobtainedusingTOCSY
NMR spectroscopy (see Fig. 5).
Compound 7 possesses a pyridyl moiety and a hydroxy group
in its structure and was reported by Issa et al.[33] These authors
assigned the peak at 8.50 ppm to the CH=N proton and singlets
at 9.70 and 9.75 ppm were ascribed to the free OH in the pyridyl
moiety and the hydrogen-bonded OH group of the naphthalene
ring, respectively. This is contrary to our interpretation of pro-
ton NMR data that showed a doublet at 15.14 ppm corresponding
to the NH proton. Additionally we observed a broad singlet at
10.93 ppm, which is attributed to the hydroxyl group associated
with the pryidyl moiety. We did, however, observe a doublet at
9.72 ppm for the CH=N proton. Furthermore, we observed a
signal at 181.2 ppm in the 13C NMR spectrum which demon-
strates the presence of a C=O group. Our experimental result
adds support to the formation of the keto tautomer, while the
one reported by Issa et al. perhaps accounts for an enol tautomer.
Additionally, we present the 15N HMBC NMR spectrum, which
further substantiates the keto tautomeric structure for this com-
pound (Fig. 6). We conclude that the compound exists in its keto
Unver et al.[32] reported the synthesis and characterization of
the trifluoromethyl-substituted compound 5, including its NMR
andX-raycrystaldata.Accordingtotheauthors, theprotonNMR
spectrum for this compound showed a singlet at 14.91 ppm as
well as another singlet at 9.74 ppm, for the OH and CH=N pro-
tons, respectively. In their 13C NMR spectral study, they assigned
the peak at 167.8 ppm for the CH=N carbon, and the signal
at 160.8 ppm for the C-OH carbon. The infrared spectral data
showed a band at 3448 cm−1, which was attributed to the OH
group in the enol tautomer. These authors also obtained a crys-
tal structure that demonstrates an enol tautomeric form for this
compound. Our 1H NMR data for this compound showed a dou-
blet at 14.62 ppm as well as a broad singlet at 9.49 ppm for the
NH and CH=N protons, respectively. Our IR data did not show a
peak at 3448 cm−1, which indicates the absence of an OH group.
The origin of the doublet obtained in the present study is unclear
in the absence of X-ray data. It is possible that the differences
in the findings are due to differences between the solvents used,
DMSO-d6 versus CDCl3, which may affect hydrogen bonding
characteristicsconsiderably. Unveretal. foundthatinDMSO-d6,

Synthesis and Characterization of Anils Exhibiting Thermochromism
1277
185
180
175
170
165
160
155
150
145
140
135
130
125
120
115
110
105
f1 [ppm]
Fig. 4. 13C NMR spectrum of compound 6 in CDCl3 at room temperature (note the peak at 180 ppm corresponding to the C=O group of the keto-enamine
tautomer).
6.5
7.0
7.5
8.0
8.5
9.0
9.5
10.0
10.5
11.0
11.5
H
1
12.0
12.5
13.0
13.5
14.0
14.5
15.0
15.5
16.0
15.5 15.0 14.5 14.0 13.5 13.0 12.5 12.0 11.5 11.0 10.5 10.0 9.5 9.0 8.5 8.0 7.5 7.0 6.5
1H [ppm]
Fig. 5. TOCSY spectrum of compound 6 in CDCl3 at room temperature.

1278
T. K. Venkatachalam, G. K. Pierens, and D. Reutens
105
110
115
120
125
130
135
140
145
f
150
155
160
165
170
175
180
185
15.5 15.0 14.5 14.0 13.5 13.0 12.5 12.0 11.5 11.0 10.5 10.0 9.5 9.0 8.5 8.0 7.5 7.0 6.5
f2 [ppm]
Fig. 6. HMBC spectrum for compound 6 in CDCl3.
H
N
O
the IR spectrum showed a strong stretching vibration band at
3416 cm−1 for the free hydroxyl group in the structure.
Compound 9 showed keto tautomeric structural features with
the NH and CH=N protons resonating as doublets. In the car-
bon NMR spectrum, the peak at 179.3 ppm was assigned to the
carbon of the C=O group. The keto tautomeric structure of the
compound is further substantiated by the X-ray crystal structure
reported by Holeck et al.,[36] with a reported C=O bond length of
1.26 Å, which is close to the normal value for a carbon-oxygen
double bond (1.22 Å). The slight increase in the bond length
observed in the X-ray crystallographic studies might be due to
a zwitter ionic species.
In the case of compound 10, the proton and carbon NMR
chemical shifts pointed towards an enol tautomer. For example,
there was no doublet at ∼15.0 ppm as seen in previous keto com-
pounds.A singlet resonance at 13.19 ppm was observed, indicat-
ing an OH group hydrogen bonded to the nitrogen on the CH=N
group. In addition we obtained another singlet at 8.68 ppm cor-
responding to the CH=N proton. The IR spectrum also showed
a band at 3502 cm−1 consistent with the enol tautomeric form.
Recently Kai et al.[37] reported the electronic structure and non-
linear optical properties of Bis(salicylaldiminatio) schiff base
compounds and their Ni(II) complexes, thus supporting an enol
tautomer.
HO
N
N
I
O
H
N
OH
II
Scheme 3.
tautomeric form both in solution and in the solid state.The X-ray
crystal structure for compound 7 published Ozek et al.[34] also
suggested a keto tautomer with a C=O bond length of 1.29 Å.
Compound 7 contains two hydroxyl groups; one associated
with the pyridyl ring and the other with the naphthyl ring. The-
oretically, a hydrogen bond can be formed with the N=C in two
ways (Scheme 3). In structure (I) the proton from the hydroxyl
group of the naphthalene ring participates in hydrogen bonding,
resulting in the formation of a stabilized six-membered ring.
In structure (II), the proton from the hydroxyl group associ-
ated with the pyridine ring is involved in a hydrogen bonded
five-membered ring.
The X-ray crystal structure reported by Meng et al.[35] for
compound 8 showed an enol tautomer with a C=O bond length
of 1.340 Å, which is in accordance to the known bond lengths
of enols. The proton and carbon NMR data of our compound
confirms the enol tautomeric form for this compound. The
CH=N proton appeared as a singlet at 9.69 ppm. Furthermore
Finally our data support a keto tautomeric structure for the
iodo-substituted compound 11. Gundazvez et al.[38] reported
a broad singlet at 15.25 ppm for this compound, which they
attributed to an OH group, suggesting the existence of an
enol tautomeric form. However, their 13C NMR chemical shifts

Synthesis and Characterization of Anils Exhibiting Thermochromism
1279
O
of the compounds, allowing a convergent interpretation of the
structures of the compounds.
H
O
O
N
H
Thermochromism
O
Many Schiff bases exhibit thermochromism. Ogawa et al.[41]
studied the properties of crystalline forms of certain anils
exhibiting thermochromism using X-ray crystallography. There
was a mixture of keto and enol tautomers in the solid state
and the proportion of each tautomer varied with temperature.
The highest proportion (90%) of the keto form was observed
at 90 K. Carles et al.[42] showed that salicylidene Schiff bases
exhibit thermochromism in the solid state. Thermochromism
was observed in most of our anils, which changed colour when
the temperature was altered (Table 1, Fig. 7). For example, com-
pound 2 converted from orange red to deep yellow when cooled
to −40◦C with dry ice.
Scheme 4. Structure of Schiff base derived from chromene based
aldehydes.
were identical to those that we observed for the compound
in the present study. Unver et al.[39] reported the X-ray crys-
tallographic studies of 1-[N-(4-iodophenyl)]aminomethylidene-
2(H)naphthalenone. Our IR spectrum did not show the free
hydroxyl group (lack of 3500 cm−1 band) in contrast to reports
in the literature.[38] We obtained a doublet for both the NH and
CH=N protons, unlike earlier reports.
In summary, we have synthesized anils which possess ther-
mochromism. 1H and 13C NMR spectroscopy suggests that only
the keto-enamine structure is formed at room temperature in sev-
eral of the compounds. We observed a doublet for the coupling
of the NH proton with the neighbouring CH proton, providing
additional insight into the keto-enamine structure of these anils.
Recently, Sashidhara et al.[40] regioselectively synthesized
and characterized the keto-amines derived from chromene
derivatives, consisting of fused benzopyranones, and concluded
that these Schiff bases existed only as keto-enamine structures.
Thereported proton NMR spectrumshowed a signal at 13.5 ppm,
which they assumed to be due to the hydrogen bonding char-
acteristics of NH protons. They also observed an 13C NMR
resonance at 179 ppm, which they attributed to the C=O group
in the keto-enamine product. Further evidence for the presence
of a keto-enamine tautomer was provided using HSQC, COSY,
and HMBC spectroscopy. The structure of the chromene-based
Schiff basesynthesizedby Sashidhara et al. is shownin Scheme 4
for clarity.
Experimental
All chemicals were obtained from Sigma Aldrich and were used
without further purification. NMR spectra were recorded in
both CDCl3 as well as DMSO-d6 and the chemical shifts are
reported relative to the chloroform peak at 7.27 ppm or DMSO
at 2.54 ppm, respectively. The NMR data was acquired on a
Bruker 900 MHz NMR spectrometer equipped with a cryoprobe.
The proton NMR spectra where acquired with a sweep width
of 18 ppm centred at 7 ppm. The carbon NMR spectra were
acquired with a sweep width of 220 ppm centred at 110 ppm.
The DQCOSY andTOCSY experiments were also acquired with
a sweep width of 18 ppm using a 90◦ pulse of 9 μs with 256
and 350 increments, respectively. The 15N HSQC spectrum was
acquired with sweep widths of 18 and 45 ppm for proton and
nitrogen, respectively, and the nitrogen centred at 118 ppm. The
15N chemical shifts were not referenced, since spectra were run
to qualitatively identify whether the proton is directly attached to
the nitrogen atom in the molecule.The raw data was usually mul-
tiplied by an exponential or shifted sine squared function before
performing the Fourier transform. UV-vis spectra were obtained
from a Lambda UV spectrometer using either methanol or chlo-
roform as the solvent. Infrared spectra were taken as KBr discs
on a Lambda FTIR spectrometer.
Effect of the Pyridyl and Benzyl Moieties
We compared the chemical shift observed in the 13C NMR
spectra for anils that contained the C=O signal. All anils con-
taining the pyridyl group showed peaks between 176.5 ppm and
181.5 ppm. Anils containing the benzyl group showed peaks in
the considerably upfield range of 165.7–168.8 ppm. However,
the latter compounds also showed a clear doublet for the NH pro-
tons as well as for the CH protons in the 1H NMR spectra. This
can only be possible if the compounds existed as keto-tautomers,
because the hydrogen bonded OH of the enolic form would only
result in a broad singlet with a chemical shift <15 ppm, which
was not observed here. Although these compounds also con-
tained doublets for the NH as well as CH protons in the proton
NMR spectra, the coupling constants were relatively small, in the
rangeof2–3 Hz, unlikethoseobservedforthepyridylsubstituted
anils (6–8 Hz).At present, the more upfield chemical shift for the
carbon associated with C=O in anils containing a benzene ring
remainstobe explained.The trifluoro-substitutedcompound and
the hydroxy-substituted compound showed the highest chemical
shift value for the C=O carbon in these anils. In the phenyl com-
pounds, introduction of iodo- and chloro-substituents shifted the
carbonyl carbon signals to ∼168 ppm. We examined the effect
that varying the solvent had on the chemical shift of individ-
ual protons of selected anils. In both CDCl3 and DMSO-d6, we
observed the doublets for the NH and CH protons demonstrating
that the polarity of the solvent did not affect the chemical shift
of these protons.
General Procedure for Synthesis of Anil Derivatives
The appropriate amine compound (0.05 mol) and the respective
aldehyde (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 pre-
cipitated anils were filtered under vacuum and washed with
copious amount of ethanol and dried under vacuum. Some of
the compounds reported in the present study have been reported
previously.[28–38] However, for many of the compounds, their
NMR and IR data does not match with the data reported in the
present study. A detailed discussion on this aspect is included in
the results and discussion section of the manuscript.
The challenge in identifying the correct structural features
of these anils is reflected in the inconsistencies in interpretation
of their spectral data. In this regard, in the present study, the
solution and solid-state spectral characteristics match for most
1: νmax(KBr)/cm−1 3341, 1625, 1609 (s), 1569 (w), 1498
(vs), 1438, 1334, 1301, 1261 (s), 1184, 1066, 1026, 979, 919,
829, 758. λmax (CHCl3)/nm 460, 454, 388, 342. δH (CDCl3,

1280
T. K. Venkatachalam, G. K. Pierens, and D. Reutens
Ϫ40°C
24°C
50°C
Fig. 7. Appearance of crystals of selected anils at various temperatures in the solid state.
900 MHz) 15.32 (d, J 2.3 Hz, 1H), 9.35 (d, J 2.0 Hz, 1H), 8.15–
8.14 (d, J 9.0 Hz, 1H), 7.83–7.82 (d, J 9.0 Hz, 1H), 7.76–7.75
(d, J 9.0 Hz, 1H), 7.55–7.54 (t, 1H), 7.45–7.44 (d, J 9.0 Hz, 1H),
7.36 (s, 1H), 7.26–7.25 (t, 1H), 7.15–7.14 (d, J 9.0 Hz, 1H),
7.01–7.00 (d, J 9.0 Hz, 1H), 3.6 (s, 3H). δH (CDCl3, 225 MHz)
166.9, 155.6, 153.8, 140.2, 135.9, 132.9, 129.3, 128.0, 127.53,
127.5, 122.1, 122.0, 121.0, 119.1, 112.6, 109.1, 56.4.
2: νmax(KBr)/cm−1 3001, 1631, 1611 (vs), 1558, 1488, 1422,
1329, 1180, 1084, 1007, 968, 836, 759. λmax (CHCl3)/nm 460,
440, 380, 340. δH (CDCl3, 900 MHz) 15.27 (d, J 3.0 Hz, 1H),
9.34 (d, J 3.0 Hz, 1H), 8.11–8.10 (d, J 9.0 Hz, 1H), 7.83–7.82
(d, J 9.0 Hz, 1H), 7.74–7.73 (d, J 9.0 Hz, 1H), 7.55–7.53 (t, 1H),
7.42–7.41 (d, J 9.0 Hz, 2H), 7.36–7.35 (t, 1H), 7.30–7.29 (d, J
9.0 Hz, 2H), 7.13–7.12 (d, J 9.0 Hz, 1H). δC (CDCl3, 225 MHz)

Synthesis and Characterization of Anils Exhibiting Thermochromism
1281
168.4, 155.8, 144.7, 136.5, 132.9, 132.0, 129.7, 129.4, 128.1,
127.4, 123.6, 121.7, 121.4, 118.9, 108.9.
(d, J 9.0 Hz, 1H), 7.72–7.71 (d, J 9.0 Hz, 1H), 7.61–7.59 (m, 2H),
7.50–7.50 (t, 1H), 7.30–7.28 (t, 1H), 7.00–6.99 (d, J 9.0 Hz, 1H),
6.94–6.93 (d, J 9.0 Hz, 1H), 6.90–6.89 (d, J 9.0 Hz, 1H), 2.60 (s,
3H). δC (CDCl3, 225 MHz) 179.6, 158.4, 151.5, 149.3, 139.3,
138.7, 134.2, 129.3, 128.4, 126,8, 123.7, 120.5, 119.2, 112.3,
108.5, 24.4.
3: νmax(KBr)/cm−1 3119, 2606, 1629, 1548, 1480, 1450,
1312, 1255, 1192, 1144, 1120, 1033, 981, 848, 818, 741. λmax
(MeOH)/nm 461, 438, 316, 298, 276. δH (CDCl3, 900 MHz)
15.36–15.35 (d, J 9.0 Hz, 1H), 9.93–9.92 (d, J 9.0 Hz, 1H), 8.51
(s, 1H), 8.14–8.13 (d, J 9.0 Hz, 1H), 7.84–7.83 (d, J 9.0 Hz,
1H), 7.77–7.76 (d, J 9.0 Hz, 1H), 7.63–7.62 (d, J 9.0 Hz, 1H),
7.52–7.51 (t, 1H), 7.33–7.32 (t, 1H), 7.08–7.07 (d, J 9.0 Hz, 1H),
6.94–6.93 (d, J 9.0 Hz, 1H). δC (CDCl3, 225 MHz) 176.7, 152.2,
151.3, 149.9, 141.0, 133.8, 129.4, 128.6, 128.2, 127.2, 124.1,
123.9, 119.4, 117.6, 116.9, 109.1.
10: νmax(KBr)/cm−1 3502 (b), 3063, 1615, 1573, 1496, 1459,
1372, 1286, 1218, 1193, 1161, 1148, 1130, 907, 828, 761. λmax
(CHCl3)/nm 451, 379, 372, 336, 271. δH (CDCl3, 900 MHz)
Enol tautomer and no keto tautomer: 13.19 (s, 1H, hydrogen
bonded OH), 8.68 (s, 1H), 7.43–7.37 (m, 5H), 7.05–7.04 (d, J
9.0 Hz, 1H), 6.98–6.96 (t, 2H). δC (CDCl3, 225 MHz) 162.3,
161.1, 147.1, 133.3, 132.2, 122.2, 119.1, 117.3.
4: νmax(KBr)/cm−1 3207, 2729, 1620 (vs), 1606, 1588, 1548,
1475, 1402, 1375, 1319, 1294, 1218, 1133, 1107, 972, 835, 765.
λmax(CHCl3)/nm 466, 441, 378, 344, 310. δH (CDCl3, 900 MHz)
15.38 (d, J 2.0 Hz, 1H), 9.93 (d, J 3.0 Hz, 1H), 8.42–8.41 (d, J
9.0 Hz, 1H), 8.14–8.13 (d, J 9.0 Hz, 1H), 7.77–7.76 (d, J 9.0 Hz,
1H), 7.70–7.69 (d, J 9.0 Hz, 1H), 7.64–7.63 (d, J 9.0 Hz, 1H),
7.53–7.51 (t, 1H), 7.33–7.32 (t, 1H), 7.13–7.12 (d, J 9.0 Hz, 1H),
6.95–6.94 (d, J 9.0 Hz, 1H). δC (CDCl3, 225 MHz) 176.4, 151.9,
151.7, 139.0, 138.2, 133.8, 129.3, 128.8, 128.5, 127.2, 124.0,
123.8, 119.4, 117.2, 109.0.
11: νmax(KBr)/cm−1 3385, 3158 3061, 1628, 1605, 1564,
1483, 1389, 1331, 1180, 1141, 1009, 970, 829, 767. λmax
(CHCl3)/nm 464, 451, 384, 340, 323. δH (CDCl3, 900 MHz)
15.24 (d, J 2.8 Hz, 1H), 9.33–9.33 (d, J 2.9 Hz, 1H), 8.10–8.09
(d, J 9.0 Hz, 1H), 7.82–7.81 (d, J 9.0 Hz, 1H), 7.77–7.76 (d, J
9.0 Hz, 2H), 7.74–7.73 (d, J 9.0 Hz, 1H), 7.54–7.53 (t, 1H), 7.37–
7.35 (t, 1H), 7.11–7.10 (d, J 9.0 Hz, 3H). δC (CDCl3, 225 MHz)
168.8, 155.5, 145.7, 138.6, 136.7, 133.0, 129.4, 128.1, 127.4,
123.7, 122.4, 121.6, 118.9, 108.9, 90.8.
5: νmax(KBr)/cm−1 3158, 1633 (vs), 1610 (vs), 1572 (s),
1471, 1404, 1373, 1338, 1293, 1184, 1128, 1002, 983, 885, 832,
756. λmax (CHCl3)/nm 451, 384, 340. δH (CDCl3, 900 MHz)
14.62 (d, J 1.5 Hz, 1H), 9.49 (bs, 1H), 8.19–8.18 (d, J 9.0 Hz,
1H), 7.91–7.90 (d, J 9.0 Hz, 1H), 7.81–7.79 (m, 2H), 7.76 (s, 2H),
7.61–7.59 (t, 1H), 7.43–7.41 (t, 1H), 7.21–7.20 (d, J 9.0 Hz, 1H).
δC (CDCl3, 225 MHz) 165.7, 159.8, 149.4, 136.9, 133.3, 133.1,
132.9, 132.8, 132.7, 129.5, 128.4, 127.8, 124.1, 123.6, 122.4,
121.2, 120.2, 119.7, 119.2, 109.2.
Acknowledgement
This research was funded by a Program Grant from the National Health and
Medical Research Council of Australia.
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6: νmax(KBr)/cm−1 3240 (b), 1634, 1591, 1541, 1477, 1335,
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