Tautocrowns: a concept for a sensing molecule with an active side-arm

Article abstract of DOI:10.1016/j.tet.2010.04.049

A new sensing molecule containing aza-15-crown-5 as a receptor and 4-(phenyldiazenyl)naphthalen-1-ol as a signal converter has been synthesized. In the free ligand, the hydrogen bonding between the tautomeric OH group and the nitrogen atom from the macrocycle fully shifts the tautomeric equilibrium towards the enol form. The complexation reverses the equilibrium as a result of the strong interaction between the metal ion captured in the macrocyclic moiety and the tautomeric carbonyl group. The complex formation is accompanied by strong bathochromic (between 60 and 100 nm) and hyperchromic effects on the absorption spectra. Obviously, the tautomeric OH/C{double bond, long}O groups play the role of an active side-arm in the complexation, supporting it via formation of a 3D cavity and giving possibility for ejection of the guest compound under stimuli to shift the tautomeric process back.

Full text of DOI:10.1016/j.tet.2010.04.049

Tetrahedron 66 (2010) 4292e4297
Contents lists available at ScienceDirect
Tetrahedron
journal homepage: www.elsevier.com/locate/tet
Tautocrowns: a concept for a sensing molecule with an active side-arm
,
a
,
a
a
Liudmil M. Antonov a , Vanya B. Kurteva , Svilen P. Simeonov , Vera V. Deneva ,
*
*
Aurelien Crochet b, Katharina M. Fromm b
a Institute of Organic Chemistry with Centre of Phytochemistry, Bulgarian Academy of Sciences, Acad. G. Bonchev str., bl. 9, 1113 Sofia, Bulgaria
b University of Fribourg, Department of Chemistry, Chemin du Musée 9, CH-1700 Fribourg, Switzerland
a r t i c l e i n f o
a b s t r a c t
Article history:
A new sensing molecule containing aza-15-crown-5 as a receptor and 4-(phenyldiazenyl)naphthalen-1-
ol as a signal converter has been synthesized. In the free ligand, the hydrogen bonding between the
tautomeric OH group and the nitrogen atom from the macrocycle fully shifts the tautomeric equilibrium
towards the enol form. The complexation reverses the equilibrium as a result of the strong interaction
between the metal ion captured in the macrocyclic moiety and the tautomeric carbonyl group. The
complex formation is accompanied by strong bathochromic (between 60 and 100 nm) and hyperchromic
effects on the absorption spectra. Obviously, the tautomeric OH/C]O groups play the role of an active
side-arm in the complexation, supporting it via formation of a 3D cavity and giving possibility for
ejection of the guest compound under stimuli to shift the tautomeric process back.
Received 15 January 2010
Received in revised form 25 March 2010
Accepted 12 April 2010
Available online 5 May 2010
Dedicated to our colleague and friend
Professor Stefan Stoyanov (1943e2009)
Keywords:
Active side-arm
Aza-crown
Ó 2010 Elsevier Ltd. All rights reserved.
Alkali metals
Alkaline earth metals
Molecular switches
Proton transfer
Tautomerism
X-ray
1. Introduction
type, number and position of the complexing crown ether hetero-
atoms; the stereochemistry imposed by the arms, which connect
Intelligent molecules, the structures and properties of which are
facilely and reversibly changeable by the application of external
physical and chemical stimuli, have attracted much recent atten-
tion relative to the development of new molecule-based devices.1
Design of artificial receptors for monitoring biologically and envi-
ronmentally important ionic species in solutions, especially heavy
and transition metal cations, is currently of great importance.2e12
The most common types of chemosensors consist of two com-
ponents: a signaling unit and a receptor linked directly or via
a spacer.13e16 Among the broad variety of species with complexing
ability, crown and aza-crown ethers have been approved as effi-
cient selective receptors in chemosensors.17e29 In general, their
complexing ability and selectivity for metal ions can be varied by
changing certain parameters such as the availability and acidity of
the passive phenolic OH side-arm; the size of the crown ether ring;
the phenolic group to the macrocycle; and the pH of media.30,31 At
the same time, the azo compounds are among the most widely
examined colouring and signaling systems due to their optical
stability.32e39
The new concept of our study is to build a tautomeric switch,
where a directed shift in the position of the tautomeric equilibrium
can be governed by hydrogen bonding between the tautomeric OH
group and the nitrogen atom in aza-crown ether in the free ligand
and by the interaction between the tautomeric carbonyl oxygen and
the complexated metal ion. Switching from one to the other tauto-
meric form under complexation gives sharp and fast changes in the
optical properties. The tautomeric OH and C]O groups play the role
of an active side-arm in the complexation, supporting it via forma-
tion of a 3D cavity and giving possibility for ejection of the guest
compound under stimuli to shift the tautomeric process back. The
practical applicability of this promising idea was recently proven by
us on a model compound possessing a piperidine receptor unit.40
In this paper, we report on the synthesis and spectral properties
of a compound with an aza-15-crown-5 fragment tethered to
4-(phenyldiazenyl)naphthalen-1-ol. To our best knowledge, this is
* Corresponding authors. Tel.: þ359 2 9606102 (L.M.A.), þ359 2 9606156 (V.B.K.);
fax: þ359 2 8700225; e-mail addresses: lantonov@orgchm.bas.bg (L.M. Antonov),
vkurteva@orgchm.bas.bg (V.B. Kurteva).
0040-4020/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2010.04.049

L.M. Antonov et al. / Tetrahedron 66 (2010) 4292e4297
4293
the first example of an active side-arm containing chemosensor
with significant bathochromic and hyperchromic effects on the
absorption maximum upon protonation or complexation.
2. Results and discussion
The target tautocrown 3 was obtained from naphthalen-1-ol 1
by applying diazo coupling/Mannich reaction sequence (Scheme 1),
which was established as the only reasonable way to convert 1 into
3. The first step was carried out by following a standard procedure
and the product 2 was further used without chromatography
purification. The incorporation of the aza-crown fragment was
achieved by p-toluenesulfonic acid catalyzed Mannich reaction in
good overall yield. When performing the synthesis via the reversed
sequence, the intermediate naphthalene-1-ol possessing an aza-
crown side-chain decomposed during the diazo coupling resulting
in a very complicated mixture.
Scheme 2. Tautomeric interconversion in 2 and between 3 and 3KeNa.
corresponds to appr. 8% (in cyclohexane) or 10% (in methylcyclox-
ehane/toluene) of the keto tautomer (Fig. 2).24,43
Scheme 1. Synthesis of tautocrown 3: (i) PhNH2, concd HCl, NaNO2, acetone/H2O;
(ii) (HCHO)n, p-toluenesulphonic acid, 1-aza-15-crown-5, benzene, 47% overall yield.
Quite surprisingly, when aq K2CO3 solution was used during the
work-up instead of deionized water, the product was isolated as
a red crystalline compound possessing unusually high melting
point (230e231 ꢀC) and absorption maximum at 518 nm. The latter
was identified as a sodium complex (2:2) of the keto form (3KeNa)
by NMR and X-ray analyses (Fig. 1).41 This fact shows that the ligand
binds sodium strongly enough to trap the negligible amount of
sodium impurity in potassium carbonate.
Figure 1. View of molecular structure of 3KeNa, ellipsoids are 50%, some H atoms are
omitted for clarity. (#: ꢁ2ꢁx, ꢁy, ꢁ1ꢁz), Na atoms are in green, N in blue, O in red and
C in black.
Figure 2. Relative stability (HF/6-31G**) of the tautomers of 2 (inset) and 3 (as free
ligand, left, and as calcium complex, right). The values of the relative energy, relative
energyþZPE correction and DDG are given in kcal/mol units.
The complex 3KeNa was converted into 3 by treating a CH3CN
solution with aq ammonia followed by extraction with cyclohexane
(Scheme 2).
According to the calculations, shown in Figure 2, the imple-
mentation of an aza-15-crown-5 moiety could lead to stabilization
of the enol form through hydrogen bonding between the OH group
and the macrocyclic nitrogen. The complexation could fully reverse
the equilibrium towards the keto form.
A comparison between the absorption spectra of 2 and 3 shows
that the tautomeric equilibrium in 3 is fully shifted towards the enol
form, displaying only a band at appr. 410 nm (Fig. 3). In contrast, in
the case of 2, the equilibrium is strongly solvent dependent and
bands belonging to both tautomers (at 410 nm for the enol form and
at 480 nm for the keto tautomer, the latter is strongly solvent de-
pendent) are always observed independing on the solvent.39,43
The tautomeric signal converter, 4-(phenyldiazenyl)naph-
thalen-1-ol (2), is well known since the 19th century as one of the
first tautomeric compounds discovered.42 As a result of the mod-
erate energy gap between the enol (E) and the keto (K) tautomeric
forms, the tautomeric equilibrium (Scheme 2) can be easily shifted
in one or another direction by changing the solvent or/and the
temperature. However, this equilibrium has never been shifted
fully to one of the tautomers. This energy gap, as predicted by the
quantum-chemical calculations, is appr. 1.5 kcal/mol, which

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L.M. Antonov et al. / Tetrahedron 66 (2010) 4292e4297
case of Mg(ClO4)2 and Ca(ClO4)2 an absorption maximum corre-
concentration
8
1
sponding to the 1:1 complex was observed initially (Fig. 5). Upon
further addition of the salt a new red shifted band appeared, cor-
responding to 2:2 complex. Upon addition of Ba(ClO4)2 directly 2:2
complex was observed. At these conditions it was impossible to
determine the stability constants of the 1:1 complexes of magne-
sium, calcium and barium complexes.
of LiClO
1 - 0M
4
-7
-6
-6
-6
-5
-5
-3
2 - 7.13.10
3 - 1.78.10
4 - 3.56.10
5 - 7.13.10
6 - 1.42.10
7 - 2.13.10
8 - 7.13.10
M
M
M
M
M
M
M
1
8
A
300
400
500
600
λ
/nm
Figure 3. Absorption spectra in dry CH3CN/3 with stepwise addition of LiClO4; 2 (A).
The absorption spectra of 3 in dry acetonitrile upon addition of
LiClO4 are shown in Figure 3. It is clear that the tautomeric equi-
librium is shifted towards the keto tautomer as a result of a com-
plexation, providing substantial red shift (71 nm) and increased
intensity of the new maximum at 486 nm. Consequently, the new
ligand exploits the tautomeric proton exchange in the signal
conversion.
The complexation abilities of 3 towards some alkali and alkaline
earth metal ions were studied and the obtained spectra of the final
complexes are given in Figure 4, indicating the reached maximal
concentrations of the corresponding salts (determined by their
solubility in dry acetonitrile). It was observed that upon stepwise
addition of the metal salts the spectral behaviour in the case of
alkali metal ions differs from that in alkaline earth ones. In the case
of LiClO4 and NaClO4 addition 1:1 complex formation was assumed
due to the rise of single new band (Fig. 3), corresponding to the shift
of the tautomeric equilibrium towards the keto tautomer.40 In the
Figure 5. Absorption spectra of 3 in dry CH3CN with stepwise addition of Mg(ClO4)2
(up) and their second derivatives (down).
It is interesting to mention that in all salts used (except Ba
(ClO4)2) the formation of 1:1 complex leads to spectral shift of appr.
70 nm, which corresponds to the appearance of the keto tautomer.
This means that the metal ion interacts with the C]O group and
stabilizes 3K form. Further addition of the metal ion causes in the
case of alkaline earth metal ions an easy shift of the process to 2:2
complex. In the case of Naþ this happens under very high excess of
the metal salt and never occurs in the case of Liþ addition.
The estimated stability constants and the absorption spectra
maxima of the complexes are collected in Table 1. As seen, the
Table 1
Absorption maxima, spectral shifts and stability constants of the complexes of 3 in
dry CH3CN
Metal ion
Complex type
lmax complex [nm]
Dl [nm]
log b
Liþ
1:1
1:1
2:2
1:1
2:2
1:1
2:2
1:1
2:2
486
492
522
476
518
491
509
d
71
77
107
61
103
76
94
5.28ꢂ0.16
d
Mg2þ
14.1ꢂ0.3
4.21ꢂ0.05
d
Naþ
Ca2þ
Ba2þ
d
15.3ꢂ0.4
d
d
Figure 4. Normalized absorption spectra of 3 (d) in CH3CN and its final complexes
with alkali and alkaline earth metal ions.
500
85
15.2ꢂ0.04

L.M. Antonov et al. / Tetrahedron 66 (2010) 4292e4297
4295
complex formation causes substantial red shift, which varies with
the metal ion nature and the complex composition. It is worth
mentioning that the stability constants show substantial im-
provement in respect of those of N-phenylaza-15-crown-5 (4,
Scheme 3),44 where the macrocycle is conjugated to the chromo-
phore system. At the same time, the differences in the stability
X-ray crystallographic data for 3KeNa clearly show that the tauto-
meric carbonyl group interacts with Naþ in a similar wayas in 7 and 8.
The possible explanation for the lower strength of the C]O group in 3
comparing to 7 is the structure of the chromophore unit. Compound 7
contains a structure with clearly defined electron-donative and
electron-withdrawing parts and the electron-withdrawing character
of the carbonyl group is reinforced by the cation upon complexation.
In the case of 3 the donative nature of the tautomeric eNHe group is
reduced by the surrounding substituents, which influences the
complexation ability of the C]O part. Obviously the simple charge
transfer mechanism cannot explain the large red shift in
3KeMedcharge transfer in 7 and proton transfer in 3 lead to differ-
ence in the red spectral shiftdfrom 57 nm in 746 to 107 nm in 3 (both
for the magnesium complex). This shows clearly how the idea pro-
posed by us could lead to design of new sensing molecules using the
tautomeric proton transfer chromophore units as signal converters.
In summary, we report herein on the design and synthesis of an
aza-15-crown-5-containing sensing molecule with an active
side-arm based on tautomeric proton exchange, which provides
enhanced complexation ability and substantial red shift in the
absorption spectra under complexation. However, the new ligand
needs improvement of the selectivity through structural modifi-
cations in the tautomeric sensing block or/and reducing the flexi-
bility of the aza-crown ether.
Scheme 3. Compounds with similar aza-15-crown-5 functionality.
constants in 3 in respect of the cation are not as profound as in 4
(Table 2). For instance, 3 shows strong complexation with Liþ and
Mg2þ, which does not fit well with the size of the aza-15-crown-5
ether cavity. The difference in the complexation of 4 with Ca2þ and
Ba2þ is one order of magnitude, while the corresponding stability
constants are very similar in 3. A possible explanation could be
found in the action of the active side-arm (the carbonyl group),
which allows the existence of two metal capturing motifs: small
(tautomeric carbonyl group, macrocyclic nitrogen and some of the
neighbouring macrocyclic oxygens) and large (tautomeric carbonyl
group and the whole aza-crown). The small capturing moiety al-
lows complexation with small atoms like magnesium and lithium,
behaviour, which is not typical for aza-15-crown-5. This conclusion
is supported by the observed negligible complexation with alkali
metal ions of the model analogue of 3 possessing a piperidine
fragment instead of aza-15-crown-5.40 The large complexation
motif allows strong complexation with large atoms, but reduces the
selectivity, because the lack of size fit effect (size of the aza-crown/
size of the metal ion) is compensated by the strong side-arm sup-
port. However, as seen from Figure 4 and Table 1, the difference in
the spectral maxima of the complexes allows detection of each of
the studied cations.
3. Experimental
3.1. Synthesis
3.1.1. General. 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. The high
performance flash chromatography (HPFC) purifications were car-
ried out on a Biotage HorizonTM system (Charlottesville, Virginia,
USA) on silica gel. The melting points were determined in capillary
tubes on MEL-TEMP 1102D-230 VAC (Dubuque, IA, USA) apparatus
without corrections. The NMR spectra were recorded on a Bruker
Avance IIþ 600 spectrometer (Rheinstetten, Germany) with fid res-
olution 0.3 Hz, i.e., 5ꢃ10ꢁ4 ppm; the chemical shifts were quoted in
parts per million in d-values against tetramethylsilane (TMS) as an
internal standard and the coupling constants were calculated in
hertz. The NMR spectra at room temperature show slow exchange
resulting in: (a) some signals for the crown ether protons appear as
broad multiplets or even as broad singlets; (b) the most part of the
aromatic CH and quaternary carbons show broad signals; some are
very weak and their detection in a reasonable time-scale is not ex-
plicit. The correct assignment of the latter was confirmed by ap-
plying 2D techniques. The mass spectra were taken on a DFS High
Resolution Magnetic Sector MS, Thermo Scientific.
Table 2
Stability constants (log b) of 1:1 complexes of 4e8
Metal ion
Solvent
444
545
645
746
847
Liþ
CH3CN
CH3CN
CH3CN
MeOH
CH3CN
MeOH
CH3CN
MeOH
CH3CN
6.2ꢂ0.2
4.05ꢂ0.05
4.3ꢂ0.1
Mg2þ
Naþ
2.5ꢂ0.3
6.20ꢂ0.05
5.10ꢂ0.04
2.4ꢂ0.1
3.22
2.86
3.0
4.33
3.78
4.20
Ca2þ
Kþ
4.84ꢂ0.03
6.7ꢂ0.1
4.20ꢂ0.01
6.74ꢂ0.08
3.97ꢂ0.07
3.1.2. Synthesis of 3. To a solution of 1-aza-15-crown-5 (3.3 mmol)
in benzene (20 ml) paraformaldehyde (3.3 mmol), p-toluenesulfonic
acid (20 mg), and then crude 242 (3 mmol) were added and the
mixture was refluxed with stirring for 3 h. The products were par-
titioned between benzene and deionized water. The organic phase
was dried over Na2SO4, and evaporated to dryness. Purification by
HPFC on silica gel: mobile phase with a gradient of polarity from
cyclohexane/acetone 90:10 to cyclohexane/acetone 20:80; 47%
overall yield of 2-((1,4,7,10-tetraoxa-13-azacyclopentadecan-13-yl)
methyl)-4-(phenyldiazenyl)naphthalen-1-ol 3; Rf 0.28 (acetone); very
low mp, non-measurable; 1H NMR (CDCl3) 2.894 (t, 4H, J 5.0,
2CH2eN crown), 3.635 (t, 4H, J 4.1, 2CH2eO crown), 3.707 (t, 4H, J 5.3,
2NeCH2eCH2eO crown), 3.743 (m, 8H, 4CH2eO crown), 4.034 (s,
2H, AreCH2eN), 7.428 (m, 1H, CHe4Ph), 7.528 (m, 3H, CHe8Ar and
CH-3 and CHe5Ph), 7.622 (t, 1H, J 7.2, CHe7Ar), 7.723 (br s, 1H,
Ba2þ
3.70ꢂ0.05
Itisinterestingtocomparethecomplexationabilityof3withthose
of4e8, wheretheaza-15-crown-5functionalityisused.Ifweconsider
5 as a pure example of an aza-15-crown-5 macrocycle, compound 6 is
an example with a methoxy passive side-arm, which increases the
complexation ability by one order of magnitude. As seen, the values,
which we obtained for 3 are similar for Naþ. Compounds 7 and 8
possessing a carbonyl side group give substantial values of the sta-
bility constants for 1:1 complexes. The reasons, which lead to for-
mation of 2:2 complexes in the case of 3 remain unknown, but the

4296
L.M. Antonov et al. / Tetrahedron 66 (2010) 4292e4297
CHe3Ar), 7.976 (d, 2H, J 6.6, CH-2 and CHe6Ph), 8.331 (d, 1H, J 8.3,
CHe9Ar), 8.933 (d, 1H, J 7.2, CHe6Ar); 13C NMR (CDCl3) 54.43
(2CH2eN crown), 59.57 (AreCH2eN), 68.61 (2NeCH2eCH2eO
crown), 70.25 (2CH2eO crown), 70.30 (2CH2eO crown), 70.66
(2CH2eO crown),113.46 (CHe3Ar),115.10 (Cquat-2),122.43 (CHe9Ar),
122.64 (CHe6Ar), 122.87 (CH-2 and CHe6Ph), 125.21 (Cquate10Ar),
125.31 (CHe8Ar), 127.19 (CHe7Ar), 129.04 (CH-3 and CHe5Ph),
129.94 (CHe4Ph), 132.70 (Cquate5Ar), 139.88 (Cquate4Ar), 153.35
(Cquate1Ph), 158.54 (Cquate1Ar); COSY cross peaks 2.894/3.707,
3.635/3.743, 7.428/7.528, 7.528/7.622, 7.528/7.976, 7.528/8.331,
7.622/8.933; HSQC cross peaks 2.894/54.43, 3.635/70.66, 3.707/
68.61, 3.743/70.25, 3.743/70.30, 4.034/59.57, 7.428/129.94, 7.528/
125.31, 7.528/129.04, 7.622/127.19, 7.723/113.46, 7.976/122.87, 8.331/
122.43, 8.933/122.64; HMBC cross peaks 2.894/54.43, 2.894/59.57,
2.89/68.61, 3.635/68.61, 3.635/70.30, 3.707/54.43, 3.707/70.25,
3.743/70.25, 4.034/54.43, 4.034/113.46 (weak), 4.034/115.10, 4.034/
158.54, 7.428/122.87 (weak), 7.528/122.64, 7.528/125.21, 7.528/
129.04, 7.528/153.35, 7.528/132.70 (weak), 7.622/122.43, 7.622/
125.31 (weak), 7.622/132.70, 7.976/122.87, 7.976/129.94,7.976/153.35
(weak), 8.331/127.19, 8.331/132.70, 8.331/158.54, 8.933/125.31; MS
(ESIþ) m/z 480 ([Mþ1]þ); C27H33N3O5þH.
3.3. Spectrophotometric titration
The absorption spectra were measured on Jasco V-570 UVevis-
NIR spectrophotometer (equipped with Julabo ED thermostat) at
constant temperature of 20 ꢀC. Spectral grade solvents were used
(Scharlau Multisolvent).
The complexation was studied in dry acetonitrile (dried with
P2O5, distilled upon CaH2 and kept with molecular sieve).52 AR
grade LiClO4 (Fluka), NaClO4$H2O (Fluka), Mg(ClO4)2 (Fluka), Ca
(ClO4)2$4H2O (Aldrich) and Ba(ClO4)2$aq (Fluka) were vacuum
dried at 60 ꢀC from 3 to 5 days depending on the case.
Due to the red shift upon complexation, the estimation of the
stability constants was performed at the maximum of the complex
using the final complex spectrum (Fig. 4).
3.4. X-ray crystallographic investigations
A crystal of 3KeNa was mounted on a loop and all geometric and
intensity data were taken from this crystal. Data collection using Mo
ꢀ
Ka
radiation (
l
¼0.71073 A) was performed at 150 K on a STOE IPDS-
II diffractometer equipped with an Oxford Cryosystem open flow
cryostat.53 Absorption correction was partially integrated in the data
reduction procedure.54 The structure was solved by SIR 2004 and
refined using full-matrix least-squares on F2 with the SHELX-97
package.55,56 All heavy atoms could be refined anisotropically. Hy-
drogen atoms were introduced as fixed contributors when a residual
electronic density was observed near their expected positions.
Crystal data: (3KeNa) C29H38N3NaO6, M¼547.61 g.molꢁ1, tri-
3.1.3. Synthesis of 3KeNa. When satd aq K2CO3 was used instead of
water during the work-up, sodium complex of the keto form of 3
(3KeNa) was isolated: 21% overall yield; orange crystals, mp
230e231 ꢀC; 1H NMR (DMSO-d6/CDCl3 1:1) 2.627 (t, 4H, J 4.7,
2CH2eN crown), 3.486 (m, 8H, 2NeCH2eCH2eO and 2CH2eO
crown), 3.570 (m, 6H, AreCH2eN and 2CH2eO crown), 3.640 (br s,
4H, 2CH2eO crown), 7.195 (m, 2H, CHe8Ar and CHe4Ph), 7.416 (m,
3H, CHe7Ar and CH-3 and CHe5Ph), 7.712 (d, 2H, J 7.5, CH-2 and
CHe6Ph), 7.991(s,1H, CHe3Ar), 8.241 (d,1H, J 8.2, CHe9Ar), 8.716 (d,
1H, J 8.3, CHe6Ar); 13C NMR (DMSO-d6/CDCl3 1:1) 53.91 (2CH2eN
crown), 58.57 (AreCH2eN), 67.34 (2NeCH2eCH2eO crown), 67.87
(2CH2eO crown), 68.40 (2CH2eO crown), 68.51 (2CH2eO crown),
119.38 (Cquate2Ar), 120.61 (CH-2 and CHe6Ph), 121.33 (CHe6Ar),
121.63 (CHe8Ar), 121.81 (CHe3Ar), 124.93 (CHe9Ar), 125.68
(CHe4Ph), 127.06 (CHe7Ar), 128.30 (Cquate10Ar), 128.83 (CH-3 and
CHe5Ph), 132.20 (Cquate4Ar), 135.59 (Cquate5Ar), 154.47 (Cquate1Ph),
177.07 (Cquate1Ar); COSY cross peaks 2.627/3.486, 3.486/3.570,
7.195/7.416, 7.195/8.241, 7.416/7.712, 7.416/8.716; HSQC cross peaks
2.627/53.91, 3.486/67.34, 3.486/68.51, 3.570/58.57, 3.570/68.40,
3.640/67.87, 7.195/121.63, 7.195/125.68, 7.416/127.06, 7.416/128.83,
7.712/120.61, 7.991/121.81, 8.241/124.93, 8.716/121.33; HMBC cross
peaks 2.627/53.91, 2.627/58.57, 2.627/67.34 (weak), 3.486/53.91,
3.486/67.34, 3.486/68.40, 3.570/53.91 (weak), 3.570/68.40, 3.570/
68.51, 3.640/68.40, 3.640/68.51, 7.195/120.61, 7.195/121.33, 7.195/
128.30, 7.195/128.83 (weak), 7.416/120.61 (weak), 7.416/124.93,
7.416/128.83, 7.416/135.59, 7.416/154.47, 7.712/120.61, 7.712/125.68,
7.712/154.47 (weak), 7.991/58.57, 7.991/132.20 (weak), 7.991/135.59,
7.991/177.07, 8.241/127.06, 8.241/135.59, 8.241/177.07 (weak), 8.716/
121.63, 8.716/128.30; MS (ESIþ) m/z 502 (Mþ); C27H33N3NaO5.
ꢀ
clinic, P-1 (Nr. 2), a¼10.5837(7); b¼11.4997(9); c¼13.740(1) A,
3
ꢀ
¼65.755(5)ꢀ,
b
¼67.068(5)ꢀ,
g
¼83.560(6) ; V¼1402.0(2) A , Z¼2,
ꢀ
a
rcalcd¼1.297 Mg mꢁ3, F(000)¼584, T¼150 K,
l
¼0.71073 A,
ꢀ
m(Mo
K
a
)¼0.120 mmꢁ1, 1.76ꢀ<
q
<26.76ꢀ, 5914 reflections of which 5902
unique and 857 observed, 243 parameters refined, GOOF (on F2)¼
0.457, R1¼SjFoeFcj/SFo¼0.0435, wR2¼0.1776 for I>2
wR2¼0.3119 for all data.
s(I) and
Compound 3KeNa (Fig. 1) crystallizes from a mixture of DMF
and ethanol in the triclinic space group P-1. The structure consists
of two units A connected via two sodium cations. Each sodium
cation is heptacoordinated, five coordination sites are filled by one
nitrogen and four oxygen atoms from a crown moiety of one ligand,
the two other coordination sites are filled by the oxygen atoms the
ketone groups of each unit A. The distances Na1eOcrown are 2.458
ꢀ
(7), 2.607(8), 2.640(8) and 2.499(6) A, respectively, for O2, O3, O4
and O5. The Na1eN3 bond is 2.589(8) A long, while Na1eO1 and
ꢀ
ꢀ
Na1eO1# are 2.353(7) and 2.301(7) A long. The two sodium cationꢀs
ꢀ
are 3.433(5) A apart. The angle formed by O1eNa1eO1# is 84.9(3)
wide, the angles N3eNa1eO1 and N3eNa1eO1# are 80.3(3) and
117.9(3)ꢀ, respectively. O1 is deprotonated with a distance C14eO1
ꢀ
ꢀ
ꢀ
of 1.26(1) A and angles of 119.9(6) and 141.2(6) for C14eO1eNa1
and C14eO1eNa1#, respectively. The azo-group composed by
ꢀ
atoms C1, N1, N2 and C7 is characterized by distances of 1.288(8) A
ꢀ
ꢀ
for N1eN2,1.43(1) A for C1eN1 and 1.39(1) A for C7eN2, the angles
C1eN1eN2 and C7eN2eN1 are 113.1(7)ꢀ and 115.2(8)ꢀ and the
torsion angle C1eN1eN2eC7 is ꢁ174.1(8)ꢀ, which is near the per-
fect conformation. This slightly distorted conformation can be
explained by the presence of hydrogen bonding between N1 and
O6, which is the O-atom of the ethanol molecule, and the distance
3.2. Quantum-chemical calculations
Ab initio HF/6-31G** calculations were performed at University
of Basel Computer Center by using the Gaussian 03 program
suite.48 This level of theory presents an acceptable compromise
between the computing costs and accordance with the experi-
mental results.49,50 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 me-
dium basis sets.51
ꢀ
O6/N1 is 2.891(9) A. The H-atom of ethanol is weakly delocalized
ꢀ
between O6 and N1 with a distance of 1.112(7) A for O6eH6A, 1.790
ꢀ
ꢀ
(6) A for N1eH6A and 169.3(5) for the angle O6eH6AeN1.
Acknowledgements
The tautomeric structures shown in Figure 2 and their metal
complexes were optimized without restrictions and then were
characterized as true minima by vibrational frequency calculations.
This work was supported by the Bulgarian National Science
Fund (Projects TK-X-1716 and UNA-17/2005), the Swiss National

L.M. Antonov et al. / Tetrahedron 66 (2010) 4292e4297
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