A. RoyChowdhury et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 124 (2014) 492–499
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of research in modern age. Of several bio relevant anions fluoride is
Preparation of receptor 1; N,N0-(ethane-1,2-diyl)bis(1H-indole-2-
carboxamide)
one of the most important one and has received much attention
due to its unique properties. It is well established in the scientific
community because of its usefulness in dental care and also in
the clinical treatment of osteoporosis [5–7]. Similar to fluoride,
acetate ion detection is also crucial keeping in mind that it is an
integral part of acetyl coenzyme-A [8]. Several groups of scientists
and researchers are working worldwide towards systematic devel-
opment of suitable organic receptors with several functional
groups especially of the kind; ꢁNH/ꢁSH/ꢁCONH2 for sensing
bio-relevant anions through H-bonding and in presence of several
other weak van der Waal’s interactions. In the last few years urea,
amide and pyrrole based receptors have gained much attention
[9–13]. The use of indoles and urea as a component for construct-
ing neutral receptor for selective anion recognition has been pre-
sented herein [14–17]. A receptor system generally consists of
two parts. One part is considered as the anion binding unit and
the other being the fluorophore part which converts the chemical
process related to binding phenomenon into optical or electro-
chemical signals [18]. Herein we are disclosing two cost effective
and easy to synthesize neutral anion binding receptors e.g.;
A solution of indole-2-carboxylic acid (0.32 g, 2 mM) and car-
bonyldiimidazole (CDI) (0.41 g, 2.55 mM) were taken in a solution
of acetonitrile (20 ml) and refluxed for 2 h. Ethylenediamine
(1 mM, 0.060 g) was added dropwise and the solution was refluxed
for 6 h at 353 K. A curdy white precipitate was appeared which was
filtered after cooling. The filtrate was washed repeatedly with
60 ml ether (3 ꢂ 20 ml fractions). The white powder of receptor 1
was finally collected after drying (Scheme 1).
Yield: (0.69 g, 80%) mp 220 °C.
(ESI-MS) m/z Calcd: for C20H18O2N4: 346 (M+H+), Found: 345
(Fig. S1); Anal. Calcd. for C20H18O2N4: C = 69.36, H = 5.20,
N = 16.18; found: C = 69.41, H = 5.29, N = 16.23; Proton (1H) NMR
(d in ppm)(400 MHz; DMSO-d6; Me4Si) 11.57(s, 2H, ANH indole),
8.651(s, 2H, ANH urea), 7.605(d, 2H, ArAH), 7.424((d, 2H, ArAH),
7.16(t, 2H, ArAH), 7.109(s, 2H, ArAH), 7.027(s, 2H, ArAH),
3.489(t, 4H, CH2): IR (cmꢁ1
) , ,
KBr: 3596 cmꢁ1 3464 cmꢁ1
3418 cmꢁ1, 1615 cmꢁ1, 1541 cmꢁ1, 1398 cmꢁ1, 789 cmꢁ1
.
N,N0-(ethane-1,2-diyl)bis(1H-indole-2-carboxamide) receptor
1
Preparation of receptor 2; 1,3-bis(4-methoxyphenyl)urea
and 1,3-bis(4-methoxyphenyl)urea receptor 2 (vide Chart 1) both
of which have shown quite interesting binding properties with
bio benign anions like fluoride, acetate and dihydrogen phosphate
effectively and selectively.
To a solution of p-anisidine(0.123 g,1 mM) in dichloromethane
solid triphosgene(0.296 g,1 mM) was added under N2. Immediately
a saturated aqueous solution of NaHCO3 (7 ml) was added drop-
wise. Effervescence of carbon dioxide was observed which gradu-
ally diminished. Stirring was continued for 12 h under nitrogen
atmosphere. The crude material thus obtained was collected and
placed in a separating funnel and washed with water several times
to remove the excess NaHCO3 (Scheme 2). The pink colored dichlo-
romethane solution was collected after washing with water and
excess water was removed by passing over anhydrous Na2SO4.
White crystals of receptor2 were obtained by slow evaporation of
the DCM-Hexane solution (Scheme 2).
Experimental
Materials
All starting materials (chemical reagents, solvents) were com-
mercially available and were of analytical grade. Solvents like
dichloromethane, acetonitrile were distilled and dried prior to
use. All tetrabutyl ammonium salt of anions like fluoride (hydrate),
chloride, nitrate, acetate and phosphate monobasic were purchased
from Sigma Aldrich chemical company and used as it was received.
Tetrabutyl ammonium bromide and iodide were procured from Alfa
Aesar chemical company. Solvents like Methanol, DMSO, water
were of spectroscopic grade and purchased from Merck India Pvt.
Ltd. and used without doing any further purification.
Yield: (0.248 g, 85%) mp 292 °C: (ESI-MS) m/z Calcd: 272 for
C15H16O3N2, Found: 273 (Fig. S2); Anal. Calcd. for C15H16O3N2:
C = 66.17, H = 5.88, N = 10.29; found: C = 66.12, H = 5.90,
N = 10.32. Proton (1H) NMR (d in ppm) (400 MHz; DMSO-d6; Me4Si)
8.352(s, 2H, ANH), 7.33(d, 4H, ArAH), 6.85(d, 4H, ArAH), 3.709(s,
6H, AOCH3); IR (cmꢁ1): 3294 cmꢁ1
, , ,
1633 cmꢁ1 1607 cmꢁ1
1565 cmꢁ1, 1506 cmꢁ1, 1169 cmꢁ1, 1030 cmꢁ1, 827 cmꢁ1
.
Results and discussion
Apparatus
Receptor 1 was synthesized in good yield (ꢃ80%) by simple
coupling of commercially available indole-2-carboxylic acid with
ethylenediamine using carbonyl diimidazole (CDI) as an amide-
coupling reagent. Receptor 2 was synthesized, by reaction of
para-anisidine with triphosgene in a mixture of dichloromethane
and saturated aqueous NaHCO3 solution under dry nitrogen atmo-
sphere. Receptor 1, was giving white colored crystalline material
and it was fully characterized by 1H NMR, UV–Vis, ESI-MS and
CHN analysis. X-ray quality single crystals of receptor 2 were ob-
tained from DCM-HXN (slow evaporation). The ORTEP and atom
numbering scheme of receptor 2 was shown in Fig. S3(a) which
authenticates its structure. The intermolecular H-bonding for the
receptor 2 was shown in Fig. S3(b).
The infrared spectra were recorded in a Perkin Elmer FT-IR
Spectrum 100 spectrophotometer. The mass spectra obtained in
Advion’s CMS Expression serial number: 3013-0140 compact mass
spectrometer. 1H NMR was recorded in a Bruker AV-400 spectrom-
eter. UV–Vis spectra were taken in SEC2000, ALS spectrophotome-
ter. Fluorescence was done in Perkin Elmer LS-45
spectrophotometer. Cyclic voltammetric experiments were exe-
cuted in Biologic’s SP-150 series instrument.
X-ray structure determination
[1,3-bis(4-methoxyphenyl)urea], receptor 2 was recrystallized
from DCM (dicholoromethane). X-ray quality single crystal of receptor
2 was obtained from slow evaporation of dichloromethane–hexane
(2:1) solution mixture of receptor 2. ORTEP and atom numbering
Chart 1.