Tautocrowns: Aza-15-crown moiety conjugated to a tautomeric schiff base

Article abstract of DOI:10.1080/00387010903260273

The spectral properties of a tautomeric Schiff base containing aza-15-crown-5 moiety, namely N-((4-methylnaphthalen-1-yl)methylene)- 4-(1,4,7,10-tetraoxa-13-azacyclopentadecan-13-yl)aniline on addition of alkali and alkaline earth metal ions were investigated. The newly synthesized ligand exhibited very interesting color changes in presence of metal ions: from yellow (free ligand) via colorlessness (enol tautomer complex) to yellow-orange (keto tautomer complex). These changes result from 2 stepwise processes: complex formation and shift of the tautomeric equilibrium between the tautomers after addition of the metal ions. Taylor & Francis Group, LLC.

Full text of DOI:10.1080/00387010903260273

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Tautocrowns: Aza-15-Crown Moiety Conjugated to a
Tautomeric Schiff Base
V. Deneva a , N. Burdzhiev b , E. Stanoeva b & L. Antonov a
a Institute of Organic Chemistry with Centre of Phytochemistry, Bulgarian Academy of
Sciences , Sofia, Bulgaria
b Faculty of Chemistry, University of Sofia , Sofia, Bulgaria
Published online: 19 Jan 2010.
To cite this article: V. Deneva , N. Burdzhiev , E. Stanoeva & L. Antonov (2010) Tautocrowns: Aza-15-Crown Moiety
Conjugated to a Tautomeric Schiff Base, Spectroscopy Letters: An International Journal for Rapid Communication, 43:1,
22-27, DOI: 10.1080/00387010903260273
To link to this article: http://dx.doi.org/10.1080/00387010903260273
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Spectroscopy Letters, 43:22–27, 2010
Copyright # Taylor & Francis Group, LLC
ISSN: 0038-7010 print=1532-2289 online
DOI: 10.1080/00387010903260273
Tautocrowns: Aza-15-Crown Moiety
Conjugated to a Tautomeric Schiff Base
V. Deneva1,
N. Burdzhiev2,
E. Stanoeva2, and
L. Antonov1
1Institute of Organic Chemistry
with Centre of Phytochemistry,
Bulgarian Academy of Sciences,
Sofia, Bulgaria
2Faculty of Chemistry, University
of Sofia, Sofia, Bulgaria
ABSTRACT The spectral properties of a tautomeric Schiff base containing
aza-15-crown-5 moiety, namely N-((4-methylnaphthalen-1-yl)methylene)-
4-(1,4,7,10-tetraoxa-13-azacyclopentadecan-13-yl)aniline on addition of
alkali and alkaline earth metal ions were investigated. The newly synthe-
sized ligand exhibited very interesting color changes in presence of metal
ions: from yellow (free ligand) via colorlessness (enol tautomer complex)
to yellow–orange (keto tautomer complex). These changes result from 2
stepwise processes: complex formation and shift of the tautomeric equili-
brium between the tautomers after addition of the metal ions.
KEYWORDS alkali and alkaline earth metal ions, aza-15-crown-5, Schiff bases,
tautomerism
INTRODUCTION
Tautomerism is a process of exchange of a proton between two (or more)
forms, leading to redistribution of the electronic density in the whole
molecule[1] and changing substantially its spectral and photophysical
properties. In the case of the tautomeric azonaphthols and related Schiff
bases 1–6 (shown in Scheme 1), the effects of the temperature, solvents,
and substituents have been clarified,[2,3] potentially creating tools to shift
the tautomeric equilibrium toward the enol (E) or keto (K) form.
Almost the same can be stated for the complexation of azacrowns conju-
gated to a chromophore: It leads to electronic redistribution in the ligand
molecule.[4] The process influences the spectral and photophysical proper-
ties of the whole molecule. The stability of the complex obtained depends
on temperature, solvent, and mainly the electronic structure of the ligand.
This means that by changing one of these factors, one can change the
complex stability and its spectral properties.
Linking an azacrown ether to a tautomeric dye could lead to a double
effect of control. On the one hand, the shift in the position of the tautomeric
equilibrium will lead to electronic rearrangement in the molecule and con-
sequently to a change of its complexation abilities. On the other hand, the
process of complexation might lead to shift of the tautomeric equilibrium
toward a particular tautomer. In fact, these two competitive processes
can provide additional tools for design of switch on–off sensor molecules.
In our best knowledge, there is limited number of systems where the crown
Received 5 April 2009;
accepted 8 June 2009.
Address correspondence to
L. Antonov, Institute of Organic
Chemistry with Centre of
Phytochemistry, Bulgarian Academy
of Sciences, Acad. G. Bonchev str.,
bl. 9, Sofia 1113, Bulgaria. E-mail:
lantonov@orgchm.bas.bg
22

SCHEME 2 Synthesis of 7 and 8.
benzene 0.216-g (93%) crystals of 1 are obtained
using the above procedure: mp 159–161ꢀC.
IR (nujol): 3550–2500 (broad, OH), 1630 (C N),
=
1
1120 (CꢁOꢁC) cmꢁ1. H NMR d (CDCl3): 3.52–3.73
(16H, m, OCH2), 3.73–3.85 (4H, m, NCH2), 5.30
(1H, s, OH), 6.70 (2H, d, Ar, J ¼ 9.1 Hz), 6.79 (1H,
d, Ar, J ¼ 8.3 Hz), 7.23 (2H, d, Ar, J ¼ 9.0 Hz), 7.49
(1H, ddd, Ar, J ¼ 1.2 Hz, J ¼ 6.9 Hz, J ¼ 8.1 Hz), 7.60
(1H, ddd, Ar, J ¼ 1.5 Hz, J ¼ 6.9 Hz, J ¼ 8.4 Hz), 7.87
(1H, d, Ar, J ¼ 8.4 Hz), 8.34 (1H, dd, Ar, J ¼ 1.1 Hz,
J ¼ 8.3 Hz), 8.70 (1H, d, Ar, J ¼ 8.3 Hz), 8.78 (1H, s,
CH). Anal. Calcd. for C27H32N2O5: C 69.81%, H
6.94%, N 6.03%; found C 69.44%, H 7.15%, N 6.61%.
SCHEME 1 Keto–enol tautomerism in compounds 1–8.
ether is linked in a way that can influence the tauto-
meric equilibrium, but the changes have never been
discussed in depth from this viewpoint.[5]
Previously we have synthesized a series of aza-
15-crown-5 (A15C5) moiety containing Schiff bases
and have shown the importance of position of crown
on the complexation abilities of the ligands.[6,7] In the
current article, we report the synthesis and the tauto-
meric changes upon complexation in a Schiff base,
where the A15C5 is conjugated to the tautomeric
system (compound 8). The properties of the new
ligand are discussed in comparison with the model
compound 7, where no complexation is possible.
4-((4-(Dimethylamino)-
phenylimino)methyl)naphthalen-
1-ol (7)
Solution of 4-hydroxy-1-naphthadehyde (0.086 g,
0.5 mmol) and N1,N1-dimethylbenzene-1,4-diamine
(0.068 g, 0.5 mmol) in benzene (25 ml) is refluxed
using Dean-Stark trap for 5 hr. After the completion
of the reaction, benzene is evaporated under redu-
ced pressure. The corresponding oil crystallized from
acetonitrile=toluene, thus yielding 0.075-g (52%)
crystals of 2: mp 187–189ꢀC.
MATERIALS AND METHODS
The synthesis of compounds 8 and 7 (the model
compound of 8 in respect to the free ligand tauto-
merism) is summarized in Scheme 2.
=
IR (nujol): 3550–2500 (broad, OH), 1630 (C N)
1
cmꢁ1 ꢂ H NMR d (CDCl3): 2.73 (6H, s, NCH3), 6.53
(2H, d, Ar, J ¼ 9.0 Hz), 6.69 (1H, d, Ar, J ¼ 8.0 Hz),
7.01 (2H, d, Ar, J ¼ 9.0 Hz), 7.22 (1H, ddd, Ar,
J ¼ 1.1, 6.8, 8.0 Hz), 7.34 (1H, ddd, Ar, J ¼ 1.5, 6.8,
8.5 Hz), 7.64 (1H, d, Ar, J ¼ 8.1 Hz), 8.07 (1H, dd,
Ar, J ¼ 1.4, 8.2 Hz), 8.69 (1H, s, Ar), 8.85 (1H, br. s,
Ar). Anal. Calcd. for C19H18N2O: C 78.59%, H
6.25%, N 9.65%; found C 78.63%, H 5.92%, N 9.94%.
Melting points were taken on a Kofler hot-stage
apparatus and are uncorrected. IR spectra were
N-((4-methylnaphthalen-1-
yl)methylene)-4-(1,4,7,10-
tetraoxa-13-azacyclopentadecan-
13-yl)aniline (8)
From
4-hydroxy-1-naphthadehyde
(0.086 g,
0.5 mmol) and 4-(1,4,7,10-tetraoxa-13-azacyclo-
pentadecan-13-yl)aniline[6] (0.171 g, 0.55 mmol),
23
Tautocrowns: Part I. Aza-15-Crown Moiety Conjugated

recorded on a Specord 75 instrument (Jena,
Germany). 1H NMR spectra (250.13 MHz) were
obtained on a Bruker Avance DRX-250 spectrometer
(Bruker Optics; Germany). The chemical shifts are
given in parts per million (d) relative to tetramethyl-
silane as internal standard. Microanalyses were
performed on VarioEL III CHNS=O (Elementar
Analysensysteme GmbH; Germany).
The UV-Vis spectral measurements were performed
on JASCO V-570 UV-Vis-NIR spectrophotometer
(JASCO, Japan), equipped with a Julabo ED5 thermo-
stat (precision 1ꢀC) at 20ꢀC, in spectral-grade solvents.
The complexation was studied in dry acetonitrile.[8] AR
grade LiClO4 (Fluka), NaClO4 ꢂ H2O (Fluka),
Mg(ClO4)2 (Fluka), Ca(ClO4)2 ꢂ 4H2O (Aldrich),
Sr(ClO4)2 (Aldrich), Ba(ClO4)2 ꢂ aq (Fluka), and BeSO4 ꢂ
2H2O (Merck) were vacuum dried at 60ꢀC from 3 to
5 days depending on the case. Spectral-grade acetoni-
trile (AcN) was dried with P2O5, distilled on CaH2, and
kept with molecular sieve.[8]
FIGURE 1 Absorption spectra of 7 (C¼1.90 ꢂ 10ꢁ5 mol=l) in etha-
nol (þ þ þ), CHCl3 (— — —), CH3COCH3 (o o o), and AcN (········).
RESULTS AND DISCUSSION
Not surprisingly, the spectral properties of 8 as
free ligand are identical with those of 7. As seen in
Fig. 1, in acetone, only the enol (E) tautomer
(Scheme 1) exists, exhibiting a strong band c.
400 nm.[9] In proton donor and=or polar solvents, a
low intensive band c. 500 nm, belonging to the keto
(K) tautomer, is observed. In general, the content of
the keto tautomer is low as it is in the related azo-
naphthols (1) with electron donative substituent in
the phenyl ring.[2]
Herzfeld and Nagy[10] showed that the addition of
CaCl2 in absolute ethanol leads to change in the posi-
tion of the tautomeric equilibrium in some Schiff
bases. Similar, weaker effects are observed upon
addition of various alkali and alkaline earth salts.
As seen from Fig. 2, the addition of NaClO4,
Mg(ClO4)2, or Sr(ClO4)2 to solution of 7 causes
changes in the intensities of the E (decrease) and K
(increase) maxima. The tautomeric equilibrium is
most sensitive to addition of Mg2þ in dry AcN. In
chloroform, such changes are observed only in
presence of Mg(ClO4)2. In addition, the shift in
the tautomeric equilibrium is not instantaneous:
The system reaches equilibrium in the frame of days.
Up to now, the effect of the alkali and alkaline earth
metal ions on the tautomeric equilibrium has
FIGURE 2 Absorption spectra of 7 in dry AcN (——, c ¼
4 4 4
6.90 ꢂ 10ꢁ5 mol=l) with addition of Mg(ClO4)2
(
,
c ¼
2.15 ꢂ 10ꢁ6 mol=l), Sr(ClO4)2
(
,
c ¼ 8.50 ꢂ 10ꢁ5 mol=l), and
&&&
NaClO4 (444, c ¼ 3.13 ꢂ 10ꢁ5 mol=l).
V. Deneva et al.
24

remained unexplained[10,11] and has had to be taken
into account in the study of the complexation of 8.
Consequently, in the case of 8, two competitive
processes could be expected with addition of metal
perchlorate: complex formation, involving the
A15C5 moiety, and shift in the tautomeric equili-
brium toward the K form as result of the metal-salt
addition. According to the spectral changes caused
upon addition, the alkali and alkaline earth ions
can be divided (see Scheme 3 for overall description
of the processes):
4. no spectral changes at all: Be2þ;
5. appearance and simultaneous rise of intensive
absorption maximum at 360 nm and low intensive
band at 460 nm (Fig. 3): Liþ and Naþ;
6. initial appearance of aforementioned maxima fol-
lowed by a decrease of intensity of the band at
360 nm and raise of the band at 460 nm with
further addition of the salt (Figs. 3 and 4): Ca2þ,
Sr2þ and Ba2þ;
FIGURE 3 Complex formation according to Scheme 3: Absorp-
tion spectra of 8 in dry AcN (——, c ¼ 1.72 ꢂ 10ꢁ5 mol=l) with
OOO
addition of LiClO4
(
,
c ¼ 1.85 ꢂ 10ꢁ4 mol=l), NaClO4 (444,
7. increase of intensity of already existing maximum
of the keto form at 500 nm, that is, no complexa-
tion (compare Figs. 2 and 4): Mg2þ.
c ¼ 9.80 ꢂ 10ꢁ4 mol=l), Ba(ClO4)2 (^^^, c ¼ 2.30 ꢂ 10ꢁ5 mol=l),
Ca(ClO4)2 (ꢃꢃꢃ, c ¼ 3.06 ꢂ 10ꢁ6 mol=l), and Sr(ClO4)2
(
, c ¼
&&&
4.10 ꢂ 10ꢁ6 mol=l).
Initially the spectral changes in all metal ions,
except Mg2þ, are equivalent (Fig. 3): appearance of
new bands at 360 nm and 460 nm and rise of their
intensity upon salt addition. Simultaneously there is
decrease of the existing bands at 400 and 500 nm
as seen in second derivative spectra of 8 (Fig. 5).
Upon further addition of the perchlorate in the case
of Ca2þ, Sr2þ and Ba2þ, the intensity of the band at
360 nm reaches maximum and begins to decrease.
At the same time, an elevation of the band at
460 nm is observed (Fig. 4).
The observed spectral changes suggest that the
complexation and the change in the tautomeric equi-
librium proceed consecutively (Scheme 3). Initially,
upon addition of the metal salt, both tautomers of
8 individually bind the metal ions without a change
in the position of the tautomeric equilibrium. In
the complex, the nitrogen atom from A15C5 is
involved,[7] effectively leading to its switch-off from
the chromophore tautomeric system. Hence, the
absorption spectra of the complexes of the tautomers
should be the same as in 6, where there is no substi-
tuent in the phenyl ring. Actually it is observed in
Fig. 6: The absorption maxima of 6 in dry AcN are
at 360 nm (E form) and 460 nm (K form), and with
addition of Sr(ClO4)2 the content of the keto tauto-
mer increases. After 8 has been fully consumed as
a free ligand, the excess of the metal salt leads to shift
of the tautomeric equilibrium toward the keto tauto-
mer complex (8K0). In the frame of such a scheme,
SCHEME 3 Complexation and tautomerism in 8.
25
Tautocrowns: Part I. Aza-15-Crown Moiety Conjugated

FIGURE 6 Absorption spectra of 6 in dry AcN (—, c ¼ 4.50 ꢂ
10ꢁ4 mol=l) with addition of Sr(ClO4)2 (- - - , c ¼ 6.68 ꢂ 10ꢁ5 mol=l).
FIGURE 4 Change in the position of the tautomeric equilibrium
between 8E0 and 8K0 (Scheme 3): Absorption spectra of 8 in dry
AcN (——, c ¼ 2.60 ꢂ 10ꢁ5 mol=l) with further addition of Mg(ClO4)2
4 4 4
(
and Sr(ClO4)2
, c ¼ 9.90 ꢂ 10ꢁ5 mol=l), Ca(ClO4)2 (ꢃꢃꢃ, c ¼ 3.06 ꢂ 10ꢁ5 mol=l),
the interaction with Liþ and Naþ is limited in the process
of complexation due to their weaker binding ability
with A15C5.[4] In the case of Ca2þ, Sr2þ, and Ba2þ,
whose sizes correspond to A15C5 cavity, the effective
complexation leads to full consumption of the free
ligand, and the further salt addition changes the equili-
brium between the tautomeric complexes 8E0 and 8K0.
The complexity of the processes in solution and
the lack of instantaneous shift of the equilibria do
not allow us to estimate the corresponding equili-
brium constants with acceptable precision. In gen-
eral, we can conclude that the complexation ability
of the new ligand 8 toward alkaline earth metal ions
is larger than that toward alkali ions. However, the
curves on Fig. 3, showing the maximal changes
achieved under addition of the metal salt and attrib-
uted to the complex formation, could be used as
indication of the binding ability of the ligand toward
the metal ions. Taking into account the concentra-
tions of the metal salts, which lead to final complex
formation (no further changes related to complexa-
tion), we can conclude the following order of
increase of the stability constants of the complexes
between ligand 8 and the metal ions: Naþ < Liþ <
Ba2þ < Sr2þ ꢀ Ca2þ, in accordance with the pre-
viously reported best complexation ability of A15C5
toward Ca2þ.[4,12]
&&&
(
, c ¼ 4.00 ꢂ 10ꢁ5 mol=l).
FIGURE 5 Second derivative spectra of 8 (- - - , c ¼ 3.20 .10ꢁ5
mol=l) with stepwise addition of Sr(ClO4)2 c ¼ 2.00 ꢂ 10ꢁ7 mol=l,
c ¼ 4.00 ꢂ 10ꢁ7 mol=l, c ¼ 8.00 ꢂ 10ꢁ7 mol=l, 2.00 ꢂ 10ꢁ6 mol=l, 4.10 ꢂ
10ꢁ6 mol=l, 8.10 ꢂ 10ꢁ6 mol=l, 2.43 ꢂ 10ꢁ5 mol=l, 4.00 ꢂ 10ꢁ5 mol=l,
8.10 ꢂ 10ꢁ5 mol=l, c ¼ 1.62 ꢂ 10ꢁ4 mol=l (—).
V. Deneva et al.
26

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CONCLUSIONS
The spectral changes upon addition of alkali and
alkaline earth metal ions in a newly synthesized
ligand where the A15C5 moiety is conjugated to a
tautomeric system were investigated. In the case of
alkali ions, a process of complexation was observed,
causing hypsochromic shift in the positions of the
tautomeric bands. The addition of Ca2þ, Sr2þ, and
Ba2þ initially leads to complex formation, but after
the free ligand has been fully consumed, a shift of
the tautomeric equilibrium toward keto tautomer
complex with the metal ion was observed with addi-
tion of excess of metal salt. The lack of instantaneous
shift of the equilibria does not allow one to estimate
the corresponding equilibrium constants with
acceptable precision, but in general we can state that
the complexation ability of the new ligand toward
alkaline earth metal ions is larger than that toward
alkali ions. This is the first discussion of the effect
of the metal-salt addition on the tautomeric equili-
brium in crown ether containing tautomeric Schiff
bases.
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ACKNOWLEDGMENTS
The authors gratefully acknowledge the financial
support of The Swiss National Science Foundation
(grant JRP IB7320-110961=1).
6. Vladimirova, M. P.; Simova, S. D.; Stanoeva, E. R.; Mitewa, M. I.
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11. There is no simple explanation of this phenomenon. It is known that
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Tautocrowns: Part I. Aza-15-Crown Moiety Conjugated