Selective colorimetric detection of Hg2+ and Mg2+ with crown ether substituted N-Aryl-9-aminobenzo[b]quinolizinium derivatives

Article abstract of DOI:10.1002/ejoc.201100329

1,4-Dioxa-7,13-dithia-10-azacyclopentadecane (AT₂15C5) or 1,4,7,10-tetraoxa-13-azacyclopentadecane (A15C5) were attached as metal-ion-binding receptor units at the ortho or para positions of the 9-amino-N-phenylbenzo[b]quinolizinium chromophore

Full text of DOI:10.1002/ejoc.201100329

FULL PAPER
DOI: 10.1002/ejoc.201100329
Selective Colorimetric Detection of Hg2+ and Mg2+ with Crown Ether
Substituted N-Aryl-9-aminobenzo[b]quinolizinium Derivatives
Maoqun Tian[a] and Heiko Ihmels*[a]
Dedicated to Professor Siegfried Hünig on the occasion of his 90th birthday
Keywords: Absorption / Chemosensors / Heterocycles / Quinolizinium ions / Solvent effects
1,4-Dioxa-7,13-dithia-10-azacyclopentadecane (AT215C5) or
1,4,7,10-tetraoxa-13-azacyclopentadecane (A15C5) were at-
tached as metal-ion-binding receptor units at the ortho or
para positions of the 9-amino-N-phenylbenzo[b]quinolizin-
ium chromophore. The addition of Hg2+ or Mg2+ to the para
isomers of the AT215C5–quinolizinium or A15C5–quinolizin-
ium conjugate, respectively, led to a blueshift of the absorp-
tion maxima of each compound because of the reduced do-
nor ability of the complexed amino group. In contrast, the
addition of Hg2+ to a solution of the ortho-AT215C5–quinoliz-
inium conjugate in H2O/MeOH mixtures induced a signifi-
cant redshift (ca. 50 nm) of the absorption maximum and en-
abled the photometric discrimination between Hg2+ and
competing thiophilic cations, such as Ag+ or Pb2+, because
the latter, as well as other competing metal ions, did not in-
duce such an effect. It is proposed that the Hg2+-induced red-
shift originates from the complexation of Hg2+ by the thia-
azacrown ether followed by deprotonation of the secondary
9-amino substituent of the aminobenzoquinolizinium unit.
The resulting amide functionality increases the donor–ac-
ceptor interplay leading to the redshifted absorption and also
coordinates to Hg2+ to form a lariat ether type complex. A
similar effect was observed upon the addition of Mg2+ to the
9-amino-N-phenylbenzo[b]quinolizinium derivative with the
A15C5 unit in the ortho position; however, this effect was
only operative in aprotic solvents, e.g. CH3CN, and with less
than 1 mol-equiv. of Mg2+
.
Introduction
Metal cations are essential components in biological and
Considering the hazardous effects of mercury in bio-
ecological media,[1,2] and a deviation from the required con- logical and ecological systems,[9] we have focused our atten-
centration of each cation may induce harmful effects on the tion on the photometric and fluorimetric detection of Hg2+
environment, namely, contamination and pollution, or on ions.[10] For this purpose, we developed the probe molecules
physiological systems, leading to serious diseases. Therefore, 1a and 2, in which an Hg2+-selective thiaazacrown ether
the development of sensitive and selective optical probes receptor was integrated into a 9-aminobenzo[b]quinolizin-
for cation detection has attracted much attention in recent ium (also known as 9-aminoacridizinium) chromophore.
years.[3] In particular, colorimetric probes allow straightfor- The latter unit was chosen, because the donor–acceptor in-
ward and inexpensive cation detection, even without so- terplay, and thus the absorption or emission properties, can
phisticated equipment, because the color change may be be modified by changing the donor properties of the exocy-
observed by the naked eye. In general, a cation may be de- clic amino functionality by Hg2+ complexation. Moreover,
tected by the optical response from the probe caused by the benzo[b]quinolizinium ion is water-soluble, which facili-
selective reaction or association, namely, the modulation of tates studies in aqueous solutions, as required for the detec-
charge-transfer efficiency by cation complexation,[4] cation- tion of physiologically relevant analytes. Indeed, the deriva-
induced aggregation or disaggregation of the probe mole- tives 1a and 2 may be used for the fluorimetric detection of
cules,[5] cis/trans isomerization upon intramolecular chela- Hg2+; however, Ag+ ions interfere with the detection, be-
tion,[6] irreversible chemical transformation of chemodosi- cause they bind to the receptor unit as well. Herein, we
meters,[7] or the analyte-induced change of a complexing present the selective colorimetric detection of Hg2+ with
site in a host–guest assembly based on keto–enol tautomer- probe 1a without interference of Ag+. For comparison, and
ism.[8]
to evaluate the generality of this approach, the crown ether
unit was modified in 1b such that it was selective towards
the alkaline earth metal cation Mg2+. Moreover, the influ-
ence of the substitution pattern on the colorimetric cation
detection was examined with isomers that had receptor
units attached at the para position of the phenyl substituent.
[a] Universität Siegen, Organische Chemie II,
Adolf-Reichwein-Strasse 2, 57068 Siegen, Germany
Fax: +49-271-740-4052
E-mail: ihmels@chemie.uni-siegen.de
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.201100329.
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Recently, we have observed that derivative 1a, which car-
ries an ortho-thiaazacrown ether substituent, has a different
optical response to the complexation of Hg2+ than isomer
1c,[10a] because the long-wavelength absorption band be-
tween 430 and 500 nm increases slightly upon addition of
Hg2+ in aqueous buffer solution. During studies of this phe-
nomenon in different solvents, we discovered a notable in-
fluence of the composition of methanol/water mixtures on
the photometric titrations. Although the absorption spectra
of 1a are essentially identical in each of the solvent mix-
tures, a significant difference of the development of the
long-wavelength absorption band upon the addition of
Hg2+ was observed in solutions with different water/meth-
anol ratios (Figure 1C and D, cf. Figure S2 in the Support-
ing Information). Specifically, with an increasing content of
methanol the decrease of the absorption band at 392 nm
and the increase of the band at 450 nm are more pro-
nounced, as demonstrated by the superposition of the ab-
sorption spectra of the 1a–Hg2+ complexes in different sol-
vent mixtures (Figure 1D). The binding constants of 1a
with Hg2+, as deduced from the corresponding binding iso-
therms, were essentially the same within the error margin
in methanol/water mixtures, that is, K = 1.0ϫ105, 1.6ϫ105
and 1.2ϫ105 m–1 (cf. Figure S2 in the Supporting Infor-
mation). In pure methanol, the binding constant is larger
by one order of magnitude (1.0ϫ106 m–1). The 1:1 stoichi-
ometry of the 1a–Hg2+ complex in buffer/methanol (1:3)
was confirmed by a Job plot (cf. Figure S3 in the Support-
ing Information). Notably, the change of absorption of 1a
upon the addition of Hg2+ is recognized by the naked eye
as a color change from yellow to red (Figure 2). The plot
Results
The 9-amino-N-arylbenzo[b]quinolizinium derivatives
1a–d were synthesized according to modified literature pro-
cedures (Scheme 1).[11] Thus, nucleophilic substitution reac-
tions of the fluoronitrobenzene derivatives 3a or 3b with the
azacrown ether derivatives 1,4-dioxa-7,13-dithia-10-azacy-
clopentadecane (AT215C5-H) and 1,4,7,10-tetraoxa-13-aza-
cyclopentadecane (A15C5-H) followed by the reduction of
the nitro functionality gave the aniline derivatives 5a–d in
54–98% yield, respectively. The subsequent nucleophilic
substitution reaction of 9-fluorobenzo[b]quinolizinium bro-
mide with 5a–d produced the corresponding crown ether
derivatives 1a–d in 10–46% yields. The structures of the new
1
compounds 1b, 1c, 1d, 4c, and 5c were confirmed by H
and 13C NMR spectroscopic analysis, mass spectrometry,
and elemental analysis data.
The spectrophotometric titrations of the para-substituted
crown ether derivatives 1c and 1d with Hg2+ (in MeOH)
and Mg2+ (in MeCN) led to a decrease of the long-wave-
length absorption band and to an increase of the intensity
of the absorption-band maximum at about 400 nm, along
with a slight blueshift (Figure 1A and B). In addition,
isosbestic points were formed during the titration. The
binding isotherms were fitted to a 1:1 binding stoichiometry
with binding constants of 2.7ϫ104 m–1 for 1c–Hg2+ and
1.9ϫ103 m–1 for 1d–Mg2+ (cf. Figure S1 in the Supporting
Information).
of the ratio of the absorption at 520 and 404 nm, A520/A404
,
versus concentration of Hg2+ is linear over a large range
and may be used for the quantitative detection of Hg2+ ions
(cf. Figure S4 in the Supporting Information). The limit of
detection is 0.10 μm, with a linear range from 0 to 75 μm.
Moreover, the redshift upon cation addition is highly selec-
Scheme 1. Synthesis of 9-amino-N-arylbenzo[b]quinolizinium derivatives 1a–d. Reagents and conditions: (i) AT215C5-H or A15C5-H,
Cs2CO3, 60 °C 12 h (4a–b), or Cs2CO3, DMF, 100 °C, 24 h (4c–d); (ii) SnCl2, EtOH, reflux, 3 h (5a and 5c) or Pd/C H2, EtOAc, room
temp., 12–24 h (5b and 5d); (iii) 9-fluorobenzo[b]quinolizinium bromide, EtOH, reflux, 12–72 h.
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Selective Colorimetric Detection of Hg2+ and Mg2+
Figure 2. Absorption of 1a (50 μm) in HEPES buffer/methanol
(1:3, 25 mm, pH = 7.0) in the presence of Hg2+ (2.0 equiv.), Ag+,
Zn2+, Cd2+, Co2+, Ni2+, Cr3+, Cu2+, Pb2+, Fe3+, and Fe2+
(10 equiv.). Inset: picture of 1a in the presence of selected metal
cations.
Complex formation between the benzo[b]quinolizinium
derivative 1a and Hg2+ was also examined by 1H NMR
spectroscopic analysis (Figure 3). In CD3OD, most proton
signals of the benzo[b]quinolizinium part are shifted to
higher field (Δδ = 0.1–0.5 ppm) upon the addition of Hg2+
(1 mol-equiv.). In contrast, a low-field shift of Δδ = 0.1 ppm
was observed for the proton signal of 8-H (Figure 3A and
B). The signals of the phenyl protons only showed slight
changes (Δδ = 0.05–0.10 ppm) of the chemical shifts upon
the addition of Hg2+ ions. In D2O, most aromatic proton
signals of 1a shift to lower field upon the addition of Hg2+
,
whereas the signal of 10-H was high-field-shifted by Δδ =
1
0.25 ppm (Figure 3C and D). In both cases, the H NMR
signals of the crown ether units are significantly broadened
Figure 1. (A) Spectrophotometric titrations of 1c (50 μm) with
Hg(OAc)2 (0–0.22 mm) in MeOH. (B) Spectrophotometric ti-
trations of 1d (50 μm) with Mg(ClO4)2 (0–2.2 mm) in MeCN. (C)
Spectrophotometric titrations of 1a (50 μm) with Hg(OAc)2 (0–
0.13 mm) in HEPES buffer (75% MeOH). (D) Absorption spectra
of the 1a–Hg2+ complex in aqueous HEPES buffer solution with
various methanol contents.
tive, because it is only induced by Hg2+, but not by the
potentially competing[12] transition-metal cations, such as
Ag+, Zn2+, Cd2+, Co2+, Ni2+, Cr3+, Cu2+, Pb2+, Fe3+, and
Fe2+ (Figure 2). Only the addition of Ag+ induced a blue-
shift of the absorption, but with almost no clear color
change.
1
Figure 3. H NMR spectra of 1a in CD3OD (A) and D2O (C) and
in the presence of Hg2+ (1 mol-equiv.) in CD3OD (B) and D2O (D).
Chloroform is the lattice solvent in the crystal of compound 1a
after recrystallization from CHCl3/EtOAc.
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M. Tian, H. Ihmels
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in the presence of Hg2+ (data not shown), which is a charac-
teristic indication of complex formation.
To assess whether the ortho substitution of an appropri-
ate receptor unit at the phenyl group of 9-amino-N-phen-
The absorption properties of the benzo[b]quinolizinium ylbenzo[b]quinolizinium derivatives may be employed in ge-
derivative 1a also change upon protonation (Figure 4). The neral for cation detection, the absorption of the azacrown-
titration of aqueous hydrochloric acid into a solution of 1a substituted derivative 1b was determined in acetonitrile in
in buffer solution led to a blueshift of the long-wavelength the presence of the perchlorate salts of potentially compet-
absorption maximum from 400 to 385 nm, along with a de- ing cations:[12] NH4+, Li+, Na+, K+, Mg2+, Ca2+, or Ba2+
crease of the broad long-wavelength absorption shoulder. A (Figure 5). Notably, only the addition of Mg2+ induced a
plot of the absorption at 440 nm versus the pH of the solu- large redshift of the long-wavelength absorption, resulting
tion fits appropriately to an acid–base equilibrium with a in a color change of the solution from yellow to red. A
pKa of 3.0 (inset in Figure 4).
numerical fit of the titration isotherm to a 1:1 binding stoi-
chiometry gave a binding constant of K = 6.5ϫ105 m–1 (cf.
Figure S6 in the Supporting Information). Among the other
cations tested only Ca2+ induced a slight redshift of the
absorption of 1b. Notably, the addition of excess Mg2+ (c
Ͼ 0.3 mm) resulted in a decrease of the absorption band at
450–600 nm, along with a significant enhancement of fluo-
rescence by a factor of 1800 (Figure 6), which was also se-
lective for Mg2+
.
Figure 4. Spectrophotometric titration of aqueous HCl (2 m) into
a solution of 1a (c = 50 μm) in Britton–Robinson buffer (pH = 1.5–
4.9). The arrows indicate the changes of the bands upon acidifica-
tion. Inset: plot of the absorption at 440 nm versus pH of the solu-
tion; numerical fit calculated for pKa = 3.0.
Figure 6. (A) Spectrophotometric titration of 1b (50 μm) with Mg2+
(70 μm–10 mm) in MeCN. (B) Spectrofluorimetric titration of Mg2+
(0.3 mm–15 mm) into a solution of 1b (10 μm) in MeCN; λex
358 nm.
=
Discussion
In general, it was observed that all crown ether–benzo-
[b]quinolizinium conjugates changed their absorption prop-
erties upon the addition of a metal cation that bound to
the crown ether receptor. However, depending on the sub-
stitution pattern (ortho vs. para substitution of the crown
ether) and on the solvent, different trends could be distin-
guished. In the case of the para-substituted derivatives 1c
and 1d, the changes of the absorption properties are charac-
Figure 5. (A) Spectrophotometric titration of Mg(ClO4)2 (0–70 μm)
into a solution of 1b (50 μm) in MeCN. Inset: image of 1b in the
absence (left) and in the presence of Mg2+ (right). (B) Absorption
of 1b (50 μm) in acetonitrile in the presence of Mg2+ (1.0 equiv.),
Li+, Na+, K+, NH4+, Mg2+, Ca2+, and Ba2+ (10 equiv.).
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Selective Colorimetric Detection of Hg2+ and Mg2+
teristic of conjugated donor–acceptor chromophores, the consequence of the ortho-substitution pattern in 1a, because
donor group of which binds to a metal cation.[4] Specifi- this effect was not observed for the para isomer 1c. A help-
cally, complexation of the metal ions by the azacrown ether ful comparison for this situation may be provided by metal
leads to a change of the substituent effect of the phenylene- complexes of the so-called ortho-Wurster’s crown 6, the or-
diamine unit, because the complexed azacrown substituent tho-dimethylamino group of which coordinates in a lariat
resembles the chloro substituent.[13] As a result, the donor– ether type complexation towards a bound metal cation.[16]
acceptor interplay with the benzo[b]quinolizinium unit is In contrast, the oxidation of the phenylenediamines induces
very weak compared with the uncomplexed compounds 1c a rotation of the pendant amino group away from the crown
or 1d, leading to a blueshift of the absorption bands and to fragment because of the repulsive electrostatic interactions
the disappearance of the low-energy charge-transfer band between the ligand radical cation and the cationic guest.
at λ Ͼ500 nm. However, because of the relatively small These specific structural conditions lead to electrochemical
blueshift of the absorption, which is hardly noticed by the properties of 6 that are significantly different from those of
naked eye, these compounds may not be considered ideal the isomeric para-Wurster’s crown. Based on these observa-
probes for colorimetric cation detection. The titration of tions, we propose that ligand 1a and Hg2+ also form two
Hg2+ into the isomeric thiaazacrown ether 1a in water also complexes with distinctly different structures. In water, a
induced a blueshift of the absorption maximum, thus indi- similar situation occurs as that in the oxidized Wurster’s
cating the participation of the amino group of the azacrown crown, that is, the complex formation introduces additional
receptor in the complexation of Hg2+. The amine–Hg2+ in- positive charge, and to avoid electrostatic repulsion the
1
teraction was further confirmed by the H NMR spectro- complexed crown unit and the benzo[b]quinolizinium unit
scopic analysis (Figure 3C and D), namely, the low-field rotate away from each other to achieve a conformation in
shift of almost all proton signals, including those of the which the secondary 9-amino group is conjugated to the
phenylenediamine unit, clearly indicated the reduced donor phenyl ring to compensate for the positive polarization. At
strength of the amino functionality upon complexation. the same time, this arrangement reduces the donor–ac-
Moreover, this assumption is in agreement with the photo- ceptor interplay between the NH group and the benzo[b]-
metric titration of acid to 1a, because the protonation of quinolizinium unit, leading to the observed blueshift of the
the amino function of the crown ether also leads to a blue- absorption (see above). With increasing methanol content
shift of the absorption maximum (Figure 4). It can be ex- of the solution, however, the accumulated charge in the
cluded that the amino group at the 9-position of the complex 1a–Hg is less stabilized by the solvent than in pure
benzo[b]quinolizinium unit is protonated, because it has water. In this case, the overall positive charge of the com-
been shown already that this position is only very weakly plex is reduced by deprotonation of the NH functionality,
alkaline in 9-amino-N-arylbenzo[b]quinolizinium deriva- with the counteranion, buffer components, or the solvent
tives.[14] Also, the pKa of 3.0 is comparable to those of sim- acting as the proton acceptor, which also enables additional
ilar azacrown receptors.[15]
stabilization by an intramolecular complexation of the
Surprisingly, an additional redshifted absorption shoul- newly formed amide functionality with the Hg2+ ion
der developed along with the blueshift of the maximum (Scheme 2). The deprotonation of the amino substituent in-
upon the addition of Hg2+ to 1a in water. The former can- creases its π-donor property and leads to the redshift of the
not be readily explained by the complexation of the crown absorption maximum (Figure 1C) and to the high-field shift
ether amino group with Hg2+. Remarkably, with increasing of the H NMR signals of the benzo[b]quinolizinium (Fig-
1
methanol content in the solution, the long-wavelength ure 3). Such a deprotonation reaction of amino or amido
shoulder develops into an absorption maximum at the ex-
pense of the blueshifted low-wavelength absorption band
(Figure 1D); most likely as a result of a more pronounced
donor–acceptor interaction. These observations indicate
two different absorbing species after complexation of Hg2+
.
The significant redshift upon addition of Hg2+, as well as
the impact of the solvent on this effect, are apparently a
Scheme 2. Proposed structures of the 1a–Hg2+ complex (cB = conjugate base).
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M. Tian, H. Ihmels
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substituents in water/alcohol, as induced by metal-ion com- NH functionality, if formed at all, may be unfavorable due
plexation, is a well-known reaction of donor–acceptor-type to the larger size of Ag+. Therefore, the addition of Ag+ to
1
dyes that carry a receptor unit close to the NH function- 1a results in an exclusive blueshift of the absorption. H
ality.[17] In the latter cases, the deprotonation also leads to NMR spectroscopic analysis provided further evidence for
a significant redshift of the absorption maxima because of different structures of the complexes 1a–Hg and 1a–Ag in
the increased donor ability of the deprotonated amine. Fur- MeOD: In contrast to the spectra obtained for the complex
ther experimental support for the formation of 1acB–Hg 1a–Hg (Figure 3), the proton signals of 1a shifted only
was provided by photometric titrations. Thus, it was shown slightly to lower field upon the addition of Ag+ (cf. Fig-
that the addition of acid to the complex between 1a and ure S18 in the Supporting Information). In particular, the
Hg2+ in a buffer solution induced a decrease of the long- latter effect was most pronounced for the crown ether frag-
wavelength absorption band and an increase of the absorp- ment, whereas the signals of the aromatic unit remained
tion maximum at 390 nm (cf. Figure S5 in the Supporting essentially unchanged, indicating that there was no signifi-
Information), resembling the absorption of 1c–Hg2+. In ad- cant complexation by the nitrogen atom of the azacrown
dition, the contribution of the intramolecular N–···Hg2+ part.[22]
complexation to the overall stabilization of the complex is
Such as in the case of 1a and Hg2+, the addition of Mg2+
indicated by the observation that such a deprotonation does to derivative 1b resulted in the same effect. Therefore, it
not take place in the complex of the para isomer 1c with may be concluded that a similar equilibrium between 1b,
Hg2+
.
1b–Mg, and 1bcB–Mg is established (Scheme 2). The selec-
It may be assumed that the tertiary amino group of the tivity of this effect for Mg2+ may be attributed to the par-
azacrown ether in 1acB–Hg is not, or only weakly, coordi- ticipation of the lariat-type amide group to form, in ad-
nated to the Hg2+ ion, as shown theoretically for aza-15- dition to complexation with the crown ether, an appropriate
crown ether complexes.[18] This decomplexation of the ni- cavity size for Mg2+ binding (Scheme 2).[23] However, when
trogen ligand would explain the small changes of the H the concentration of Mg2+ exceeds that of 1b, the redshifted
1
NMR signals of the o-phenylenediamine upon complex- absorption decreases, whereas a new blueshifted band de-
ation, because the tertiary amino group of the azacrown velops. Presumably, at higher Mg2+/1b ratios, an additional
ether is conjugated with the phenyl ring, acting as a π-do- Mg2+ ion binds to the complex 1bcB–Mg, most likely at the
nor, whereas the deprotonated amine is twisted towards the amide functionality. The redistribution of amide ligands in
plane of the benzene ring, so that it has solely σ-acceptor Mg complexes has been shown to operate in solution, and
properties. This situation resembles that in o-phenylenedia- the driving force for this process may be the strong N–Mg
mine in which one amino group is also decoupled from π- interaction.[24] The blueshift of the absorption and emission
conjugation.[19]
maxima indicates the reduced donor ability of the nitrogen
It should be stressed that the determined apparent bind- atom in this complex, which is likely to result from steric
ing constant, K, between 1a and Hg2+ is the combination strain and a twist of the benzo[b]quinolizinium unit about
of the complex association constant, K1, and the acidity the C–N bond such that conjugation with the nitrogen atom
constant, Ka, of the two-step equilibrium between 1a, 1a– at C-9 is significantly reduced. The increased emission in-
Hg, and 1acB–Hg (Scheme 2). Although these binding con- tensity may be the result of reduced conformational flexibil-
stants were not further dissected, it is apparent that the af- ity of the complex, because it has been shown that the low
finity of the crown ether ligand towards Hg2+ is sufficient emission quantum yield of 9-amino-N-arylbenzo[b]quinol-
to enable the selective detection of this ion with the probe izinium derivatives is mainly caused by the torsional relax-
molecule 1a. Interestingly, the isosbestic points are main- ation.[14] Unfortunately, the large excess of Mg2+ required
tained during titration, which is somewhat unusual, because to induce this light-up effect restricts the application of this
it is commonly assumed that an isosbestic point is formed effect for sensitive fluorimetric detection of Mg2+
during spectrophotometric titrations if one starting material
.
is converted exclusively into one single product quantita-
tively. In contrast, it has been demonstrated that isosbestic
points may even be observed if multiple products are
Conclusions
formed, as long as the fractional yield of the absorbing
A tool for the selective colorimetric detection of cations
products is constant.[20] By considering the constant pH based on complexation-induced deprotonation and subse-
during the titration of Hg2+ with 1a, it may be assumed quent lariat-type chelation was developed. Specifically,
that in this case fractional conversion to 1a–Hg and 1acB– benzo[b]quinolizinium derivative 1a represents one of the
Hg is constant during the titration, thus leading to the few chemosensors that allows discrimination between Hg2+
isosbestic points.
and competing thiophilic cations, such as Ag+ or Pb2+ ions,
The high affinity of Hg2+ and Ag+ for the crown ether which often interfere with Hg2+ detection.[25] At the same
unit in 1a[21] causes the selective change of the absorption time, derivative 1b enables the selective colorimetric detec-
of 1a. Nevertheless, the lower charge of Ag+, compared tion of Mg2+. Furthermore, the variation of the receptor or
with Hg2+, leads to less positive charge in a complex with chromophore unit may offer the opportunity to extend this
1a, so that deprotonation, such as that in 1a–Hg, does not approach for the optical detection of other biologically or
take place. Also, chelation in 1a–Ag by the deprotonated ecologically relevant cations.
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Selective Colorimetric Detection of Hg2+ and Mg2+
(13.3 g, 94.3 mmol), the corresponding azacrown ether
(13.3 mmol), and Cs2CO3 (5.66 g, 16.0 mmol) in DMF (35 mL) was
stirred under nitrogen at 100 °C for 24 h. After cooling to room
temp., the reaction mixture was concentrated to dryness, and water
(200 mL) was added to the residue. The mixture was extracted with
CH2Cl2 (4ϫ100 mL). The combined organic layers were washed
by brine and dried with Na2SO4. After evaporation of the solvent,
the product was purified by column chromatography (SiO2; EtOAc/
hexane, 1:2) and subsequent recrystallization from EtOH.
Experimental Section
General Instrumentation and Materials: The melting points were
measured with a melting point apparatus (Büchi 510 K). Mass
spectra (ESI in the positive-ion mode, source voltage 6 kV) were
recorded with a Finnigan LCQ Deca instrument; only m/z values
in the range of 100–2000 units were analyzed. NMR spectra were
measured with Bruker Avance 400 (1H: 400 MHz; 13C: 100 MHz)
and Varian NMR System 600 (1H: 600 MHz; 13C: 150 MHz) spec-
trometers at 20 °C; chemical shifts are given in ppm (δ) values (in-
ternal standards TMS for 1H and 13C NMR spectra). 1H NMR
chemical shifts of 1a in the absence and presence of Hg2+ are given
relative to solvent signals: δ = 3.31 ppm for CHD2OD, δ =
4.79 ppm for HDO. Elemental microanalyses of all new com-
pounds were performed with a HEKAtech EuroEA combustion
analyzer by H. Bodenstedt (Institut für Organische Chemie, Uni-
versität Siegen). Absorption spectra were recorded with a Varian
Cary 100 double-beam spectrophotometer; fluorescence emission
spectra were recorded with a Varian Cary Eclipse fluorescence
spectrometer. The pH of aqueous solutions was measured with a
calibrated pH meter (Qph 70, VWR). TLC analysis of benzo[b]-
quinolizinium derivatives was performed on silica-gel sheets (Ma-
cherey–Nagel Polygram Sil G/UV254), eluent: CHCl3/MeOH
(90:10, v/v). All commercially available chemicals were reagent
grade and used without further purification. Diethyl ether and
THF were distilled from sodium wire; DMF, DMSO, and dichloro-
methane were dried with calcium hydride and vacuum-distilled
prior to use; ethanol was purified by rectification. Other solvents
were analytical or HPLC grade and used without further purifica-
tion. Distilled water was used for experiments. The term “purified
water” refers to e-Pure™ water (resistivity 18 MΩcm–1). 9-Fluo-
robenzo[b]quinolizinium bromide and AT215C5-H were prepared
according to literature procedures.[26] For the identification of
known compounds, the spectroscopic data were compared with the
literature data. Metal cations were employed as the corresponding
perchlorate salts, except for photometric Hg2+ titrations, which
were performed with Hg(OAc)2. Control experiments showed no
difference in the optical response with different counterions,
Hg(ClO4)2 versus Hg(OAc)2.
N-(4-Nitrophenyl)-1,4-dioxa-7,13-dithia-10-azacyclopentadecane
(4c): Yield 1.22 g (25 %), yellow powder; m.p. 129–132 °C. 1H
3
NMR (400 MHz, CDCl3): δ = 2.78 (t, J = 5 Hz, 4 H, CH2), 2.91
3
3
(t, J = 8 Hz, 4 H, CH2), 3.67 (s, 4 H, CH2), 3.75 (t, J = 8 Hz, 4
3
3
H, CH2), 3.82 (t, J = 5 Hz, 4 H, CH2), 6.59 (d, J = 9 Hz, 2 H,
CHar), 8.12 (d, J = 9 Hz, 2 H, CHar) ppm. 13C NMR (400 MHz,
3
CDCl3): δ = 29.2 (2 CH2), 31.5 (2 CH2), 52.1 (2 CH2), 70.6 (2
CH2), 74.4 (2 CH2), 110.4 (2 CHar), 126.4 (2 CHar), 137.1 (Cq),
151.7 (Cq) ppm. MS (ESI+): m/z (%) = 373 (20) [M + H]+.
C16H24N2O4S2 (372.50): calcd. C 51.59, H 6.49, N 7.52, S 17.22;
found C 51.62, H 6.49, N 7.51, S 17.23.
N-(4-Nitrophenyl)-1,4,7,10-tetraoxa-13-azacyclopentadecane (4d):
1
Yield 4.07 g (90%), yellow oil. H NMR (400 MHz, CDCl3): δ =
3
3.60–3.72 (m, 16 H, CH2), 3.79 (t, J = 4 Hz, 4 H, CH2), 6.59–6.66
(m, 2 H, CHar), 8.07–8.14 (m, 2 H, CHar) ppm.[16]
General Procedure for the Reduction of N-(Nitrophenyl)-1,4,7,10-
tetraoxa-13-azacyclopentadecane: A suspension of 4b or 4d
(2.00 mmol) and 10% Pd/C (0.2 mmol) in ethyl acetate (10 mL) was
stirred under hydrogen at room temp. for 24 h. The solid catalyst
was removed by filtration and rinsed with ethyl acetate. The filtrate
was concentrated under reduced pressure to yield the desired prod-
uct.
N-(2-Aminophenyl)-1,4,7,10-tetraoxa-13-azacyclopentadecane (5b):
1
Yield 608 mg (98%), colorless oil. H NMR (400 MHz, CDCl3): δ
= 3.17 (t, 3J = 5 Hz, 4 H, CH2), 3.49 (t, 3J = 5 Hz, 4 H, CH2),
3.60–3.64 (m, 4 H, CH2), 3.66–3.72 (m, 8 H, CH2), 6.63–6.72 (m,
3
4
2 H, CHar), 6.90–6.96 (m, 1 H, CHar), 7.08 (dd, J = 8, J = 1 Hz,
1 H, CHar) ppm.[16]
N-(4-Aminophenyl)-1,4,7,10-tetraoxa-13-azacyclopentadecane (5d):
General Procedure for the Reaction of 1-Fluoro-2-nitrobenzene with
the Azacrown Ether AT215C5-H or A15C5-H: A suspension of 3a
(4.12 g, 29.2 mmol), Cs2CO3 (2.98 g, 9.14 mmol), and the corre-
sponding azacrown ether (3.80 mmol) was stirred under nitrogen
at 60 °C for 72 h. After cooling the reaction mixture to room temp.,
water (100 mL) was added, and the aqueous layers were extracted
with chloroform (3ϫ100 mL). The combined organic layers were
washed with brine (100 mL) and dried with MgSO4. After evapora-
tion of the solvent in vacuo, the product was isolated by column
chromatography (SiO2; dichloromethane/hexane, 1:1).
1
Yield 590 mg (95%), colorless oil. H NMR (400 MHz, CDCl3): δ
3
= 3.50 (t, J = 6 Hz, 4 H, CH2), 3.61–3.69 (m, 12 H, CH2), 3.72 (t,
3J = 6 Hz, 4 H, CH2), 6.53–6.59 (m, 2 H, CHar), 6.61–6.66 (m, 2
H, CHar) ppm.[16]
General Procedure for the Reduction of N-(Nitrophenyl)-1,4-dioxa-
7,13-dithia-10-azacyclopentadecane: A suspension of 4a or 4c
(3.22 mmol) and SnCl2·H2O (4.28 g, 19.0 mmol) in ethanol (40 mL)
was stirred under nitrogen at 90 °C for 3 h. After cooling the reac-
tion mixture to room temp., the solvent was evaporated in vacuo,
and ethyl acetate (150 mL) was added. An aqueous solution of
Na2CO3 (5%) was added to adjust the pH to 8. The aqueous solu-
tion was extracted with ethyl acetate (3ϫ100 mL), and the com-
bined organic layers were washed with brine (100 mL) and dried
with MgSO4. The solvent was evaporated in vacuo. The product
was obtained by recrystallization from ethyl acetate/hexane.
N-(2-Nitrophenyl)-1,4-dioxa-7,13-dithia-10-azacyclopentadecane
(4a): Yield 1.20 g (85%), orange oil. 1H NMR (400 MHz, CDCl3):
3
δ = 2.72 (t, J = 5 Hz, 4 H, CH2), 2.84–2.88 (m, 4 H, CH2), 3.39–
3.41 (m, 4 H, CH2), 3.67 (s, 4 H, CH2), 3.77 (t, 3J = 5 Hz, 4 H,
4
4
CH2), 6.95–6.99 (m, 1 H, CHar), 7.18 (dd, J = 1, J = 1 Hz, 1 H,
4
CHar), 7.39–7.44 (m, 1 H, CHar), 7.67 (dd, J = 1, 4J = 1 Hz, 1 H,
CHar) ppm.[10a]
N-(2-Aminophenyl)-1,4-dioxa-7,13-dithia-10-azacyclopentadecane
(5a): Yield 596 mg (54 %), orange needles; m.p. 101–103 °C. 1H
NMR (400 MHz, CDCl3): δ = 2.75–2.81 (m, 8 H, CH2), 3.18–3.22
N-(2-Nitrophenyl)-1,4,7,10-tetraoxa-13-azacyclopentadecane (4b):
Yield 917 mg (71%), orange oil. 1H NMR (400 MHz, CDCl3): δ =
3.44 (t, 3J = 6 Hz, 4 H, CH2), 3.59–3.70 (m, 16 H, CH2), 6.89–6.94
3
(m, 4 H, CH2), 3.69 (s, 4 H, CH2), 3.79 (t, J = 6 Hz, 4 H, CH2),
3
4
4.10–4.32 (br. s, 2 H, NH2), 6.69–6.72 (m, 2 H, CHar), 6.91–6.95
(m, 1 H, CHar), 7.33 (dd, J = 8, J = 1 Hz, 1 H, CHar), 7.37–7.43
4
4
(m, 1 H, CHar), 7.04 (dd, J = 1, J = 1 Hz, 1 H, CHar) ppm.[10a]
3
4
(m, 1 H, CHar), 7.63 (dd, J = 8, J = 2 Hz, 1 H, CHar) ppm.[16]
General Procedure for the Reaction of 1-Fluoro-4-nitrobenzene with
the Azacrown Ether AT215C5-H or A15C5-H: A suspension of 3b
N-(4-Aminophenyl)-1,4-dioxa-7,13-dithia-10-azacyclopentadecane
(5c): Yield 783 mg (71%), brown prisms; m.p. 94–97 °C. H NMR
1
Eur. J. Org. Chem. 2011, 4145–4153
© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
4151

M. Tian, H. Ihmels
FULL PAPER
(400 MHz, CDCl3): δ = 2.76 (t, J = 5 Hz, 4 H, CH2), 2.88 (t, J
3
3
(CHar), 125.8 (CHar),127.8 (Cq), 130.2 (CHar), 130.3 (CHar), 134.0
(CHar), 137.9 (Cq), 138.2 (CHar), 139.0 (Cq), 145.0 (Cq), 151.8 (Cq)
3
= 8 Hz, 4 H, CH2), 3.28 (br. s, 2 H, NH2), 3.53 (t, J = 8 Hz, 4 H,
CH2), 3.64 (s, 4 H, CH2), 3.81 (t, J = 5 Hz, 4 H, CH2), 6.57 (d, ppm. MS (ESI+): m/z (%) = 520 (100) [M]+. C16H26N2O2S2·H2O
3
3J = 8 Hz, 2 H, CHar), 6.65 (d, J = 8 Hz, 2 H, CHar) ppm. 13C (574.20): calcd. C 60.66, H 6.32, N 7.32, S 11.17; found C 60.47,
3
NMR (400 MHz, CDCl3): δ = 29.6 (2 CH2), 31.0 (2 CH2), 52.3 (2 H 6.38, N 7.30, S 11.19.
CH2), 70.7 (2 CH2), 74.1 (2 CH2), 114.2 (2 CHar), 117.0 (2 CHar),
9-{[4-(1,4,7,13-Tetraoxa-10-azacyclopentadecyl)phenyl]amino}-
137.0 (Cq), 140.6 (Cq) ppm. MS (ESI+): m/z (%) = 343 (3) [M +
benzo[b]quinolizinium Chloride (1d): Yield 335 mg (32%), purple
H]+. C16H26N2O2S2 (342.52): calcd. C 56.11, H 7.65, N 8.18, S
18.72; found C 56. 23, H 7.67, N 8.08, S 18.72.
1
needles; m.p. 246–248 °C. H NMR (400 MHz, MeOD): δ = 3.71–
3
3.56 (m, 16 H, CH2), 3.78 (t, J = 6 Hz, 4 H, CH2), 6.74–6.81 (m,
General Procedure for the Reaction of 9-Fluorobenzo[b]quinolizinium
Bromide with Donor-Substituted Aniline Derivatives: A solution of
9-fluorobenzo[b]quinolizinium bromide (556 mg, 2.00 mmol) and
the corresponding aniline derivative (2.00 mmol) in ethanol (5 mL)
was stirred under reflux under nitrogen for 72 h. After cooling the
reaction mixture to room temp., it was passed through an ion ex-
change resin (DOWEX®1 ϫ8 Cl–). The product was purified by
2 H, Ph-H), 7.08 (d, 4J = 2 Hz, 1 H, 10-H), 7.21–7.14 (m, 2 H, Ph-
3
4
H), 7.40–7.33 (m, 1 H, 3-H), 7.51 (dd, J = 9, J = 2 Hz, 1 H, 8-
H), 7.64–7.58 (m, 1 H, 2-H), 7.93 (d, 3J = 9 Hz, 1 H, 1-H), 8.08
3
3
(d, J = 9 Hz, 1 H, 7-H), 8.10 (s, 1 H, 11-H), 8.67 (d, J = 7 Hz, 1
H, 4-H), 9.50 (s, 1 H, 6-H) ppm. 13C NMR (100 MHz, CD3OD):
δ = 53.7 (CH2), 70.0 (CH2), 71.1 (CH2), 71.4 (CH2), 72.3 (CH2),
100.3 (CHar), 113.5 (CHar), 119.0 (CHar), 119.9 (CHar), 123.3 (Cq),
column chromatography (SiO2; CHCl3/MeOH, 10:1) and subse- 126.3 (2 CHar), 126.9 (CHar), 128.6 (Cq), 130.8 (CHar), 131.0
quent recrystallization from MeOH/ethyl acetate.
(CHar), 134.4 (CHar), 138.6 (CHar), 139.6 (Cq), 140.9 (Cq), 147.7
(Cq), 154.1 (Cq) ppm. MS (ESI+): m/z (%) = 489 (100) [M]+.
C29H34ClN3O4·0.5H2O (533.06): calcd. C 65.34, H 6.62, N 7.88;
found C 65.53, H 6.57, N 7.95.
9-{[2-(1,4-Dioxa-7,13-dithia-10-azacyclopentadecyl)phenyl]amino}-
benzo[b]quinolizinium Chloride (1a): Yield 116 mg (10%), orange-
red needles; m.p. 240–241 °C. 1H NMR (400 MHz, MeOD): δ =
2.68–2.75 (m, 8 H, CH2), 3.25–3.28 (m, 4 H, CH2), 3.64 (s, 4 H,
CH2), 3.72 (t, 3J = 5 Hz, 4 H, CH2), 7.19–7.24 (m, 2 H, Ph-H),
7.32–7.34 (m, 1 H, Ph-H), 7.41 (d, 4J = 2 Hz, 1 H, 10-H), 7.45–
7.47 (m, 1 H, 3-H), 7.56–7.58 (m, 1 H, Ph-H), 7.67–7.78 (m, 2 H,
2-H, 8-H), 7.91 (s, 1 H, CHCl3), 8.06 (d, 3J = 9 Hz, 1 H, 1-H), 8.20
Supporting Information (see footnote on the first page of this arti-
1
cle): H and 13C NMR spectra of all new compounds, spectra of
photometric titrations, and Job plot.
3
3
Acknowledgments
(d, J = 9 Hz, 1 H, 7-H), 8.35 (s, 1 H, 11-H), 8.77 (d, J = 7 Hz, 1
H, 4-H), 9.67 (s, 1 H, 6-H) ppm.[10a]
Generous support by the Deutsche Forschungsgemeinschaft is
gratefully acknowledged.
9-{[2-(1,4,7,13-Tetraoxa-10-azacyclopentadecyl)phenyl]amino}-
benzo[b]quinolizinium Perchlorate (1b): Fully characterized as the
perchlorate salt, which was obtained by the addition of a saturated
aqueous solution of sodium perchlorate to the solution of the chlo-
ride in methanol, and subsequent recrystallization of the precipitate
from MeCN. Yield: 165 mg, 14%, orange prisms; m.p. 207–208 °C.
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1H NMR (400 MHz, CD3OD): δ = 3.21 (t, J = 5 Hz, 4 H, CH2),
3
3.53 (t, J = 5 Hz, 4 H, CH2), 3.57–3.61 (m, 4 H, CH2), 3.67–3.70
(m, 4 H, CH2), 3.74 (s, 4 H, CH2), 7.15–7.19 (m, 1 H, Ph-H), 7.21–
7.26 (m, 1 H, Ph-H), 7.39 (dd, 3J = 8, 4J = 1 Hz, 1 H, Ph-H), 7.44–
7.50 (m, 1 H, 3-H), 7.66–7.73 (m, 3 H, 2-H, 10-H, Ph-H), 7.99 (dd,
4
3
3J = 9, J = 2 Hz, 1 H, 1 H, 8-H), 8.08 (d, J = 9 Hz, 1 H, 1-H),
8.17 (d, 3J = 9 Hz, 1 H, 7-H), 8.37 (s, 1 H, 11-H), 8.79 (d, 3J =
7 Hz, 1 H, 4-H), 9.65 (s, 1 H, 9-H) ppm. 13C NMR (100 MHz,
CD3OD): δ = 55.7 (CH2), 69.7 (CH2), 71.1 (CH2), 71.3 (CH2), 71.4
(CH2), 102.1 (CHar), 120.0 (CHar), 120.4 (CHar), 120.6 (CHar),
123.8 (Cq), 124.9 (CHar), 125.6 (CHar), 126.0 (CHar), 127.0 (CHar),
129.7 (CHar), 130.2 (CHar), 130.9 (CHar), 134.4 (CHar), 137.6 (Cq),
138.6 (CHar), 139.5 (Cq), 140.6 (Cq), 144.2 (Cq), 151.2 (Cq) ppm.
MS (ESI+): m/z (%) = 489 (100) [M]+. C29H34ClN3O8 (588.05):
calcd. C 59.23, H 5.83, N 7.15; found C 59.01, H 5.85, N 7.22.
9-{[4-(1,4-Dioxa-7,13-dithia-10-azacyclopentadecyl)phenyl]amino}-
benzo[b]quinolizinium Chloride (1c): Yield 511 mg (46%), dark pur-
1
ple prisms; m.p. 240–244 °C. H NMR (600 MHz, [D6]DMSO): δ
= 2.75 (t, 3J = 5 Hz, 4 H, CH2), 2.83 (t, 3J = 8 Hz, 4 H, CH2),
3
3
3.55–3.64 (m, 8 H, CH2), 3.71 (t, J = 5 Hz, 4 H, CH2), 6.71 (d, J
4
3
= 9 Hz, 2 H, Ph-H), 7.13 (d, J = 2 Hz, 1 H, 10-H), 7.23 (d, J =
3
4
9 Hz, 2 H, Ph-H), 7.44–7.48 (m, 1 H, 3-H), 7.58 (dd, J = 9, J =
2 Hz, 1 H, 8-H), 7.66–7.70 (m, 1 H, 2-H), 7.99 (d, J = 9 Hz, 1 H,
3
1-H), 8.16 (d, 3J = 9 Hz, 1 H, 7-H), 8.36 (s, 1 H, 11-H), 8.82 (d, 3J
= 7 Hz, 1 H, 4-H), 9.69 (s, 1 H, NH), 9.81 (s, 1 H, 6-H) ppm. 13C
NMR (150 MHz, [D6]DMSO): δ = 29.6 (2 CH2), 31.0 (2 CH2),
51.8 (2 CH2), 70.5 (2 CH2), 73.5 (2 CH2), 98.9 (CHar), 112.7 (2
CHar), 118.0 (CHar), 119.0 (CHar) 121.5 (Cq), 125.0 (2 CHar), 125.8
4152
www.eurjoc.org
© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Org. Chem. 2011, 4145–4153

Selective Colorimetric Detection of Hg2+ and Mg2+
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Received: March 8, 2011
Published Online: May 27, 2011
Eur. J. Org. Chem. 2011, 4145–4153
© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
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