Coumarin-based chromogenic receptor for Ni2+ in aqueous medium exhibiting a reconfigurable logic gate pattern

Article abstract of DOI:10.1002/ejoc.201100511

Coumarin-based receptor R1 recognizes Ni²⁺ at sub-micromolar levels in aqueous ethanolic medium (60:40) at physiological pH and exhibited INH logic function at pH values below 7. Additionally, this system showed reconfiguration to AND as well as TRANSFER logic functions in aqueous and dry dimethyl sulfoxide, respectively. The complexation of Ni²⁺ with R1 constituted the key step in its recognition, which was excellently demonstrated through single-crystal XRD analysis.

Full text of DOI:10.1002/ejoc.201100511

FULL PAPER
DOI: 10.1002/ejoc.201100511
Coumarin-Based Chromogenic Receptor for Ni2+ in Aqueous Medium
Exhibiting a Reconfigurable Logic Gate Pattern
Rakesh K. Mishra[a] and K. K. Upadhyay*[a]
Keywords: Cations / Nickel / Receptors / Sensors / UV/Vis spectroscopy / Logic gate
Coumarin-based receptor R1 recognizes Ni2+ at sub-micro- dry dimethyl sulfoxide, respectively. The complexation of
molar levels in aqueous ethanolic medium (60:40) at physio-
logical pH and exhibited INH logic function at pH values
below 7. Additionally, this system showed reconfiguration to
AND as well as TRANSFER logic functions in aqueous and
Ni2+ with R1 constituted the key step in its recognition, which
was excellently demonstrated through single-crystal XRD
analysis.
such as half-adder, full-adder, half-subtractor, full-sub-
tractor, complementary IMP/INH etc. have also been re-
ported over the last couple of years.[9] The reconfigurability
of these molecular sensors in terms of logic operations is
quite interesting and raises their impact further. This flexi-
bility may involve one or more parameters, such as the type
and charge of metal ions, the presence or absence of other
ions, the pH, and solvent etc.
Introduction
The chromogenic sensing of ions through synthetic che-
moreceptors is gaining momentum in view of their crucial
roles in our lives. The Ni2+ ion is an essential micronutrient
that plays a number of crucial roles in biochemical reactions
occurring in living organisms.[1] Any change in these reac-
tions may lead to serious consequences.[2] Hence, the bio-
chemistry of Ni2+ has been under intense investigation for
last few years by a number of research groups.[3] In addition
to its biological applications, Ni2+ has also been used as a
catalyst for a variety of reactions,[4] however, there are so
far only a few reports on the chromogenic sensing of
3-(2-{(E)-2-[1-(2-Oxo-2H-chromen-3-yl)ethylidene]-
hydrazin-1-yl}-1,3-thiazol-4-yl)-2H-chromen-2-one
(1),
which is the focus of this communication, is the first exam-
ple of a coumarin-based chromogenic receptor for Ni2+ ex-
hibiting a reconfigurable logic gate pattern. Most of the
previously reported chromogenic receptors for Ni2+ require
more complex synthetic procedures than the present model.
The present study on visual Ni2+ sensing is strongly sup-
ported by the single-crystal XRD studies of the key step
i.e., complex formation between R1 and Ni2+. Most of the
comparable studies on Ni2+ reported so far relied on spec-
troscopic evidence or, in a few cases, physico-chemical
methods. Hence, the present study has an edge over many
other similar studies for Ni2+ previously reported. The pres-
ent work is a part of our continuing effort over the last
couple of years to synthesize optical receptors for biolo-
gically relevant ions through simple reaction protocols.[10]
Ni2+ [5]
This prompted us to synthesize a new coumarin-
.
based chromogenic receptor capable of recognizing Ni2+ in
aqueous medium at physiological pH.
The parallels between the optical output of a receptor
with a particular analyte and the truth tables of a particular
logic gate have opened a new vista in the development of
molecular machines. These receptors are being examined as
miniature machines with the potential to signal the presence
of a particular analyte at very low concentration levels. The
optical responses of a chemoreceptor can be fitted into the
truth table of a suitable logic gate to generate the corre-
sponding logic function for that particular receptor.[6]
A
number of properly designed chemoreceptors are being
used as functional components where on–off functions can
be switched by external stimuli such as light, pH, electrical,
or chemical inputs.[7] A modest number of logic functions
have so far been proposed by a number of groups.[8] Besides
these individual logic functions, their combinatorial forms,
Results and Discussion
The synthesis of 1 may be conveniently illustrated as
shown in Scheme 1. The receptor was obtained by adding
an equimolar mixture of compound 2 with 3 and allowing
the reaction mixture to stir at approximately 60 °C for
around 2.5 h, leading to the isolation of a crude product as
a yellowish precipitate. The solid was filtered and dried un-
der vacuum, and R1 was finally obtained as a solid product
[a] Department of Chemistry, Faculty of Science,
Banaras Hindu University,
Varanasi 221005, India
Fax: +91-542-236-8127
E-mail: drkaushalbhu@yahoo.co.in
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.201100511.
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upon basification of an aqueous suspension of the crude
product with aq. NaHCO3. Compounds 1, 2, and 3 were
synthesized according to reported methods.[11–13]
Figure 2. Absorbance profile of R1 in the presence of different
metal ions.
Scheme 1. Synthetic route to R1. Reagents and conditions: (i) Br2,
AcOH, reflux, 2 h; (ii) thiosemicarbazide, EtOH, reflux, 4 h; (iii) 2
and 3, EtOH, stirring, 60 °C, 2.5 h.
Absorption bands with their maxima at 270 and 372 nm
due to π–π* and n–π* transitions, respectively, were ob-
served in the UV/Vis spectrum of R1. The visual appear-
ance of 25 μm solutions of R1 upon separate additions of
1 equiv. each of Ni2+, Na+, K+, Ca2+, Ba2+, Al3+, Pb2+
,
Mn2+, Co2+, Cu2+, Zn2+, Cd2+, Hg2+ (as their chloride
salts) is shown in Figure 1.
Figure 1. Color change of solutions of R1 (25 μm) in the presence
of different metal ions (pH 7.5Ϯ0.1; 10 mm HEPES; EtOH/H2O,
3:1).
Figure 3. Change in UV/Vis absorption pattern of R1 (25 μm) as a
function of amount of Ni2+ added (0–50 μm, pH 7.5Ϯ0.1; 10 mm
HEPES; EtOH/H2O, 3:1). The corresponding binding curve at
508 nm is shown in the inset.
It can be seen that, excepting Ni2+ and Cu2+, no other
ion produced any significant visible change in R1. The cor-
responding UV/Vis absorption pattern also supported the (Ϯ0.500, R2 = 0.99452), showing high binding affinity for
observed changes (Figure 2).
Ni2+. The limit of detection (LOD) was determined by ap-
The UV/Vis titrimetric studies were performed by adding plying the IUPAC protocol[14] as 1.42ϫ 10–7 m with a lin-
0.0–2.0 equiv. of Ni2+ concomitantly to the R1 solution in earity range of 1.50ϫ 10–6 to 6.25ϫ 10–6 m (Figure 4a).
aqueous ethanol at physiological pH (25 μm; pH 7.4Ϯ0.1;
The Job’s plot experiment also showed 2:1 stoichiometry
10 mm HEPES; EtOH/H2O, 3:1). The 270 and 370 nm ab- between R1 and Ni2+ (Figure 4b). The same was further
sorption bands of R1 showed bathochromic and hypsoch- confirmed by the ESI-MS spectrum of the R1–Ni2+ com-
romic shifts of 8 and 40 nm, respectively, while a new band plex in solution, which showed a peak at m/z = 915.4 that
at 508 nm developed due to strong ligand-to-metal charge excellently matched the expected mass (Supporting Infor-
transfer (LMCT). The entire family of spectra with three mation, Figure S4).
clear isosbestic points at 418, 348, and 300 nm is shown in
Figure 3.
Finally, the binding mode, stoichiometry, and sites of in-
teraction were revealed by XRD studies of single-crystals of
The binding constant for R1–Ni2+ was determined by the R1–Ni2+ complex. The X-ray structure of the complex
non-linear fitting of the UV/Vis titration data in a 2:1 (H/G, showed a distorted octahedral geometry around Ni2+ (Fig-
Host/Guest) binding equation, yielding β21 = 8.60ϫ 109 m–2 ure 5).
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Coumarin-Based Chromogenic Receptor for Ni2+
Figure 5. ORTEP plot showing single-crystal X-ray diffraction
pattern of the R1–Ni complex (hydrogen atoms are omitted for
clarity).
nitrogen atoms and a double bond between the carbon and
1
nitrogen atoms. This was also supported by H NMR ti-
tration studies between R1 and tetrabutylammonium acet-
ate, which showed intramolecular exchange of a proton be-
tween the endocyclic (thiazole ring) and exocyclic nitrogen
atoms (Supporting Information, Figure S8). Important
bond lengths and bond angles for the complex are given in
Table 1.
Table 1. Selected coordination bond lengths and angles for R1 with
esds in parentheses.
Bond lengths [Å]
Ni(1)–N(1)
Ni(1)–N(3)
Ni(1)–N(4)
Ni(1)–N(6)
Ni(1)–O(2)
2.016(8)
2.062(7)
2.024(7)
2.088(9)
2.128(8)
Ni(1)–O(6)
N(1)–N(2)
N(4)–N(5)
C(12)–N(2)
C(35)–N(5)
2.091(8)
1.363(11)
1.370(11)
1.353(14)
1.309(13)
Bond angles [°]
O(6)–Ni(1)–N(6) 163.7(3)
O(6)–Ni(1)–O(2) 87.9(3)
O(6)–Ni(1)–N(3) 85.6(3)
O(6)–Ni(1)–N(4) 86.1(3)
O(6)–Ni(1)–N(1) 90.1(3)
N(6)–Ni(1)–N(4) 78.8(3)
O(2)–Ni(1)–N(3) 163.4(3)
O(2)–Ni(1)–N(4) 90.8(3)
O(2)–Ni(1)–N(1) 85.0(3)
N(3)–Ni(1)–N(4) 104.0(3)
N(3)–Ni(1)–N(1) 79.7(3)
N(4)–Ni(1)–N(1) 174.4(3)
Figure 4. (a) Absorbance at 508 nm as a function of Ni2+ concen-
tration within the 1.50ϫ 10–6 to 6.25ϫ 10–6 m range using 25 μm
of R1. (b) Job’s plot experiment showing 2:1 stoichiometry between
R1 and NiCl2.
The UV/Vis responses of R1 towards Ni2+ in the pres-
ence of one equivalent of the aforementioned ions were also
studied. The corresponding absorbance profiles observed at
508 nm were not significantly perturbed by the majority of
ions, with the exception of Cu2+, with which the reduction
in intensity was approximately 66% (Figure 6). The same
may be understood in terms of the equilibrium shown in
Equation (1), in which L is the deprotonated form of R1;
the equilibrium involves transmetallation of R1–Ni2+ with
Cu2+ in accordance with Irving–Williams series considera-
tions.[15]
As can be seen, a single nickel ion is surrounded by two
receptor units, with each unit coordinating at three sites. An
ORTEP view of the Ni2+-directed 3D helical architecture
of R1 is shown in Figure 5. A distorted octahedral position-
ing of the donors around Ni2+ can be seen in Figure S6
of the Supporting Information. The XRD data indicated a
common distance of approximately 1.36 Å between N(1)–
N(2) and between N(4)–N(5) in both of the units of R1
coordinated with Ni2+. On the other hand, the C(12)–N(2)
and C(35)–N(5) distances were 1.35 and 1.31 Å, respec-
tively. The data indicated a single bond between the two
(1)
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R. K. Mishra, K. K. Upadhyay
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the water content progressively affected the sensing phe-
nomenon because of precipitation of the receptor. Another
possible reason may be the competing nature of water and
R1 towards Ni2+. On the basis of the single-crystal XRD
data described above, as well as on IR spectroscopic studies
(Supporting Information, Figure S10), the sensing of Ni2+
by R1 may be summarized by the equilibrium shown in
Equation (2) involving complexation between the species.
Figure 6. Spectrophotometric responses of R1 (25 μm) to selected
metal ions (25 μm) at 508 nm. dark gray bars: R1 + Mn+, light gray
bars: R1 + Mn+ + Ni2+
.
(2)
The change in the visible color of a solution of R1 in-
duced by Cu2+ was altogether different to that observed
upon addition of Ni2+ (Figure 2), hence this is not an inter-
ference in the real sense, at least for the qualitative visual
differentiation between Ni2+ and Cu2+
.
UV/Vis titrimetric studies were performed by concomi-
tant addition of Cu2+ to a solution of R1 in aqueous etha-
nol at physiological pH (25 μm; pH 7.5Ϯ0.1; 10 mm
HEPES; EtOH/H2O, 3:1). The entire family of spectra with
three clear isosbestic points at 396, 341, and 300 nm is
shown in Figure 7. Determination of the binding constant
for R1–Cu2+ suffered from a high degree of inaccuracy be-
cause of poor fitting of the corresponding titration data
into the 1:1 binding equation (Supporting Information, Fig-
ure S6). The Job’s plot experiment also indicated the same
[G]/[H] stoichiometry (Figure 7, inset). Because no single
crystal of the corresponding R1–Cu2+ complex could be de-
veloped, binding details could not be obtained.
Figure 7. Change in UV/Vis absorption profile of R1 (25 μm) as a
function of the amount of Cu2+ added (0–50 μm, pH 7.5Ϯ0.1;
10 mm HEPES; EtOH/H2O, 3:1), and a Job’s plot (inset).
Figure 8. (a) Effect of water on the absorption intensity of R1
(25 μm; pH 7.5Ϯ0.1; 10 mm HEPES) at 508 nm in the presence of
Ni2+ (1.0 equiv.); (b) effect of pH (25 μm; EtOH/H2O, 3:1.) on the
absorption at 508 nm. Solid sphere (᭹): only R1, star (*): in the
presence of Ni2+ (1.0 equiv.).
Living systems involve aqueous media, therefore, poten-
tial applications must generally be water soluble. Because
R1 showed selective sensing of Ni2+ in aqueous ethanolic
The HEPES buffer used in the sensing process continu-
medium, a water-tolerance study was also performed (Fig- ally removed H+, thus driving the equilibrium in the for-
ure 8a). As can be seen, the maximum water tolerance limit ward direction i.e., towards complexation.
for R1 was established to be approximately 60% for visual
Because the above studies showed that R1 underwent de-
as well as spectral recognition of Ni2+. Further increases in protonation during its binding with Ni2+, the effect of pH
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Coumarin-Based Chromogenic Receptor for Ni2+
variation was also studied. In the pH range 4.0–7.0 there
was no significant response, whereas higher pH values rang-
ing from 7.0–12.0 favored Ni2+ binding (Figure 8b) due to
deprotonation of R1 in the basic medium.
Because the pH of the solution also affected the binding
of Ni2+ to R1, to explore the possibility of logic gate opera-
tions with the present system the effect of H+ as one of the
inputs was studied. In aqueous ethanolic solution there was
almost no response in the pH range of 4.0–7.0, hence an
INH logic function was exhibited for inputs of H+ and Ni2+
(Figure 9; Supporting Information, Figure S7).
Figure 10. Color and spectral changes on addition of In1 and In2
in DMSO and the corresponding truth table along with the
TRANSFER logic scheme.
Figure 9. The truth table and corresponding logic scheme for INH
in aqueous EtOH (25 μm; EtOH/H2O, 3:1).
addition of Ni2+ helps drive the equilibrium towards the
right, leading to visual/spectral color change. The corre-
sponding translation of the above operations in terms of a
logic gate resulted in an AND logic function (Figure 11;
Supporting Information, Figure S9).
The same molecular device, that is, R1, also exhibited
reconfiguration in terms of its logic gate behavior on chang-
ing of the sensing medium and taking anions as one of the
inputs. In dry dimethyl sulfoxide (DMSO), R1 showed no
absorbance at 575 nm in the absence of any chemical input,
whereas the addition of AcO–/F– (In1) lead to the appear-
ance of a broad band at 575 nm and the color of the solu-
tion turned blue. This result may be understood in terms of
anion-induced deprotonation[16] of R1, which was further
supported by 1H NMR titration between R1 and tetra-
butylammonium acetate (Supporting Information, Fig-
ure S8). On the other hand, the addition of one equivalent
of Ni2+ (In2) to R1 lead no clear visual or spectral changes.
Nevertheless, addition of Ni2+ to R1 already having In1
(AcO–/F–, hence deprotonated R1) led to a clear spectral
change in the form of blue shifting of the 575 nm band to
535 nm accompanied by color change from blue to purple.
These set of spectral changes fit the truth table of the
TRANSFER logic function (Figure 10).
Figure 11. The truth table and corresponding logic scheme in aque-
ous DMSO (90:10, 25 μm).
Conclusions
In aqueous DMSO (water/DMSO, 10:90), no visual/
spectral changes occurred upon the separate addition of
either In1 or In2, whereas the system responded to In2 in
the form of a broad band centered at 508 nm in presence
of In1 (i.e., AcO–/F–). This may be understood in terms of
the equilibria in Equations (3) and (4), where A– = AcO–/
F–.
We have developed a coumarin-based visual receptor for
Ni2+ in aqueous medium that operates at physiological pH.
To the best of our knowledge, this is the first report of any
coumarin-based chromogenic receptor for Ni2+ with a re-
configurable logic gate pattern. The optical responses of R1
fit the truth tables of a number of logic gates, such as INH,
TRANSFER, and AND functions. In dry DMSO, the sys-
tem exhibited TRANSFER logic behavior, which changes
to AND in aqueous DMSO (10:90). Moreover, in aqueous
ethanolic medium, R1 exhibited INH logic patterns at pH
values below 7. Thus, R1 in its various reconfigurations
would be a unique addition to the series of optical receptors
(3)
(4)
The competing nature of water towards A– might delay capable of performing logic gate operations. The single-
the approach of equilibrium 3 and, hence, no visual/spectral crystal XRD studies of the R1–Ni2+ complex proved be-
responses were obtained as mentioned above. In contrast, yond doubt the chemical interaction between them.
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Experimental Section
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Materials and General Methods: All titration experiments were con-
ducted at room temperature. For the UV/Vis titrations, a 1.0ϫ
10–3 m stock solution of R1 was prepared; 2.5ϫ 10–5 m solutions
were subsequently prepared by diluting a suitable aliquot of stock
solution with EtOH/H2O. The metal-binding studies were per-
formed by separate concomitant additions of Ni2+/Cu2+ to R1, giv-
ing sufficient time to develop the color after each addition. The
actual titrations between R1 and Ni2+ were performed in water/
ethanol mixtures (25:75). The maximum water-tolerance limit was
also determined in this way.
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[3]
[4]
R1 was prepared as previously reported[17] and characterized by
1
IR, H and 13C NMR spectroscopic methods, along with mass de-
termination with LC-MS (Supporting Information, Figures S1–
S3). Yield 83%. IR (KBr): ν
= 3429, 3142, 2932, 1720, 1609,
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˜
max
1569, 1450, 1377, 1236, 1139, 1025, 958, 859, 751, 635, 578 cm–1.
1H NMR (400 MHz, [D6]DMSO): δ = 2.30 (s, 3 H, CH3), 7.48–
7.38 (m, 4 H, ArH), 7.67–7.63 (m, 2 H, ArH), 7.89–7.80 (m, 3 H,
ArH), 8.19 (s, 1 H, ArH), 8.60 (s, 1 H, ArH), 11.45 (s, 1 H,
NH) ppm. 13C NMR (100 MHz, [D6]DMSO): δ = 16.1, 111.2,
115.8, 115.9, 118.8, 119.1, 124.7, 126.4, 128.7, 129.1, 131.7, 132.2,
138.1, 140.7, 152.2, 153.2, 158.7, 159.0, 168.7 ppm. MS (ESI): m/z
= 430.2 [C23H15N3O4S = 429.1].
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Synthesis of R1–Ni2+ Complex: A methanolic solution (10 mL) of
Ni(CH3COO)2·4H2O (0.025 g, 0.1 mmol) was added dropwise to a
methanolic solution (5 mL) of R1 (0.043 g, 0.1 mmol), and the
solution was stirred for 4 h. A dark-red precipitate was observed,
which was filtered and dried in vacuo. Rectangular dark colored
crystals of the R1–Ni2+ complex that were suitable for X-ray data
collection were obtained after a few days by slow diffusion of a
CH2Cl2 solution of the complex covered with a layer of petroleum
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ether. Yield (0.080 g, ca. 88%). IR (KBr): νmax = 1719 (C=O), 1656
˜
(C=N), 1600, 1552, 1517, 1442, 1374, 1319, 1226, 1175, 1080, 1029,
960, 849, 756, 609, 460 cm–1. MS (ESI): m/z
= 915.4
(C46H28N6NiO8S2 = 915.5). UV/Vis (DMSO): 300, 333, 535 nm;
UV/Vis (MeOH): 300, 333, 508 nm.
Crystallographic Data for R1–Ni2+ Complex: C46H28N6NiO8S2;
monoclinic; a = 7.261(5) Å, b = 37.762(5) Å, c = 14.736(5) Å, α =
90.000(5)°, β = 102.022(5)°, γ = 90.000(5)°; V = 3952(3) Å3; T =
293(2) K; space group C1c1; Z = 4; OXFORD CCD-based dif-
fractometer; λ = 0.71073 Å (Mo-Kα); θmax = 26.190°. The final R1
values were 0.0574 [IϾ2σ(I)]. The final wR(F2) values were 0.1640
[IϾ2σ(I)]. The goodness of fit on F2 was 1.123. CCDC-787811
contains the supplementary crystallographic data for this paper.
These data can be obtained free of charge from The Cam-
bridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/
data_request/cif.
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Acknowledgments
The authors are thankful to Prof. Thomas M. Fyles, University of
Victoria, Canada, and Prof. A. P. de Silva, Queen’s University of
Belfast, Ireland, for their useful suggestions during the preparation
of this manuscript. R. K. M. is thankful to the Council of Scientific
and Industrial Research (CSIR), New Delhi, India, for financial
support in the form of SRF (NET).
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© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Received: April 12, 2011
Published Online: July 14, 2011
Eur. J. Org. Chem. 2011, 4799–4805
© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
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