Ferrocenylbenzobisimidazoles for recognition of anions and cations

Article abstract of DOI:10.1021/ic400365c

The preparation of 2,7-disubstituted benzobisimidazoles decorated with substituents displaying different electrooptical properties is described. The presence of redox, chromogenic, and fluorescent groups at the heteroaromatic core, which acts as ditopic binding site, made these receptors potential candidates as multichannel probes for ions. The triad 4 behaves as a selective redox and fluorescent chemosensor for HSO₄^- and Hg ²⁺ ions, whereas receptor 5 acts as a redox and chromogenic chemosensor molecule for AcO^- and SO₄^2- anions. The change in the absorption spectra is accompanied by a color change from yellow to orange, while sensing of Zn²⁺, Hg²⁺, and Pb ²⁺ cations is carried out only by electrochemical techniques. Receptor 6 exhibits a remarkable cathodic shift of the oxidation wave only in the presence of AcO^-, H₂PO₄^-, and HP₂O₇^3- anions, whereas addition of Pb ²⁺ induces an anodic shift. A new low energy band in the absorption spectra, which is responsible for the color change from colorless to pale yellow, and an important increase of the monomer emission band is observed only in the presence of H₂PO₄^-, and HP ₂O₇^3- anions. The most salient feature of the receptor 6 is its ability to act as a multichannel (redox, chromogenic, and fluorescent) chemodosimeter for Cu²⁺, and Hg²⁺ metal cations.

Full text of DOI:10.1021/ic400365c

Article
pubs.acs.org/IC
Ferrocenylbenzobisimidazoles for Recognition of Anions and Cations
María Alfonso, Alberto Tarraga,* and Pedro Molina*
́
́
Departamento de Química Organica, Facultad de Química, Campus de Espinardo, Universidad de Murcia, E-30100 Murcia, Spain
S
* Supporting Information
ABSTRACT: The preparation of 2,7-disubstituted benzobisi-
midazoles decorated with substituents displaying different
electrooptical properties is described. The presence of redox,
chromogenic, and fluorescent groups at the heteroaromatic
core, which acts as ditopic binding site, made these receptors
potential candidates as multichannel probes for ions. The triad
4 behaves as a selective redox and fluorescent chemosensor for
−
HSO4 and Hg2+ ions, whereas receptor 5 acts as a redox and
chromogenic chemosensor molecule for AcO− and SO4
2−
anions. The change in the absorption spectra is accompanied
by a color change from yellow to orange, while sensing of Zn2+,
Hg2+, and Pb2+ cations is carried out only by electrochemical
techniques. Receptor 6 exhibits a remarkable cathodic shift of
the oxidation wave only in the presence of AcO−, H2PO4 , and HP2O7 anions, whereas addition of Pb2+ induces an anodic
shift. A new low energy band in the absorption spectra, which is responsible for the color change from colorless to pale yellow,
−
3−
and an important increase of the monomer emission band is observed only in the presence of H2PO4 , and HP2O73− anions. The
−
most salient feature of the receptor 6 is its ability to act as a multichannel (redox, chromogenic, and fluorescent) chemodosimeter
for Cu2+, and Hg2+ metal cations.
Benzobisimidazoles type I with two linear opposed imidazole
rings represent a new class of versatile fluorescent compounds,5
and they have been used as starting material for the preparation
of Janus bis(carbenes).6 Recently, benzobisimidazolium salts
have also been shown to act as a redox partner7 and sensor for
CO28 whereas the ferrocene-substituted derivative behaves as a
highly selective multichannel chemosensor molecule for acetate
anion,9 and for Pb(II) and Zn (II) metal cations.10 However,
the chemistry of the angular benzobisimidazole isomer II
remains almost unexplored,11 (Chart 1). Only the 2,7-
bisferrocene derivative has been proved to be a selective
redox and fluorescent chemosensor molecule for hydrogen
sulfate and Hg(II) ions.12
Ferrocene unit has largely proved to be a simple and
remarkable redox-signaling unit. Thus, preparation and sensing
properties of ferrocene derivatives have been recently
reviewed.13 In such ferrocene-containing ligands, cation binding
at an adjacent receptor site induces a positive shift in the redox
potential of the ferrocene/ferrocenium redox couple. The
redox-active ferrocene moiety has also been exploited in the
electrochemical sensing of anions; these receptors are expected
to show cathodic shifts in their redox process when complexed
to an anion. Keeping this in mind, decoration of an imidazole
derivative with a ferrocene unit could be a valuable way for the
electrochemical detections of anions or cations. In this context,
INTRODUCTION
■
The design of receptors that contain two quite different binding
sites for the complexation of cationic and anionic guest species
is a new emerging and topical field of supramolecular
chemistry.1 Among different types of chemosensors converting
the ion recognition in physical recordable signals, colorimetric/
chromogenic chemosensors are especially attractive because the
ion recognition can be easily monitored by ion-complexation-
induced changes in UV−vis absorption spectra, which would
allow the so-called “naked eye” detection of ions. Among
organic dyes used as signaling units in the development of
chromogenic receptors, dinitrophenyl derivatives have been
widely used. However, fluorescence sensors make the best
choice, since they are qualified with high sensitivity, fast
response, and inexpensive installations, too. As fluorogenic
groups, photoactive pyrenyl substituents are very attractive
because of long fluorescence lifetime, pure blue fluorescence,
strong and well characterized emissions, and their chemical
stability.2 Formation of the self-assembled complex results in a
remarkable change in the fluorescence emission intensities of
the pyrene excimer and monomer.3
Because of its amphoteric nature the imidazole ring behaves
as an excellent hydrogen bond donor moiety in synthetic anion
receptor systems, and the acidity of the NH proton of the
imidazole can be tuned by changing the electronic properties of
the imidazole substituents. On the other hand, the presence of
a donor pyridine-like nitrogen atom within the ring, capable of
selectively binding cationic species also converts the imidazole
derivatives into excellent metal ion sensors.4
Received: February 12, 2013
© XXXX American Chemical Society
A
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In this context, we report now the preparation, redox and
electronic properties as well as anion and cation sensing
behavior of a new family of ferrocene receptors, in which the
metallocene unit is directly linked to a π-extended imidazole
ring such as 1,6-dihydrobenzobisimidazole II. At first, this
structural motif combines the redox activity of the ferrocene
group with the binding ability of the imidazole ring in a highly
preorganized system. Our synthetic methodology allows the
decoration of the 7-position of this ring system with units
displaying different physical properties. The insertion of
subunits with different optoelectronic characteristics within
the ditopic fluorogenic itself heteroaromatic core may represent
an important “added value” to this class of receptors.
Chart 1
The presence of multiple binding sites in the designed new
structural motifs, the multiresponsive character of the receptors,
and the ability of the imidazole rings to act as a favorable
binding site for anions and cations in the recognition event are
most noteworthy, allowing their use as multisignaling molecular
ion chemosensors.
several examples of ferrocenylimidazole derivatives have been
successfully used as chemosensor of several kind of ions.14
The presence of at least a ferrocenyl group linked to a 2-
position of one imidazole ring in benzobisimidazole type II
plays a double key role in the recognition event. First, it has
been found in several types of aza-substituted ferrocenes that
high values of the redox potential shift on coordination are
related to the inverse iron−nitrogen separation.15 Considering
the close proximity in structures type II between the binding
sites of the imidazole ring and the 2-substituted ferrocene unit,
important potential shifts of the redox wave of the ferrocene/
ferrocenium redox couple should be expected upon either
coordination with metal cations or hydrogen-bonded complex
formation with anions. Second, it has been recently reported,16
that electron-rich substituents appended to the 2-position of
the imidazole ring increase availability of the N-base electron
density, which is an effective means of improving the N-base
donor strenght (cation coordination), with concomitant
increase of the acidity of the N−H protons of the imidazole
ring because of the electron-withdrawing character of the metal
ion (anion complexation).
EXPERIMENTAL SECTION
■
General Procedure for the Preparation of 2,7-Disubstituted-
1,6-dihydrobenzo[1,2-d:3,4-d′]bisimidazoles 4, 5, and 6. To a
solution of 2-ferrocenyl-4,5-diamino-1H-benzo[d]imidazole (0.20 g,
0.60 mmol) 317 in nitrobenzene (10 mL) the appropriate aldehyde
(0.60 mmol) and acetic acid (0.5 mL) were added. The resulting
mixture was stirred at 60 °C for 12 h and then a saturated solution of
NaHCO3 was added until pH = 7. Then, H2O (50 mL) was added,
and the solution was extracted with CH2Cl2 (3 × 50 mL). The organic
layer was dried over anhydrous Na2SO4 and concentrated to dryness
to give a brown residue which was purified on a silica gel column using
dicloromethane/hexane/methanol (9:1:0.5) as eluent to afford the
corresponding final products, which were crystallized from the
adequate solvent.
2,7-Diferrocenyl-1,6-dihydrobenzo[1,2-d:3,4-d′]bisimidazole
4. Yield: 43% (0.13 g); mp >300 °C (d). 1H NMR (300 MHz,
MeOD): δ 4.05 (s, 10H); 4.41 (st, 4H); 5.00 (st, 4H); 7.32 (s,
2H).13C NMR (75 MHz, MeOD) δ: 153.6(C); 135.6 (C); 128.3 (C);
a
Scheme 1. Synthesis of Receptors 4, 5, and 6
a
Reagents and conditions: (a) C6H5NO2, AcOH, 60 °C; (b) NaBH4, EtOH, CoCl2·6H2O; (c) ferrocenecarboxaldehyde, C6H5NO2, AcOH, 60 °C;
(d) 2,4-dinitrobenzaldehyde, C6H5NO2, AcOH, 60 °C; (e) 1-pyrenecarboxaldehyde, C6H5NO2, AcOH, 60 °C.
B
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109.9 (CH); 74.3 (C); 71.3 (CH); 70.8 (CH); 68.4 (CH). ESI-MS:
527 (M++1). Anal. Calc. for C28H22Fe2N4. C, 63.91; H, 4.21; N, 10.65.
Found: C, 63.70; H, 4.45; N, 10.37.
bonding. If basicity of the anion is high enough to deprotonate
the receptor, one observes appearance of a new intense
absorption band in the visible region of the electronic spectrum,
and the disappearance of NMR signals of abstracted receptor
protons, and an upfield shift of the signals of the adjacent
protons of the receptor.22 The titration experiments were
further analyzed using the computer program Specfit.23
Electrochemical Study. The reversibility and relative
oxidation potential of the ferrocene/ferrocenium redox couple
in receptors 4−6 were determined by cyclic voltammetry (CV),
Osteryoung square-wave voltammetry (OSWV), and linear
sweep voltammetry (LSV) in solutions of the appropriate
solvent containing 0.1 M [(n-Bu)4N]PF6 as supporting
electrolyte.
As expected, the CV of the bis(ferrocene) receptor 4, shows
a reversible two-electron oxidation wave at E1/2 = 623 mV,
versus the decamethylferrocene (DMFc) redox couple,
indicating that the two metal centers are electronically
decoupled. Whereas, the electrochemical response of 5 and 6
showed a reversible one-electron oxidation peak at E1/2 = 630
mV and E1/2 = 675 mV, respectively, versus DMFc (Table 1).
A possible way to reveal the formation of hydrogen-bonded
complexes under electrochemical titration conditions is to
suppress the deprotonation process by adding a small amount
2-Ferrocenyl-7-(2,4-dinitrophenyl)-1,6-dihydrobenzo[1,2-
d:3,4-d′]bisimidazole 5. Yield: 46% (0.14 g); mp = 215−217 °C
(d). 1H NMR (400 MHz, CD3CN) δ: 4.12 (s, 5H); 4.36 (st, 2H); 4.88
(st, 2H); 7.52 (s, 2H), 7.99 (d, J = 8.4 Hz, 1H); 8.37 (dd, J = 1.6, 8.4
Hz, 1H); 8.55 (d, J = 1.6 Hz, 1H). 13C NMR (100 MHz, CD3CN) δ:
68.0 (CH); 70.4 (CH); 70.7 (CH); 73.0 (C); 74.7(C); 120.6 (CH);
127.3 (CH); 130.6 (C); 132.7 (CH); 144.4 (C); 148.4 (C); 149.4
(C); 153.0 (C). ESI-MS: 509.2 (M+ + 1). Anal. Calc. for
C24H16FeN6O4. C, 56.71; H, 3.17; N, 16.53. Found: C, 56.50; H,
3.42; N, 16.22.
2-Ferrocenyl-7-(2-pyrenyl)-1,6-dihydrobenzo[1,2-d:3,4-d′]-
1
bisimidazole 6. Yield: 41% (0.13 g); mp = 240−243 °C (d). H
NMR (400 MHz, THF-d8) δ: 4.11 (s, 5H); 4.38 (bs, 2H); 5.09 (bs,
2H); 7.43 (bs, 2H); 8.03 (t, J = 7.8 Hz, 1H); 8.13 (d, J = 9.0 Hz, 1H);
8.16 (d, J = 9.0 Hz, 1H); 8.19 (d, J = 9.6 Hz, 1H); 8.22 (d, J = 9.0 Hz,
1H); 8.24 (d, J = 7.8 Hz, 1H); 8.31 (d, J = 7.8 Hz, 1H); 8.55 (bs, 1H);
9.70 (bs, 1H). Because of solubility problems, the 13C NMR could not
be carried out. HR MS (m/z), calculated for C34H22FeN4: 543.1273
(M++1); found: 543.1276. Anal. Calc. for C34H22FeN4. C, 75.29; H,
4.09; N, 10.33. Found: C, 75.51; H, 3.83; N, 10.50.
RESULTS AND DISCUSSION
■
Synthesis. Here, we describe the utility of an appropriate
diamino-benzothiadiazole 1 as an ideal starting material for
building novel angular 2,7-disubstituted benzobisimidazoles
decorated with groups displaying different photophysical
properties.
Table 1. Characteristic Electrochemical Data of the Free
Receptors 4−6 and Their Metal and Anion Complexes
Our synthesis started with the preparation of the 4,5-
diamino-2,1,3-benzothiadiazole18 1 and further condensation
with ferrocenecarboxaldehyde to give 7-ferrocenyl-8-H-
imidazo[4,5-e]-2,1,3-benzothiazole19 2 in 48% yield. Depro-
tection of the two amino groups by ring-opening of the
thiadiazole ring in compound 2 by reduction with NaBH4 in
the presence of CoCl2 provided 2-ferrocenyl-4,5-diamino-1H-
benzo[d]imidazole17 3 in 65% yield. Formation of the second
imidazole ring was achieved by reaction with ferrocenecarbox-
aldehyde, 2,4-dinitrobenzaldehyde, and 1-pyrenecarboxalde-
hyde to give the 2,7-disubstituted benzobisimidazoles 4
(43%), 5 (46%), and 6 (41%), respectively (Scheme 1).
Sensing Properties. The chemosensor behavior of
compounds 4, 5, and 6 toward a variety of cations (Li+, Na+,
K+, Mg2+, Ca2+, Ni2+, Cu2+, Zn2+, Cd2+, Hg2+ and Pb2+), as their
perchlorate or triflate salts, (Li+, K+, Mg2+, Ni2+, Cd2+ and Pb2+
were added as perchlorate salts, while Na+, Ca2+, Cu2+, Zn2+,
and Hg2+ were added as triflate salts) and anions (F−, Cl−, Br−,
a
a
compound
E1/2 (ΔE1/2
)
compound
[5·AcO−]
E1/2 (ΔE1/2)
b
540 (−90) ;
c
4
623
561 (−69)
b
470 (−160) ;
c
[4·Hg2+]
[4·Zn2+]
[4·Pb2+]
[4·HBF4]
[4·AcO−]
[4·H2PO4
870 (247)
690 (67)
[5·H2PO4
]
−
567 (−63)
b
735 (105) ;
c
−
[5·HSO4
]
709 (79)
b
490 (−140) ;
c
2−
875 (252)
895 (275)
[5·SO4
]
546 (−84)
b
404 (−226) ;
c
3−
[5·HP2O7
[5·F−]
[5·OH−]
6
]
493 (−137)
b
b
370 (−260) ;
c
487 (−136) ;
c
550 (−73)
579 (−51)
b
430 (−193) ;
c
−
]
355 (−275)
529 (−94)
b
667 (44) ;
c
−
[4·HSO4
]
675
687 (64)
[4·SO4
]
495 (−128)
[6·Zn2+]
[6·Pb2+]
747 (72)
2−
b
365 (−258) ;
c
3−
[4·HP2O7
]
800 (125)
450(−173) ;
c
AcO−, NO3 , HSO4 , H2PO4 and HP2O7 added as TBA+
salts and SO42− as tetramethylammonium salt) was investigated
by linear sweep voltammetry (LSV), cyclic voltammetry (CV),
and Osteryoung square-wave voltammetry20 (OSWV), as well
−
−
−
3−
480 (−143)
b
368 (−255) ;
c
[4·F−]
[6·Cd2+]
730 (55)
524 (−99) ;
b
573 (−50)
b
490 (−185) ;
c
[4·OH−]
5
269 (−354)
[6·AcO−]
1
552 (−123)
as through UV−vis, fluorescent and H NMR spectroscopic
b
475 (−200) ;
c
−
630
[6·H2PO4
]
techniques. It is worth mentioning that these recognition
studies were carried out in CH3CN and EtOH solutions for
receptors 4 or in CH3CN solutions in the case of 5. However,
the limited solubility of 6 in those solvents forced us to develop
such studies in tetrahydrofuran (THF) solutions.
540 (−135)
b
508 (−167) ;
c
[5·Hg2+]
[5·Zn2+]
837 (207)
720 (90)
[6·SO4
]
2−
525 (−150)
b
3−
[6·HP2O7
[6·F−]
]
450 (−225) ;
b
295 (−380) ;
c
511 (−164)
In recent years alternative mechanisms for several types of
anion-receptor interactions21 have been developed. If the
basicity of the anion is insufficient to induce deprotonation of
the receptor, one observes formation of a hydrogen-bonded
complex manifested in a red-shift of the receptor absorption
band and a downfield shift or often disappearance of NMR
signals of the receptors protons involved in the hydrogen
b
523 (−152) ;
b
[5·Pb2+]
880 (250)
870 (240)
355 (−320) ;
c
558 (−117)
[5·HBF4]
[6·OH−]
315 (−360)
a
b
ΔE1/2 = E1/2complex − E1/2 free freceptor, mV vs DMFc. In the absence of
acetic acid. In the presence of 20 equiv of acetic acid.
c
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of acetic acid.24 In preliminary experiments, we found that
addition of up to 20 equiv of acetic acid affected neither CV nor
OSWV of receptors 4, 5, and 6.
In all the cases, titration with the strong base Bu4NOH,
which definitely leads to deprotonation, induced a remarkable
cathodic shift of the oxidation peak (ΔE1/2 = −354 mV, for 4,
ΔE1/2 = −275 mV, for 5, and ΔE1/2 = −360 mV, for 6) (Table
1).
Zn2+ induced the appearance, in the OSWV, of a new oxidation
peak at a remarkably more positive potential (Table 1). In the
case of Pb2+ a new oxidation peak was also observed in the
OSWV at E1/2 = 875 mV. However, the reduction peak in the
CV of the complex 4·Pb2+ appears at a potential which is almost
the same as that observed for the free receptor. This
experimental result could be explained in the following way:
once the oxidation of the ferrocene unit present in the formed
complex 4·Pb2+ has taken place, such species is almost
completely disrupted and therefore the Pb2+ ion is expelled
from the receptor. As a consequence, the reduction will take
place on the radical cation 4+ derived from the free receptor 4.
In other words, this result shows that the 4·Pb2+ complex
undergoes reversible electrochemically induced complexation/
decomplexation processes on a time scale faster than the
electrochemical experiment (see Supporting Information,
Figure S8).
The anion binding properties of 4, in CH3CN, demonstrated
−
−
that only addition of AcO−, HSO4 , and H2PO4 anions
promotes remarkable responses. On the other hand, addition of
F−, and HP2O7 anions induced the same change in the
3−
oxidation peak as that observed when 1 equiv of Bu4NOH
(ΔE1/2 = −354 mV) was added, which is indicative of the fact
that such anions only promote the deprotonation of the free
receptor (see Supporting Information, Figures S4−S5). Never-
theless, the results obtained on the stepwise addition of AcO−,
Interestingly, while addition Zn2+, Pb2+, and Hg2+ metal
cations to 4 promotes the formation of the corresponding
complexes, addition of Cu2+ induces the oxidation of the
ferrocene moiety present in the free receptor.
−
−
HSO4 , and H2PO4 anions revealed three different electro-
chemical behaviors. For the AcO− anion a typical “two wave
behavior”25 was observed, with the appearance of a second peak
at more negative potential (ΔE1/2 = −136 mV) together with
−
With reference to the electrochemical titration studies carried
out by using receptor 5, it is worth highlighting that addition of
the corresponding to the free receptor. In the case of H2PO4
anion a “shifting behavior” was observed, appearing as a second
redox peak, negatively shifted (ΔE1/2 = −193 mV), compared
−
−
2−
AcO−, H2PO4 , HSO4 , and SO4 anion, in the presence of
acetic acid, induced the same kind of perturbation in the
oxidation potential as those observed with receptor 4. These
electrochemical data also suggest that the interaction of the
−
to the free receptor. Remarkably, for HSO4 anion a new
oxidation peak emerged at more positive potential (ΔE1/2
=
+44 mV) (Table 1). The electrochemical behavior observed for
−
HSO4 anion with receptor 5 also involves an initial proton
−
HSO4 ion is consistent with a guest-to-host proton transfer
transfer followed by hydrogen-bond formation and subsequent
−
reaction by the moderately strong acidic HSO4 anion,
anion coordination. Addition of F− and HP2O7 anions
3−
accompanied by hydrogen-bonding and electrostatic interaction
with the guest anion: proton transfer is followed by hydrogen-
bond formation and subsequent anion coordination.26
definitely lead to deprotonation and induced a strong cathodic
shift of the oxidation peak.
Likewise, addition of the above-mentioned metal cations to
an electrochemical solution of receptor 5 in CH3CN show that
only the addition of Hg2+, Pb2+, and Zn2+ induced the
appearance, in the OSWV, of a new oxidation peak at a
remarkably more positive potential (Table 1). Again, the
potential of the reduction peak in the CV of [5·Pb2+] also
resembles that of the free receptor, and addition of Cu2+ metal
cation promotes the same oxidation effect observed when the
receptor 4 was used.
This assumption is supported by the following results: (a)
upon protonation of receptor 4 with HCl the redox peak was
2−
shifted anodically (ΔE1/2 = 270 mV); (b) when SO4 anion
was added, the redox peak was shifted cathodically (ΔE1/2
=
2−
−128 mV), and (c) upon addition of SO4 anion to the
electrochemical solution of the protonated receptor [4·H+] a
cathodically shifted oxidation peak appears at virtually the same
−
potential as that observed for the [4·HSO4 ] complex (E1/2
=
667 mV) (Figure 1)
The electrochemical recognition abilities of 6 were also
investigated in the presence of anions and cations. The results
obtained demonstrate that the oxidation wave associated to the
free receptor underwent a cathodic shift upon addition of
On the other hand, the results obtained on the stepwise
addition of substoichiometric amounts of the above-mentioned
set of metal cations show that only the addition of Hg2+ and
AcO−, H2PO4, and SO4 anions although the magnitude of
2−
those shifts and the electrochemical behavior were dependent
on the anion studied. Thus, addition of substoichiometric
amounts of SO42− anion revealed a typical “two wave behavior”,
with the appearance of a second peak at more negative
potential together with the one corresponding to the free
receptor (see Supporting Information, Figure S17). By contrast,
the addition of the other oxoanions gave rise to a “shifting
behavior”, in which a second redox peak, cathodically shifted
when compared to the free receptor, appears (Figure 2 and
Table 1).
3−
Addition of HP2O7 and F− to 6 induced the progressive
appearance of two new oxidation peaks cathodically shifted. To
rule out possible deprotonation processes of the free ligand, we
have also carried out other electrochemical experiments either
by adding Bu4NOH to the electrochemical solution of the free
receptor or by titrating with these anions in the presence of
acetic acid, in which case the deprotonation process is
Figure 1. Evolution of the OSWV of 4 (c = 1 × 10−4 M) in CH3CN/
n-Bu4NPF6, versus decamethylferrocene (DMFc) redox couple,
−
scanned at 0.1 V s−1 in the presence of (a) HSO4 (purple); (b)
HCl (red); (c) SO42− (green); (d) initial addition of HCl followed by
2−
addition of SO4 (blue).
D
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whereas addition of HP2O7 or F− to an electrochemical
solution of receptor 6, under the same acidic conditions, gave
only rise to one oxidation peak, cathodically shifted (see
Supporting Information, Figure S18). These data clearly
suggest that the perturbations observed upon addition of
3−
−
AcO−, H2PO4 , and SO42− anions to the free receptor 6 should
be associated to a recognition event, involving the formation of
a hydrogen-bonded complex between 6 and such anions. On
the other hand, the results obtained upon titration with
HP2O73− and F− anions showed that in the absence of an acidic
medium both a deprotonation and a recognition process should
simultaneously taking place, while in the presence of a small
amount of acetic acid only the corresponding hydrogen-bonded
complexes are formed (Table 1).
The results obtained from the electrochemical metal cation
binding studies of 6 show that only the progressive addition of
Zn2+, Cd2+, and Pb2+ metal cations promote a significant anodic
shift of the ferrocene oxidation peak in the free receptor. Once
again, the [6·Pb2+] complex formed is almost completely
disrupted when such complex undergoes electrochemical
oxidation and, consequently, the Pb2+ is then expelled from
the receptor. Moreover, the substantial shift toward cathodic
currents observed in the corresponding LSV of the free
receptor upon addition of Cu2+ and Hg2+ constitutes a relevant
evidence of the oxidant character of these metal cations toward
6 (see Supporting Information, Figures S19−S20).
Figure 2. Evolution of the OSWV of 6 (c = 2 × 10−4 M in THF),
using n-Bu4NPF6 as supporting electrolyte and scanned at 0.1 V s−1,
upon addition of increasing amounts of (a) [(n-Bu)4N]AcO and (b)
[(n-Bu)4N]H2PO4.
prevented. First, titration with the strong base Bu4NOH gave
rise to the appearance of a new oxidation peak, cathodically
shifted. This magnitude is quite similar to that obtained for the
cathodic shift of one of the oxidation peaks resulting upon
addition of HP2O73− and F−, but different as those derived from
the addition of AcO−, H2PO4 , and SO4 anions. Second,
titrations in the presence of 20 equiv of AcOH almost did not
affect the results obtained in the electrochemical titrations
−
2−
Absorption and Emission Study. Ion recognition proper-
ties of the receptor 4 toward metal cations and anions have also
been studied by using absorption and emission techniques. The
carried out by using AcO−, H2PO4 , and SO4 anions alone,
−
2−
Table 2. Characteristic UV-vis Data for the Receptor 4−6 and Representative Metal and Anion Complexes
a
b
c
receptor
UV−vis λmax (10−3 ε)
IP
Kas
Dlim
d
4
304 (1.721); 445 (0.068)
e
[4·Zn2+]
[4·Cd2+]
[4·Cu2+]
[4·Hg2+]
[4·HBF4]
304 (1.776); 460 (0.098)
304 (1.830); 455 (0.100)
319
319
e
e
f
366 (0.574); 430 (0.469); 915 (0.117)
304 (1.504); 455 (0.094)
290; 335
i
9.46 × 104
1.28 × 105
5.26 × 10−6
f
304 (1.705); 445 (0.112)
319; 337
d
−
[4·H2PO4
]
304 (1.588); 448 (0.061)
d
2−
[4·SO4
]
304 (1.394); 448 (0.066)
e
5
307 (1.099); 390 (0.694)
e
i
[5·Pb2+]
310 (1.317); 358 (sh)
372
355
320
403
403
403
418
1.02 × 10−5
e
[5·HBF4]
358 (sh); 455 (0.088)
e
[5·Cu2+]
374 (0.854); 923 (0.061)
e
i
i
i
[5·AcO−]
307 (1.099); 419 (0.681)
4.80 × 104
4.99 × 105
9.97 × 104
7.48 × 10−6
5.03 × 10−6
6.53 × 10−6
e
−
[5·H2PO4
[5·SO4
]
307 (1.096); 413 (0.675)
e
2−
]
307 (1.064); 428 (0.676)
e
3−
[5·HP2O7
]
310 (1.047); 485 (0.925)
e
[5·OH−]
310 (1.058); 485 (0.887)
313; 342; 418
g
6
300 (1.685); 374 (1.589)
g
j
[6·Zn2+]
351 (1.468); 455 (0.057)
315; 360; 440
1.09 × 1012
1.05 × 10−5
6.41 × 10−6
g
i
[6·Pb2+]
365 (1.719); 455 (0.067)
300; 315; 375; 435
3.28 × 106
g
[6·Hg2+]
362 (1.742); 455 (0.072); 940 (0.070)
360 (1.462); 940 (0.075)
g
[6·Cu2+]
290
g
[6·AcO−]
[6·H2PO4
[6·HP2O7
310 (1.435); 385 (1.442); 400 (sh, 1.341)
310 (1.436); 385 (1.420); 400 (sh, 1.267)
307 (1.426); 385 (1.443); 400 (sh, 1.301)
330; 385
330; 385
310; 330; 385
310; 330; 385
330; 385
g
−
]
g
h
i
]
3.57 × 104
2.28 × 105
4.36 × 10−6
9.61 × 10−6
3−
g
h
i
[6·F−]
[6·OH−]
303 (1.449); 385 (1.468); 400 (1.253)
g
385 (1.362); 405 (sh, 1.235)
a
b
c
d
e
f
g
ε in dm3 mol−1 cm−1. Isosbestic points in nm. Detection limits in M. CH3CN or EtOH solution. CH3CN solution. EtOH solution. THF
h
i
j
solution. In the presence of 20 equiv of acetic acid. In M−1. In M−2.
E
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−
UV−vis absorption spectrum of receptor 4 in CH3CN (c = 5 ×
10−5 M) and EtOH at the same concentration, exhibits two
bands at λ = 304 nm (ε = 1721 M−1 cm−1) and 445 nm (ε = 68
M−1 cm−1) (Table 2 and Supporting Information, Table S1).
The addition of Mg2+, Ni2+, Zn2+, Cd2+, Hg2+, and Pb2+ elicited
the same optical response: the progressive appearance of a new
LE band red-shifted by Δλ = 5−15 nm and well-defined
isosbestic points indicative of the presence of only two
absorbing species in the solution were found (Supporting
Information, Figure S21). However, in EtOH solution only the
addition of Hg2+ cations induced the red-shift of the LE band
by Δλ = 10 nm (Supporting Information, Figure S24). Binding
HSO4 anion, the results indicating the formation of 1:1
complex with association constant Ka = 4.19 × 104 M−1 and
detection limit 1.57 × 10−6 M.
Interestingly, the protonated species [4·H+], formed by
addition of 1 equiv of HBF4 to the free ligand, showed the same
emission spectrum as those obtained by addition of both Hg2+
−
and HSO4 ions, although with a CHEF = 156 (Figure 3).
Moreover, the emission band of [4·H+] undergoes a gradual
increase in intensity only upon addition of SO42− ion, reaching
a maximum when 1 equiv of this anion was added (CHEF =
1.83). This result clearly indicates that the protonated species
[4·H+] could be used for the selective detection of SO4 ion,
2−
taking into account that the only addition of this anion to the
free ligand does not promote any change in its emission
spectrum (Figure 3).
The stoichiometries proposed from absorption and fluo-
rescent data were further confirmed by mass spectrometry. The
́
assays using the method of continuous variations (Jobs plot)
suggest a 1:1 binding model (Supporting Information, Figure
S25). Titration studies of receptor 4 toward the set of anions
−
under study revealed that only F−, AcO−, SO42−, H2PO4 , and
3−
HP2O7 decrease in the intensity of the HE absorption band
−
ESI mass spectrum of receptor 4 in the presence of HSO4
(see Supporting Information, Figure S26).
shows a peak at m/z 623.5 corresponding to the 1:1 complex,
whereas the ESI and ESITOF mass spectra in the presence of
Hg2+ cations show a peak at m/z 1451 corresponding to the
complex [4·Hg2+]2. The relative abundance of the isotopic
clusters was in good agreement with the simulated spectra of
the complexes (see Supporting Information, Figures S32−S35).
The UV−vis absorption spectrum of receptor 5 in CH3CN
(c = 5 × 10−5 M) exhibits two bands at λ = 307 nm (ε = 1099
M−1 cm−1) and 390 nm (ε = 694 M−1 cm−1). The addition of
Zn2+, Hg2+, and Pb2+ elicited the same optical response: the
progressive appearance of a new LE band blue-shifted (Δλ =
−32 nm), and the initial yellow solution of the receptor
becomes colorless (Figure 4). The presence of well-defined
Assessments of the ion affinities also came from observing
the extent to which the fluorescence intensity of the receptor 4
was affected in the presence of cations and anions (see
Supporting Information, Figure S28). As expected, receptor 4
showed a weak fluorescence in EtOH (c = 10−5 M), revealing
that the excitation spectrum at λ = 330 nm is an ideal excitation
wavelength. The emission spectrum displays a structureless
band centered at λ = 405 nm, with a rather low quantum yield
(Φ = 10−3) with reference to anthracene as standard (Φ = 0.27
0.001).27 Receptor 4 in EtOH showed a large CHEF
(Chelation-Enhanced Fluorescence Effect)28 when Hg2+ cation
was added. Thus addition of 1 equiv of Hg2+ cation induced a
red-shift (Δλ = 10 nm) of the emission band which is also
accompanied by a remarkable increase in the intensity of such
emission band (CHEF = 225), and by a 60 fold increase in the
quantum yield (Φ = 6 × 10−2) (see Supporting Information,
Figure S29). From the fluorescence titration data, a 1:1 binding
mode is deduced, and the association constant was calculated to
be 3.95 × 105 M−1. The detection limit29 was found to be 6.58
× 10−6 M.
After addition of 1 equiv of HSO4− anion to a solution of the
receptor 4 in EtOH, the emission band red-shifted (Δλ = 13
nm), with a concomitant increase in the intensity of the
emission band (CHEF = 486), and in the fluorescence
quantum yield which increased by a factor of 100 (Φ =
10−1) (Figure 3). The stoichiometry of the complex was also
determined by the changes in the fluorogenic response of 4 and
Figure 4. Changes in the absorption spectra of 5 (black) (c = 5 × 10−5
M in CH3CN) upon addition of increasing amounts of: Pb(ClO4)2;
Zn(OTf)2 and Hg(OTf)2 (deep red); [(n-Bu)4N]AcO, [(n-
Bu)4N]2SO4 and [(n-Bu)4N]H2PO4 (deep blue); [(n-Bu)4N]F, [(n-
Bu)4N]3HP2O7 and [(n-Bu)4N]OH (deep green). Inset: visual
features observed upon addition of metal cations, anions, and upon
deprotonation.
isosbestic points indicates the existence of only two absorbing
species in the solution. Binding assays using the method of
continuous variations (Job’s plot) suggest a 1:1 binding model
for Pb2+ (Supporting Information, Figure S37). Electrospray
mass spectra also confirmed such stoichiometries because they
show the appropriate molecular ion peaks with the relative
abundance of the isotopic clusters being in good agreement
with the simulated spectra of the complexes formed (see
Supporting Information, Figures S38−S41).
Figure 3. Evolution of the emission spectrum of 4 (black), upon
−
addition of (a) HSO4 (purple); (b) HBF4 (red); (c) initial addition
2−
of HBF4 followed by addition of SO4 (green); (d) Hg2+ (blue).
Titration studies of receptor 5 toward the set of anions under
−
2−
−
Inset: visual features observed upon addition of HSO4 anion.
study revealed that only AcO−, H2PO4 , and SO4 ions
F
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−
induced the progressive appearance of a new LE band red-
of the complexes [6·A] (A = AcO−, and H2PO4 ) which show
molecular ion peaks at m/z 601.5 and 639.0, respectively.
Moreover, the relative abundance of the isotopic clusters was in
good agreement with the simulated spectra of the complexes
(see Supporting Information, Figures S52−S54).
−
shifted (Δλ = 29 nm for AcO−, Δλ = 23 nm for H2PO4 , and
2−
Δλ = 38 nm for SO4 ion) (Figure 4). A well-defined
isosbestic point at λ = 403 nm indicated that a neat
interconversion between the uncomplexed and complexed
species occurs. The new LE band is responsible for the change
of color, from yellow to orange, which can be used for “naked
eye” detection of these anions. From analysis of the spectral
titrations data and electrospray mass spectra studies (see
Supporting Information, Figures S43−S47), 1:1 binding
stoichiometries were determined and the binding constants
were determined (Table 2). The absorption spectrum of 5 in
When titrations were carried out in the presence of AcOH,
only F−, AcO−, and HP2O73− anions promoted the same slight
bathochromic shift of the LE band observed under neutral
conditions (see Supporting Information, Figure S55).
When the titration experiments were carried out with metal
cations, the receptor 6 only responded to the presence of Zn2+,
Cd2+, and Pb2+. In these cases, a new and weak absorption band
centered at 455 nm developed on titration, and sharp isosbestic
points were observed (see Supporting Information, Figure
S56). The titration profiles could be fitted to a 1:1
(ligand:metal cation) model for Cd2+ and Pb2+ and to a 2:1
stoichiometry for Zn2+ (Supporting Information, Figure S57).
The oxidation experienced by receptors 4 and 5 in the
presence of Cu2+ and 6 upon addition of Cu2+ and Hg2+ has
also been confirmed by UV−vis experiments. The resulting
spectra obtained from the oxidation of 4 and 5 with Cu2+ and
from 6, both with Cu2+ and Hg2+, gave rise to a new band in the
region 915−940 nm, which can be ascribed to the formation of
a ferrocenium ion30 (see Supporting Information, Figure S58).
The changes in the fluorescence spectrum of 6 upon addition
of metal cations and anions were also examined. Receptor 6 (c
= 5 × 10−6 M in THF) shows a weak and structureless
monomer emission band at 449 nm (Φ = 0.015) when excited
at 380 nm. On addition of increasing amounts of anion,
receptor 6 only shows reliable changes upon addition of
H2PO4− and HP2O73− anions, consisting in the appearance of a
new and more intense red-shifted emission band (Δλ = 22 nm,
3−
the presence of either F− or HP2O7 displayed the same
changes as those observed with Bu4NOH: the appearance of a
new red-shifted band (Δλ = 95 nm), which is responsible for
the strong red color of the deprotonated species 5-H+ (Figure
4).
The recognition capacity of 6 toward anions and cations was
also studied by observing changes in their UV−vis absorption
and emission spectra. When a THF solution of 6 (c = 5 × 10−5
M) was titrated with AcO−, H2PO4 , HP2O73−, and F− anions,
−
the absorption band centered at 374 nm gradually decreased
and two new lower energy absorption bands emerged at 385
nm (ε = 1440 M−1 cm−1) and 400 nm (ε = 1315 M−1 cm−1),
with clear isosbestic points. The new LE absorption band
appearing at 400 nm is responsible for the color change of the
solution from colorless to pale yellow. As an example, Figure 5
−
Φ = 0.111 for H2PO4 , and Δλ = 36 nm, Φ = 0.206, for
HP2O73−) (Figure 6). Similar results were also obtained when
Figure 5. Changes in the absorption spectra of 6 (c = 5 × 10−5 M in
THF) upon addition of increasing amounts of [(n-Bu)4N]AcO until
1.8 equiv were added. Inset: visual features observed after addition of
AcO− anions.
displays the spectra obtained on titration with [Bu4N]AcO,
although these spectral features are the same as those observed
for the other mentioned anions (see Supporting Information,
Figure S50).
Figure 6. (a) Changes in the emission spectra of 6 (c = 5 × 10−6 M in
THF) in the presence of 2 equiv of the indicated species. Emission is
monitored at λexc = 380 nm. (b) Visual changes observed in the
fluorescence of THF solutions of 6 (left) after addition of [(n-
Bu)4N]H2PO4 (middle) and [(n-Bu)4N]3HP2O7 (right).
The Job’s plots display a maximum absorption change when
the molar fraction of A versus [6 + A] (A = AcO−, and
H2PO4−) is 0.5, indicative of the formation of the
corresponding complexes with 1:1 stoichiometry (see Support-
ing Information, Figure S51). The association constants were
calculated (Table 2) from these titration data. Taking into
account that upon addition of F− and HP2O73− only hydrogen-
bonded complex formation takes place in the presence of acetic
acid, the Job’s plots were obtained under such conditions and
demonstrated that binding between receptor 6 and these anions
also occurs through a 1:1 stoichiometry. From these titration
data, the apparent association constants were calculated. In the
such titration processes were carried out in the presence of
AcOH, confirming the recognition event taken place between
receptor 6 and those anionic species (see Supporting
Information, Figures S59−S61 and Table S2). The 1:1
stoichiometry of the complexes was also confirmed by
fluorescence Job’s plots, the calculated association constants
being Kas = 4.26 × 105 M−1 for the case of H2PO4 and Kas =
−
2.60 × 106 M−1 for HP2O7 anions.
3−
−
case of AcO− and H2PO4 such stoichiometry has also been
The fluorogenic metal ion binding properties of 6
demonstrate that the fluorescence emission was only affected
further confirmed by analysis of the electrospray mass spectra
G
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by the presence of Cu2+ and Hg2+ metal cations as a
consequence of the oxidation of the ferrocene. In these cases,
the change in emission of 6 also results in both an increase of
the fluorescence intensity (CHEF = 6 for Cu2+ and CHEF = 5
for Hg2+) and a deep change in intensity of the blue color of the
receptor solution (Figure 7). Such interesting results may be
was added, which indicate that this anion interacts with the free
ligand promoting a protonation process on the imidazole
nitrogen atoms.
1
Moreover, the H NMR spectra of 4 upon addition of
increasing amounts of HBF4 also displayed the same set of
signals as those observed for the species formed upon addition
−
of both Hg2+ and HSO4 ions (see Supporting Information,
Figures S64−S66 and Table S3). Consequently, we assume that
−
addition of HSO4 ions promotes, in a first stage, the N-
protonation of the imidazole moiety which is then followed by
the recognition of the formed SO42− anion by such protonated
species, as it is illustrated in Figure 9.
Figure 7. Changes in the emission spectra of 6 (c = 5 × 10−6 M in
THF) in the presence of Cu2+ or Hg2+ cations. Inset: visual changes
observed upon oxidation of 6 by these metal cations.
−
Figure 9. Proposed binding mode for the HSO4 anion.
explained by quenching of the fluorescence of the pyrene
subunit in the neutral triad 6 by the ferrocene moiety31 which is
responsible for the weak emission band observed. However,
after oxidation of 6, the electron-donating ability of the
ferrocene moiety is reduced and, as a result, the electron
transfer is arrested leading to a fluorescence enhancement. So
that, receptor 6 behaves as a multichannel chemodosimeter of
these metal cations. Also, as the ferrocene/ferrocinium pair
transformation in 6 can be reversibly carried out, the possibility
of obtaining a redox-fluorescent switch, based on the triad 6, is
opened.
For compound 5, the detailed evolution of every proton shift,
upon addition of the appropriate anion and cation is shown in
Figures 10a and 10c, respectively, and in Supporting
1H NMR Study. To gain some insight about the plausible
1
binding modes operating in these recognition processes, a H
NMR titration study of 4 in CH3OD solutions was also carried
out (Figure 8). The results obtained in this study demonstrated
Figure 10. Evolution of the 1H NMR spectrum of the free receptor 5
(b), upon addition of 1 equiv of [(n-Bu)4N]AcO (a) and 1 equiv of
Pb(ClO4)2 (c).
Figure 8. Evolution of the 1H NMR spectrum of 4 upon addition of 1
equiv of HSO4 , HBF4, and Hg(OTf)2.
−
a similar trend upon addition of both Hg2+ and HSO4− ions to
the free ligand. Thus, the progressive addition of Hg2+ to the
solution of 4 in CH3OD causes the simultaneous downfield
shift of the ferrocene protons and those signals associated to
the central ring protons. The maximum downfield shifts were
observed when addition of 1 equiv of Hg2+ was reached: ΔδHCp
= 0.19 ppm, ΔδHα = 0.21 ppm, ΔδHβ = 0.36 ppm, and ΔδHPh
= 0.36 ppm. More importantly, similar changes in the chemical
Information, Figures S67−S70 as well as in Supporting
Information, Table S4. In general, (Figure 10c) protons H4
and H5, appearing at a very close chemical shift in the free
receptor (Figure 10b), are split into two clear doublets, shifted
to higher field, in the presence of increasing amounts of the
anions (AcO−, H2PO4 , HSO4 , and SO42−). On the other
hand, the effect of the anion complexation in the ferrocene
protons is dependent on the type of the studied proton. Thus, a
shielding effect is observed for both the Hβ, within the
−
−
−
shifts of these protons were also observed when HSO4 anion
H
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monosubstituted Cp unit, and those within the unsubstituted
Cp moiety, while a very pronounced deshielding is observed for
the Hα protons. Anion complexation is also accompanied by a
increase of the monomer emission band is observed only in the
−
presence of H2PO4 anions, whereas the fluorescent response
toward the metal cations Zn2+, Cd2+, and Pb2+ was silent.
However, in the presence of the Cu2+ and Hg2+ metal cations a
remarkable increase of the monomeric emission band was
observed. In other words, receptor 6 behaves as a fluorescent
chemodosimeter of these metal cations.
1
significant downfield shift of the H NMR signals for the H5′
and H6′ protons of the dinitrophenyl fragment, while the signal
for H3′ proton underwent a slight upfield shift.
Regarding the 1H NMR cation titration processes it is worth
mentioning the downfield shift detected for all the ferrocenyl,
aromatic and heteroaromatic protons present in the free ligand
as a result of the cation complexation (Figure 10c and
Supporting Information, Figure S70). Interestingly, the
ASSOCIATED CONTENT
* Supporting Information
General experimental comments, NMR spectra, electro-
chemical, UV−vis, fluorescence, and H NMR titration data.
This material is available free of charge via the Internet at
http://pubs.acs.org.
■
S
−
1
protonation effect observed upon addition of HSO4 anion to
the free ligand 4, under the 1H NMR titration conditions, is not
observed in receptor 5. This fact is consistent with the lower
basicity of the nitrogen atom belonging to the imidazole ring,
bearing the electron withdrawing dinitrophenyl substituent.
AUTHOR INFORMATION
Corresponding Author
*E-mail: pmolina@um.es (P.M.), atarraga@um.es (A.T.).
■
1
Because of the limited solubility of 6, the H NMR titration
experiments were carried out in THF-d8 where 6 presents a
higher solubility. In particular, a THF-d8 solution of 6 was
−
Notes
titrated with H2PO4 , which was added up to 1.2 equiv (see
The authors declare no competing financial interest.
Supporting Information, Figure S71). An analysis of such
spectra revealed the following most significant changes: (i) the
splitting of the broad signal at δ = 7.43 ppm, corresponding to
the protons H4 and H5 protons of the heterocyclic ring, into
two doublets [δ = 7.43 (Δδ = 0 ppm) and δ = 7.54 ppm (Δδ =
+0.11 ppm)]; (ii) a different behavior of the protons belonging
to the ferrocene moiety: while the protons within the
unsubstituted Cp ring are unaffected, the Hα are downfield
shifted (Δδ = +0.30 ppm) and the Hβ are slightly highfield
shifted (Δδ = −0.14 ppm).
ACKNOWLEDGMENTS
We gratefully acknowledge the financial support from
MICINN-Spain, Project CTQ2011/27175, FEDER and
■
́
Fundacion
Region
Ayudas a Grupos de Excelencia de la Region
́
Sen
́
eca (Agencia de Ciencia y Tecnologia de la
́
de Murcia) project 04509/GERM/06 (Programa de
de Murcia, Plan
́
́
Regional de Ciencia y Tecnologia 2007/2010). One of us M.A.
also thanks to the MICINN for a FPI fellowship.
CONCLUSIONS
REFERENCES
■
■
The synthesis of the high flat rigid 2,7-disubstituted
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1
copy. Receptor 5 behaves as a redox and chromogenic
chemosensor molecule for AcO−, H2PO4 , and SO4 anions.
The change in the absorption spectra is accompanied by a color
change from yellow to orange, which allows their potential
“naked eye” detection, whereas Zn2+, Hg2+, and Pb2+ cations
induced a color change to colorless. Electrochemical data
−
2−
−
support that the behavior of the HSO4 anion is identical to
that observed toward receptor 4. Receptor 6 behaves as redox
(6) Khramov, D. M.; Boydston, A. J.; Bielawski, C. W. Angew. Chem.,
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I
dx.doi.org/10.1021/ic400365c | Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
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J
dx.doi.org/10.1021/ic400365c | Inorg. Chem. XXXX, XXX, XXX−XXX