Selective iodide chemosensing through a redox-active Cu-corrole

Article abstract of DOI:10.1039/c4ra04191g

An electron deficient Cu(iii)-corrole (B) has been identified as a highly selective colorimetric sensor for iodide. A one-electron redox couple between iodide and Cu(iii)-corrole serves as the basis for selectivity. The proposed mechanism has been supported by UV-vis and ¹⁹F NMR studies and further by unambiguous identification of Cu(ii) species through EPR.

Full text of DOI:10.1039/c4ra04191g

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Selective iodide chemosensing through a redox-
active Cu-corrole†
Cite this: RSC Adv., 2014, 4, 28417
Biju Basumatary, Masood Ayoub Kaloo, Vineet Kumar Singh, Ruchika Mishra,
*
Muthuchamy Murugavel and Jeyaraman Sankar
Received 6th May 2014
Accepted 16th June 2014
DOI: 10.1039/c4ra04191g
www.rsc.org/advances
An electron deficient Cu(III)-corrole (B) has been identified as a highly preparation protocols. On the other hand, colorimetric detec-
selective colorimetric sensor for iodide. A one-electron redox couple tion through molecular scaffolds has been proposed as a
between iodide and Cu(III)-corrole serves as the basis for selectivity. powerful alternative for fast and in situ detection due to the
The proposed mechanism has been supported by UV-vis and 19F NMR sensitivity and simplicity of the approach. However, designing a
studies and further by unambiguous identification of Cu(II) species chemosensor for selective recognition of iodide is challenging
through EPR.
task owing to its low charge density, large ionic radius and weak
hydrogen bonding abilities.6
To date, most of the chemosensor approaches for selective
iodide sensing are mainly dominated by uorescence quench-
ing response due to heavy atom effect of iodide and size selec-
tive discrimination of iodide among other anions. These
fabrications are based upon amide/urea, bis-imidazolium and
benzimidazole derivatives, quantum-dot-based probes and
conjugated polymers.7 Very rare reports exist wherein, iodide
recognition has been achieved through visual-colorimetric
means.8
With immense interest and motivation from our previous
work regarding new anion-sensing approach for uoride,9 we
attempted another strategy for selective halide sensing based
upon electron decient and redox-active Cu(III)-corrole. Owing
to a direct pyrrole–pyrrole linkage in corroles, unique spectral
properties along with ability to stabilize higher oxidation states
of metals,10a they act as materials of choice for perceiving ion
sensing through molecular tools.10b–d Further, an interesting
metal center reduction observed in Cu(III)-corrole by Kadish and
coworkers11 through electrochemical means, followed by Gross
et al.12 with Na2S2O4, prompted us to exploit the electron de-
cient Cu(III) centre in b-octabromo-meso-tris(pentauorophenyl)
corrole (B) for anion recognition properties. We could achieve a
Exploration and design of new functional materials with novel
applications in the eld of anion recognition has attracted
considerable attention over the past few decades.1 This is
certainly owing to the decisive role of anions in the myriad of
processes involved in biology, chemistry, industry and the
environment.2
Among various anions, iodide subsists as an indispensable
entity due to its key role in various physiological activities such
as brain function and cell metabolism. It is also an essential
micronutrient for normal human growth and plays vital role in
proper functioning of thyroid.3 According to world health
organization (WHO), deciency of iodide exist as a major cause
for mental retardation. Further, iodine is an essential element
in sea water in the form of iodide and iodate species. In addi-
tion to this, environmental iodine is used for synthesizing many
organic chemicals, manufacture of certain dyes and medicine.4
Owing to its fundamental roles in day-to-day life and crucial
physiological relevance, various methods in the form of gas
chromatography5a atomic absorption spectrometry,5b capillary
electrophoresis,5c chemiluminescence,5d polarography,5e volta-
metry,5f ion-selective electrodes5g–i and electrostatic ion chro-
matography5j are proposed and in operation for iodide/iodine
analysis. However, most of these techniques are time
consuming and require sophisticated instrumentation,
demand expertise requirements along with complicated sample
Department of Chemistry, Indian Institute of Science Education and Research Bhopal,
Indore bypass road, Bhopal, India-462030. E-mail: sankar@iiserb.ac.in; Fax: +91-755-
6692392; Tel: +91-755-6692-334
† Electronic supplementary information (ESI) available: Synthesis, NMR,
LR-GCMS and absorption spectra. See DOI: 10.1039/c4ra04191g
Scheme 1 Synthesis of Chemosensor B.
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highly selective iodide sensing through colorimetric signals
visible to naked-eyes. To the best of our knowledge this presents
a lone example wherein selective iodide recognition has been
achieved through a one-electron redox couple between iodide
and Cu(III)-corrole.
The corrole B was obtained in a two-step process (Scheme 1).
5,10,15-Tris(pentauorophenyl)corrole (A) obtained through a
reported procedure,13a,b was metallated with copper(II) acetate
hydrate under reux conditions, which on subsequent bromi-
nation provides B.13c The compounds were characterized by 19F
NMR, LRMS-ESI and UV-vis spectroscopy (S1–S5).
Compound A shows a split Soret band at 407 nm and broad
Q-type band in 520–650 nm, whereas B displays a single Soret
band at 438 nm and broad Q-type band shied to 560–700 nm
in dichloromethane (DCM). Split Soret band in A occurs due to
the reduced symmetry in the presence of three sterically crow-
ded inner NH protons.14 Metallation of B with Cu(III) induces
planarity and in turn a single intense Soret band. Red shied
(30 nm) Soret band in case of B is attributed to a strong elec-
tronic pull of eight peripheral beta-pyrrolic bromine atoms.
To test the ability of B towards anion sensing, experiment
preceded with addition of selective anions such as Fꢀ, Clꢀ, Brꢀ,
Iꢀ, SO42ꢀ, NO3ꢀ, CH3COOꢀ and PO42ꢀ in the form of tetra butyl
ammonium (TBA) salts in dichloromethane (DCM). Among
which, only Iꢀ produced a drastic colorimetric change. This was
further monitored through absorption spectroscopy. Here
addition of 0–4 equivalents of Iꢀ resulted in signicant changes
of absorption band at 424 nm, while rest of the anions inter-
acted weakly, even with ten-fold additions to that of iodide
(Fig. 1 and 2 and ESI† Fig. 6).
Fig. 2 Bar graph presenting high selectivity of B for iodide in presence
of various common anions in DCM. Observations were made with
(1.7 mM) of B upon addition of 4 equivalents of Iꢀ (0.6 mM) and 32
equivalents of Fꢀ (0.6 mM) up to saturation level. Rest of the anions
have been added up to 10-fold concentrations with respect to iodide.
induced a noticeable difference via an increase in the intensity
of absorption around 290 nm and 360 nm (inset Fig. 1) region,
whereas with Fꢀ, a decrease in the intensity was observed (Fig. 8
in ESI†). With the help of calibration curve, detection limit of
7 mM was calculated for iodide (Fig. 10 in ESI†).
In order to unravel the interaction, 19F NMR titration of B in
CDCl3 was carried out with TBAI in DCM. Addition of iodide to
B shows an initial broadening of uorine peaks along with
An initial red shi of 424 nm to 441 nm was observed fol-
lowed by a continual enhancement in the intensity upon further
additions, while enhancement of 596 nm was directly observed
as a function of anion concentrations. The given changes were
visibly marked from yellow-brown to green (Fig. 5) color of the
solution. Notably, in the case of Fꢀ, a further 5 nm red shi was
observed in higher concentrations (Fig. 8 in ESI†), while other
anions remained silent with respect to any notable shi or
change in absorption prole even at higher concentrations. The
changes are probably an outcome of electrostatic interaction
of Fꢀ with copper centre. Importantly, addition of iodide to B
Fig. 3 19F NMR spectral changes of B (1.2 mM) in CDCl3 upon addition
of various equivalents of TBAI in DCM at 25 ꢁC. Ortho, meta and para
fluoride peaks are assigned.
Fig. 4 (a) EPR spectra of B in DCM at 120 K with addition of iodide
(10 eq.). Instrument setting: modulation frequency 100 kHz, modula-
Fig. 1 Changes in the absorption spectra of compound B (1.7 mM) tion amplitude 19 G and Center field 3400 G. (b) Cyclic voltammogram
upon addition of 0–4 equivalents of iodide in DCM. Inset showing the of B in DCM at 25 ꢁC, 0.1 M TBAP as supporting electrolyte, scan rate ¼
changes in short wavelength region of B as a result of iodine.
0.1 V sꢀ1
.
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seems a probable basis for the given selectivity. This triggers the
single-electron redox couple in the medium under investiga-
tion. We hope the proposed strategy expose new doors in che-
mosensor designs for discrimination of iodide and other
anionic species by ne tuning of its redox potential.
Acknowledgements
Fig. 5 Proposed mechanism of iodide interaction and visible colori-
metric changes.
Authors acknowledge DST, New Delhi for nancial support and
UGC, New Delhi (BB, RM), CSIR, New Delhi (MM) for research
fellowships. We thank IISER Bhopal for the infrastructure.
slight upeld shied para-F and meta-F peaks (Fig. 3). Later
attributes to the formation of a paramagnetic species in the
system. The absence of a characteristic absorption at very short
UV and long wavelength region ruled out any possible para-
magnetic contribution from Cu(II)-corrole p-cation radical.15
To understand the existence of paramagnetic species and
explore the given mechanism of interaction, EPR experiments
were carried out. Spectra of B (Fig. 4a) in presence of excess
iodide at 120 K, indicates the formation of Cu(II)-corrole anion.
The signal is centered at g ¼ 2.035 and with four hyperne
splitting due to 63Cu (acu ¼ 230 G). The nine superhyperne
splitting due to four 14N atom of reduced Cu(II)-corrole anion
was observed, with aN ¼ 17 G. The given observations were quite
similar with that observed by Kadish and coworkers11 by elec-
trochemical reduction for [OEC(Cu)]ꢀ, where g ¼ 2.092 G and
acu ¼ 227 G. The current observations match with the
measurements reported by Gross et al.12 Here, chemical
reduction by sodium hydrosulte results in the existence of
[Cu(tdcc)]ꢀ species with g ¼ 2.033 and acu ¼ 207.12 G.
Notes and references
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observed in the case of Brꢀ/Br2 at 0.95 V and 1.25 V (ESI
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