Controlled shift in the tautomeric equilibrium of 4-((phenylimino)methyl) naphthalen-1-ol

Article abstract of DOI:10.1016/j.molstruc.2012.11.055

4-((Phenylimino)methyl)naphthalen-1-ol and 4-((phenylimino)methyl)-2- (piperidin-1-ylmethyl)naphthalen-1-ol have been synthesized and their tautomeric properties were investigated using molecular spectroscopy (UV-Vis absorption/emission and NMR), X-ray cr

Full text of DOI:10.1016/j.molstruc.2012.11.055

Journal of Molecular Structure 1036 (2013) 267–273
Contents lists available at SciVerse ScienceDirect
Journal of Molecular Structure
journal homepage: www.elsevier.com/locate/molstruc
Controlled shift in the tautomeric equilibrium
of 4-((phenylimino)methyl)naphthalen-1-ol
V. Deneva a, Y. Manolova a, L. Lubenov a, V. Kuteva a, F.S. Kamounah b, R. Nikolova c, B. Shivachev c,
L. Antonov a,
⇑
a Institute of Organic Chemistry with Centre of Phytochemistry, Bulgarian Academy of Sciences, Acad. G. Bonchev Str., bl. 9, 1113 Sofia, Bulgaria
b Department of Science, Systems and Models, Roskilde University, Universitetsvej 1, DK-4000 Roskilde, Denmark
c Institute of Mineralogy and Crystallography, Bulgarian Academy of Sciences, Acad. G. Bonchev Str., bl. 107, 1113 Sofia, Bulgaria
h i g h l i g h t s
"
"
"
"
"
4-((Phenylimino)methyl)-2-(piperidin-1-ylmethyl)naphthalen-1-ol was studied.
The controlled shift of the tautomeric equilibrium through structural modifications was achieved.
The inclusion of the piperidine ring leads to shift of the equilibrium to the enol tautomer.
Upon protonation the equilibrium shifts to the keto tautomer in a controlled and reversible manner.
The salt addition leads to shift in the tautomeric equilibrium, independly on the kind of salt used.
a r t i c l e i n f o
a b s t r a c t
Article history:
4-((Phenylimino)methyl)naphthalen-1-ol and 4-((phenylimino)methyl)-2-(piperidin-1-ylmethyl)naph-
thalen-1-ol have been synthesized and their tautomeric properties were investigated using molecular
spectroscopy (UV–Vis absorption/emission and NMR), X-ray crystallographic analysis and quantum-
chemical calculations. The results show that with the implementation of a flexible piperidine ring a con-
trolled shift in the tautomeric equilibrium will be achieved upon protonation/deprotonation in acetoni-
trile. The addition of a metal salt also shifts the tautomeric equilibrium, but at very high concentrations,
which is caused not by a complex formation, but a special effect of the salt addition.
Ó 2012 Elsevier B.V. All rights reserved.
Received 31 October 2012
Accepted 26 November 2012
Available online 3 December 2012
Keywords:
Tautomerism
Schiff bases
Alkali and alkaline earth metal salts
Molecular spectroscopy
X-ray crystallography
1. Introduction
Recently, we have shown that through suitable structural mod-
ifications in 1-phenylazo-4-naphthol (1) molecule the individual
The tautomerism being a process of exchange of a proton be-
tween two (or more) forms, leads to redistribution of the electronic
density in the whole molecule [1] and, consequently, to substantial
change in the spectral and photophysical behavior. In most of the
cases, when a tautomeric mixture exists in solution at standard
laboratory conditions, the shift in the position of the equilibrium
in a controlled manner remains a challenge. Although, in the case
of azonaphthols and related Schiff bases, the changes in the solvent
composition, temperature, pH and the salts addition have been
proven [2,3] to affect the position of the tautomeric equilibrium
between the enol and keto form (respectively E and K, Scheme 1),
they have never been sufficient to provide pure, single tautomers.
tautomeric forms can be isolated [4]. This is achieved by linking
a piperidine unit, which stabilizes the enol tautomer through
hydrogen bonding (Scheme 2, compound 3). Upon protonation or
complexation [5] the equilibrium is fully shifted to the keto tauto-
mer. The attachment of the piperidine unit via a spacer leads to the
fact that the conjugated tautomeric chromophore is not affected,
i.e. the measured spectral changes are result of the changes in
the tautomeric state. In this way, the concept for controlled shift
in the position of the tautomeric equilibrium in azo-naphthols
has been proven by compound 3 and related aza-crown ethers
[5]. Practically, the shift in the tautomeric equilibrium causes sub-
stantial changes in the absorption spectra, providing a background
for design of sensing molecules.
In the current communication we report the synthesis and the
controlled shift in the tautomeric equilibrium in the Schiff base
4, where the piperidine moiety is linked to a tautomeric system
⇑
Corresponding author. Tel.: +359 2 9606102.
E-mail address: lantonov@orgchm.bas.bg (L. Antonov).
0022-2860/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.molstruc.2012.11.055

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V. Deneva et al. / Journal of Molecular Structure 1036 (2013) 267–273
melting point system. The NMR spectra were recorded on a Bruker
Avance II+ 600 spectrometer (Rheinstetten, Germany) at 25 °C; the
chemical shifts were quoted in ppm in d-values against tetrameth-
ylsilane (TMS) as an internal standard and the coupling constants
were calculated in Hz. The assignment of the signals was con-
firmed by applying 2D techniques. The spectra were recorded as
10ꢀ1 M solutions in deuterochloroform except NOESY experiments
where 3 ꢁ 10ꢀ2 M solutions were used in order to avoid transmo-
lecular interactions. The low resolution mass spectra were taken
on a HP 5973 Mass Selective Detector, the high resolution mass
spectra on a DFS High Resolution Magnetic Sector MS, Thermo
Scientific.
Scheme 1.
2.2. 4-((Phenylimino)methyl)naphthalen-1-ol 2
A mixture of 4-formyl-naphthalen-1-ol (1 mmol) and aniline
(2 mmol) was grinded in a mortar for 5 min. The orange solid
formed was washed with water and then with small portions of
aq. ethanol. The pure product, obtained by flash chromatography
on basic Al2O3 by using CH2Cl2 as a mobile phase, was identical
on m.p. [7] and NMR spectra [3] with the literature data.
2.3. 4-((Phenylimino)methyl)-2-(piperidin-1-ylmethyl)naphthalen-1-
ol 4
(Scheme 3): To a solution of piperidine (2.2 mmol) in benzene
(15 ml) paraformaldehyde (2.2 mmol), p-toluenesulfonic acid
(15 mg), and then crude 4-((phenylimino)methyl)naphthalen-1-ol
(2 mmol) were added and the mixture was refluxed with stirring
for 1.5 h. The products were partitioned between benzene and
water. The organic phase was dried over Na2SO4, evaporated to
dryness, and purified by HPFC on silica gel by using a mobile phase
with a gradient of polarity from CH2Cl2 to acetone–CH2Cl2 1:1 or by
flash chromatography on basic Al2O3 by using a mobile phase with
a gradient of polarity from CH2Cl2 to acetone–CH2Cl2 1:9; The
product is not stable enough on silica gel and decomposed during
flash chromatography; 76% overall yield; Rf 0.46 (acetone–CH2Cl2
1:1); yellow solid, m.p. 150.4–150.6 °C; 1H NMR (600 MHz) 1.734
(bs, 6H, CH2-3, CH2-4 and CH2-5 piperidine), 2.257 (bs, 2H, ½ of
CH2-2 and CH2-6 piperidine), 3.104 (bs, 2H, ½ of CH2-2 and CH2-6
piperidine), 3.936 (s, 2H, ArACH2AN), 7.264 (tt, 1H, J 1.4, 7.4, CH-
4 Ph), 7.297 (dd, 2H, J 1.2, 8.4, CH-2 and CH-6 Ph), 7.451 (ddd,
2H, J 1.8, 7.4, 8.4, CH-3 and CH-5 Ph), 7.554 (ddd, 1H, J 1.1, 6.7,
8.2, CH-8 Ar), 7.627 (ddd, 1H, J 1.4, 6.7, 8.5, CH-7 Ar), 7.880 (s,
1H, CH-3 Ar), 8.379 (dd, 1H, J 0.6, 8.2, CH-9 Ar), 8.898 (d, 1H, J
8.5, CH-6 Ar), 9.053 (s, 1H, CH@N), 9.616 (bs, 1H, OH);13C NMR
23.87 (CH2-4 piperidine), 25.78 (CH2-3 and CH2-5 piperidine),
53.91 (CH2-2 and CH2-6 piperidine), 61.91 (ArACH2AN), 113.75
(Cquat-2), 116.60 (Cquat-4), 120.94 (CH-2 and CH-6 Ph), 122.06
(Cquat-10), 122.82 (CH-9 Ar), 123.23 (CH-6 Ar), 125.07 (CH-8 Ar),
125.27 (CH-4 Ph), 127.58 (CH-7 Ar), 129.11 (CH-3 and CH-5
Ph),130.63 (CH-3 Ar), 132.35 (Cquat-5), 153.18 (Cquat-1 Ph), 158.25
(Cquat-1), 159.28 (CH@N); COSY cross peaks 7.264/7.451, 7.297/
7.451, 7.554/7.627, 7.554/8.379, 7.627/8.898; NOESY cross peaks
2.257/3.104, 2.257/3.936, 3.104/3.936, 3.936/7.880, 7.264/7.451,
7.297/7.451, 7.297/9.053, 7.554/7.627, 7.554/8.379, 7.627/8.898,
Scheme 2.
in the same manner. It is expected that along with changes of the
absorption spectra, changes in the emission spectra can be
achieved with shift of the position of the tautomeric equilibrium,
giving an option for development of fluorescent sensors [6], which
are fundamentally impossible in the case of 3.
2. Experimental part
2.1. Synthesis
All reagents were purchased from Aldrich, Merck and Fluka and
were used without any further purification. Fluka silica gel/TLC-
cards 60778 with fluorescent indicator 254 nm were used for TLC
chromatography and Rf-values determination. Merck Silica gel 60
(0.040–0.063 mm) was used for flash chromatography purification
of the products. The melting points were determined in capillary
tubes on SRS MPA100 OptiMelt (Sunnyvale, CA, USA) automated
Scheme 3.

V. Deneva et al. / Journal of Molecular Structure 1036 (2013) 267–273
269
Fig. 1. Change of therelative energy (HF/6-31Gꢃꢃ) of the tautomers of 2, 4 and 4H+. The values of
DE, DE + ZPE and DDG are given in kcal/mol units.
acetonitrile was dried with P2O5, distilled upon CaH2 and kept with
molecular sieve. The protonation was made with 96% sulfuric acid,
monitoring the pH of the solution by using of Metrohm 654 pH m.
4. Quantum-chemical calculations
4.1. Ab initio
HF/6-31Gꢃꢃ calculations were performed by using the Gaussian
09 program suite [9]. This level of theory presents an acceptable
compromise between the computing costs and accordance with
the experimental results [10]. The well-known weakness of the
HF method is the neglect of electron correlation, but it is also quite
common that basis set incompleteness introduces errors opposite
to that, leading in some cases to fortuitously good HF results with
medium basis sets [11]. The tautomeric structures of 2 and 4 (neu-
tral and protonated) were optimized without restrictions and then
were characterized as true minima by vibrational frequency
calculations.
Scheme 4.
7.880/9.053, 8.898/9.053; HSQC cross peaks 1.734/23.87, 1.734/
25.78, 2.257/53.91, 3.104/53.91, 3.936/61.91, 7.264/125.27,
7.297/120.94, 7.451/129.11, 7.554/125.07, 7.627/127.58, 7.880/
130.63, 8.379/122.82, 8.898/123.23, 9.053/159.28; HMBC cross
peaks 1.734/23.87, 1.734/25.78, 1.734/53.91, 3.936/53.91, 3.936/
113.75, 3.936/116.60 (weak), 3.936/122.06 (weak), 3.936/130.63,
3.936/158.25, 7.264/120.94, 7.297/120.94, 7.297/125.27, 7.297/
129.11 (weak), 7.297/153.18 (weak), 7.451/120.94, 7.451/125.27
(weak), 7.451/129.11, 7.451/153.18, 7.554/122.06, 7.554/123.23,
7.627/122.82, 7.627/132.35, 7.880/116.60 (weak), 7.880/132.35,
7.880/158.25, 8.379/127.58, 8.379/132.35, 8.379/158.25, 8.898/
116.60 (weak), 8.898/122.06, 8.898/125.07, 8.898/132.35 (weak),
9.053/116.60 (weak), 9.053/122.06, 9.053/130.63, 9.053/132.35,
9.053/153.18; HRMS (4H+, ESI positive mode) 345.1998.
5. X-ray diffraction
Diffraction data were collected at 290 K by
x-scan technique,
on an Agilent Diffraction SuperNova Dual four-circle diffractome-
ter equipped with Atlas CCD detector using mirror-monochroma-
tized MoKa radiation from micro-focus source (k = 0.7107 Å). The
determination of cell parameters, data integration, scaling and
absorption correction were carried out using the CrysAlis Pro pro-
gram package [12]. The structures were solved by direct methods
(SHELXS-97) [13] and refined by full matrix least-square proce-
dures on F2 (SHELXL-97) [13]. The non-hydrogen atoms were
refined anisotropically and hydrogen atoms were placed at ideal-
ized positions and refined using the riding model. A summary of
the fundamental crystal and refinement data is provided in
Table S1 [14].
3. Spectroscopy
The UV–Vis spectral measurements were performed on JASCO
V-570 UV–Vis–NIR spectrophotometer, equipped with a Julabo
ED5 thermostat (precision 1 °C) at 20 °C, in dry acetonitrile [8].
AR grade LiClO4 (Fluka), NaClO4ꢂH2O (Fluka), Mg(ClO4)2 (Fluka),
Ca(ClO4)2ꢂ4H2O (Aldrich), Ba(ClO4)2ꢂaq (Fluka) were vacuum dried
at 60 °C for 3 days. All the solvents used were spectral grade and

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V. Deneva et al. / Journal of Molecular Structure 1036 (2013) 267–273
ppm
7.2
7.4
7.6
7.8
8.0
8.2
8.4
8.6
8.8
9.0
9.2
ppm
9.2 9.0 8.8 8.6 8.4 8.2 8.0 7.8 7.6 7.4 7.2
Fig. 3. Partial NOESY spectrum of 4 in deuterochloroform.
of 3, the alcohols are the only exception, probably due to their abil-
ities to break the OHꢂ ꢂ ꢂNpipridine bonding and to form stable bridge
solvates due to their double proton acceptor and proton donor
nature.
The NMR spectra of 4 in deuterochloroform at room tempera-
ture show sharp and well defined signals characteristic for the sin-
gle enol-tautomer. NOESY interactions of azomethyne proton
(Fig. 3) indicate that both enol forms 4E-1 and 4E-2 coexist in
the solution, which is consistent with free rotation around the sin-
gle CnaphACazomethyne bond. The latter is in agreement with our pre-
vious theoretical calculations [3], showing that the relative energy
Fig. 2. Absorption spectra of 2 (a) and 4 (b) in different solvents: cyclohexane and
toluene (line), acetonitrile (crosses) and acetone (rhombs).
6. Results and discussion
The quantum-chemical calculations, shown in Fig. 1, explain the
expected switching of the tautomeric equilibrium in 4 upon pro-
tonation/deprotonation (Scheme 4). As seen, the availability of
the piperidine unit in 4 leads to stabilization of the enol form
(4E) comparing to the tautomeric backbone 2, which is achieved
by formation of intramolecular hydrogen OHꢂ ꢂ ꢂNpipridine bonding.
The process of protonation of the piperidine nitrogen causes for-
mation of a new hydrogen bonding, namely C@O ꢂ ꢂ ꢂ HNpþipridine
,
which shifts the equilibrium to the keto tautomer (4H+, which is
equivalent to 4K).
The theoretical results are supported by the absorption spectra
of 2 and 4 shown in Fig. 2. In Fig. 2a both tautomers of 2 are
evident, the enol at ꢄ350 nm and the keto form at ꢄ440 nm. The
latter is supported by the tendency of formation of keto-dimers
in solution as already shown [3]. In the case of 4 only enol tauto-
mer exist in all tested solvents (Fig. 2b), which confirms that the
theoretical model shown in Fig. 1 works. In this case as in the case
Fig. 4. Single-crystal X-ray structure of 4. Displacement ellipsoids are drawn at the
50% probability level (the hydrogen atoms are represented by circles of arbitrary
radii).

V. Deneva et al. / Journal of Molecular Structure 1036 (2013) 267–273
271
Fig. 6. Part of the crystal structure of 4, showing the intramolecular hydrogen
bonds and the weak CAHꢂ ꢂ ꢂ and CAHꢂ ꢂ ꢂO interactions (indicated by dashed lines).
p
Fig. 5. A graphical illustration of the (a) overlay of the two independent molecules
present in the ASU (molecule B in dark/blue) and (b) the angle between the phenyl
and naphthalene rings. (For interpretation of the references to color in this figure
legend, the reader is referred to the web version of this article.)
bonds could be located. However, several weak CAHꢂ ꢂ ꢂ
p (C121A
H121ꢂ ꢂ ꢂCg1, C227AH227ꢂ ꢂ ꢂCg1, Cg1: the centroid of C15/C16/
C17/C18/C19/C110 ring) and C148AH148ꢂ ꢂ ꢂO11 interactions
stabilize the three-dimensional packing (Fig. 6). Nevertheless, the
packing of the molecules in the crystal structure is probably far
from optimal as voids of 2.2 Å diameter can be traced (Fig. S1).
As suggested from Fig. 1, the protonation of the piperidine
nitrogen is a suitable tool to switch the equilibrium from pure enol
to pure keto form (Scheme 4). The protonation, as shown in Fig. 7,
proceeds easily (pKa = 5.5 0.3) and leads to decrease of the band
at 340 nm, belonging to 4E, and to appearance and rise of a new
band at ꢄ540 nm, which is attributed to the keto tautomer. This
transition is controlled and reversible, allowing exact tautomer
composition at given acidity to be achieved.
Compound 4 in neutral condition, upon excitation at maximum
of the enol form, emits at 410 nm, showing broad red-tiled band.
The same is observed upon excitation of 2 at the same condition.
The Stokes shift is around 4000 cmꢀ1, which cannot be attributed
to excited state proton transfer as in related Schiff bases possessing
intramolecular hydrogen bonding [2]. Upon protonation, as seen
from Fig. 8 the band at 410 nm sharpens slightly blue shifting
and at the same time a weak band at ꢄ500 nm appears and rises
proportionally to the increase of the acidity. Obviously this band
belongs to the keto tautomer, because it can be clearly seen upon
excitation at 460 nm. This is one more evidence that the proton-
ation of 4 causes shift in the position of the tautomeric equilibrium,
which leads to appearance of new emission maxima, belonging to
the keto tautomer.
between these two forms is around 0.5 kcal/mol. This behavior is a
feature of 2 and 4, caused by the steric hindrance between H6 and
the azomethyne H atom, which is, for instance, suppressed by the
stabilizing intramolecular hydrogen bonding in 2-((phenylimi-
no)methyl)naphthalene-1-ol and related Schiff bases [15,16].
Crystals of 4 suitable for single crystal X-ray analysis were
grown from benzene. An ORTEP view is reported on Fig. 4 together
with the atomic labeling scheme. Compound 4, C23H24N2O, crystal-
lizes as plates in the centrosymmetric P ꢀ 1 space group (No. 2)
with two molecules per asymmetric unit. The structural features
(bond distances and angles) of the two molecules, present in the
ASU, are comparable (Table S2) and are similar to those of related
compounds [17,18]. The piperidine moieties in both molecules
adopt a chair conformation with C2N-C in equatorial position
(puckering amplitude (Q) 0.5728, 0.5614 and h 177.84°, 4.5° for A
and B respectively) [19]. The phenyl moieties are nearly planar
(rms of 0.0041 and 0.0091 Å for molecule A and B respectively).
The naphthalene moieties are also close to planarity with rms of
0.0210 and 0.0119 Å). The main structural difference in A and B
is associated with the relative rotation of the phenyl around the
CAN bond (Fig. 5). The angle between the mean planes of the phe-
nyl and naphthalene rings is 35.65(6)° vs. 44.93(6)° in A and B
respectively, which corresponds to the values estimated from
quantum-chemical calculations and spectral measurements [16].
As seen the enol form is stabilized by an intramolecular hydrogen
bond O11AH11Oꢂ ꢂ ꢂN122 and O21AH21Oꢂ ꢂ ꢂN222 in its 4E-2
isomer (Scheme 5 and Fig. 4). No additional typical hydrogen
Scheme 5.

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V. Deneva et al. / Journal of Molecular Structure 1036 (2013) 267–273
Fig. 7. Protonation of 4 in acetonitrile.
Fig. 9. Absorption spectra of 2 and 4 in dry acetonitrile with maximum addition of
Mg(ClO4)2 – pMe = 0.15, Ba(ClO4)2 – pMe = 0.09, Ca(ClO4)2 – pMe = 0.15, LiClO4
pMe = 0.15.
–
rising band and its maximal value is achieved with essentially
the same salt concentration, irrespective of the salt used.
In our opinion this process cannot be explained as complex for-
mation, but also this is not a process of deprotonation of the Schiff
bases, because the spectra of the deprotonated compounds are red
shifted. It can be stated that the process does not result from dis-
sociation of the salt, because the metal ion addition leads to stron-
ger change in the tautomeric equilibrium in solvents where this
dissociation is weak. In water or in solvents with substantial water
content no such effect has been observed. Consequently, only
undissociated ionic compounds enhance the solvent effect, by pro-
moting stabilization of the keto form, but the exact mechanism re-
mains unknown and will be object of further studies.
7. Conclusions
The concept for controlled shift in the position of the tautomeric
equilibrium through structural modifications has been shown
in the case of 4-((phenylimino)methyl)-2-(piperidin-1-ylmethyl)
naphthalen-1-ol, comparing to 4-((phenylimino)methyl)naphtha-
len-1-ol. It has been shown by molecular spectroscopy in solution
and by X-ray analysis in solid state that the inclusion of the piper-
idine ring leads to shift of the equilibrium to the enol tautomer,
where the OH group is stabilized by a hydrogen bonding with
the piperidine nitrogen atom. Upon protonation the equilibrium
shifts to the keto tautomer in a controlled and reversible manner.
The effect of the salt addition also leads to shift in the tauto-
meric equilibrium, which does not depend on the kind of salt used.
This phenomenon happens at relatively high salt concentrations
and cannot be related to complex formation or deprotonation of
the Schiff base.
Fig. 8. Emission spectra of protonation of 4 in dry acetonitrile with excitation at
360 nm (flat line pH = 6.185 and dashed line pH = 5.705) and inset with excitation
at 460 nm.
According to the investigations of Herzfeld and Nagy [20] the
addition of metal salts (mainly CaCl2 in absolute ethanol) leads
to change in the position of the tautomeric equilibrium in some
Schiff bases, providing an additional tool to shift the tautomeric
equilibrium. Although, by applying a number of assumptions, they
have attempted to interpret this salt effect [21], no satisfactory
explanation has been found to clarify this phenomenon. In our case
similar effects are observed upon addition of various alkali and
alkaline earth perchlorates. As seen from Fig. 9 the salts addition
in acetonitrile leads to similar effects in both 2 and 4. Obviously
the salt does not form complexes using the cavity formed by piper-
idine nitrogen and the tautomeric CAOH/C@O groups as it happens
in 3 [4,5], because the same spectral change in the intensities of the
E (decrease) and K (increase) maxima irrespective on the com-
pound structure and on the type and salt concentration is
observed. In addition, there are no changes in the position of the
Acknowledgements
The financial support by The Bulgarian Science Fund (Project
BG051PO001/3.3-05 Science & Business) and German Academic
Exchange Service DAAD (Project HTC02/227) is Gratefully
acknowledged. In addition, we acknowledge the access to compu-
tational facilities and experimental resources (Projects RNF01/
0110, UNA-17/2005, DRNF-02-13/2009, and DRNF-02/01).

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273
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Appendix A. Supplementary material
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.molstruc.2012.
11.055.
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