Selective iodide chemosensing through a redox-active Cu-corrole

Abstract

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.

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Exploration and design of new functional materials with novel applications in the field 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), deficiency 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 addition 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 chromatography^5a atomic absorption spectrometry,^5b capillary electrophoresis,^5c chemiluminescence,^5d polarography,^5e voltametry,^5f ion-selective electrodes^5g–i and electrostatic ion chromatography^5j 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 preparation protocols. On the other hand, colorimetric detection through molecular scaffolds has been proposed as a powerful alternative for fast and in situ detection due to the sensitivity and simplicity of the approach. However, designing a chemosensor for selective recognition of iodide is challenging 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 fluorescence quenching response due to heavy atom effect of iodide and size selective 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 fluoride,⁹ we attempted another strategy for selective halide sensing based upon electron deficient 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 deficient Cu(III) centre in β-octabromo-meso-tris(pentafluorophenyl)corrole (B) for anion recognition properties. We could achieve a 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(pentafluorophenyl)corrole (A) obtained through a reported procedure,^13a,b was metallated with copper(II) acetate hydrate under reflux conditions, which on subsequent bromination provides B.^13c The compounds were characterized by ¹⁹F NMR, LRMS-ESI and UV-vis spectroscopy (S1–S5).

Scheme 1 Synthesis of Chemosensor B.

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 shifted to 560–700 nm in dichloromethane (DCM). Split Soret band in A occurs due to the reduced symmetry in the presence of three sterically crowded inner NH protons.¹⁴ Metallation of B with Cu(III) induces planarity and in turn a single intense Soret band. Red shifted (30 nm) Soret band in case of B is attributed to a strong electronic 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 F⁻, Cl⁻, Br⁻, I⁻, SO₄²⁻, NO₃⁻, CH₃COO⁻ and PO₄²⁻ in the form of tetra butyl ammonium (TBA) salts in dichloromethane (DCM). Among which, only I⁻ produced a drastic colorimetric change. This was further monitored through absorption spectroscopy. Here addition of 0–4 equivalents of I⁻ resulted in significant changes of absorption band at 424 nm, while rest of the anions interacted weakly, even with ten-fold additions to that of iodide (Fig. 1 and 2 and ESI† Fig. 6).

Fig. 1 Changes in the absorption spectra of compound B (1.7 μM) upon addition of 0–4 equivalents of iodide in DCM. Inset showing the changes in short wavelength region of B as a result of iodine.

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 μM) of B upon addition of 4 equivalents of I⁻ (0.6 mM) and 32 equivalents of F⁻ (0.6 mM) up to saturation level. Rest of the anions have been added up to 10-fold concentrations with respect to iodide.

An initial red shift of 424 nm to 441 nm was observed followed 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 F⁻, a further 5 nm red shift was observed in higher concentrations (Fig. 8 in ESI†), while other anions remained silent with respect to any notable shift or change in absorption profile even at higher concentrations. The changes are probably an outcome of electrostatic interaction of F⁻ with copper centre. Importantly, addition of iodide to B induced a noticeable difference via an increase in the intensity of absorption around 290 nm and 360 nm (inset Fig. 1) region, whereas with F⁻, a decrease in the intensity was observed (Fig. 8 in ESI†). With the help of calibration curve, detection limit of 7 μM was calculated for iodide (Fig. 10 in ESI†).

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, modulation amplitude 19 G and Center field 3400 G. (b) Cyclic voltammogram of B in DCM at 25 °C, 0.1 M TBAP as supporting electrolyte, scan rate = 0.1 V s⁻¹.

Fig. 5 Proposed mechanism of iodide interaction and visible colorimetric changes.

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 fluorine peaks along with slight upfield shifted 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 paramagnetic contribution from Cu(II)-corrole π-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 = 2.035 and with four hyperfine splitting due to ⁶³Cu (a_cu = 230 G). The nine superhyperfine splitting due to four ¹⁴N atom of reduced Cu(II)-corrole anion was observed, with a_N = 17 G. The given observations were quite similar with that observed by Kadish and coworkers¹¹ by electrochemical reduction for [OEC(Cu)]⁻, where g = 2.092 G and a_cu = 227 G. The current observations match with the measurements reported by Gross et al.¹² Here, chemical reduction by sodium hydrosulfite results in the existence of [Cu(tdcc)]⁻ species with g = 2.033 and a_cu = 207.12 G.

Hence the given peculiar selectivity of B correlates to one-electron oxidation of iodide by highly electron deficient Cu(III) centre (Fig. 5), via a strong electron-withdrawing trigger facilitated by bromine atoms. Enhanced absorbance band at 290 nm and 350–370 nm in B (Fig. 1 inset) in presence of I⁻ indicates formation of I₂ in the solution under consideration.¹⁶ Importantly, the given color change is irreversible in nature even with addition to excess equivalent of water, which exclude possibilities of any weak electrostatic interaction.

Further, cyclic voltammogram (CV) of B shows first metal centre reduction from Cu(III) to Cu(II) at half wave potential, E_1/2 = 0.57 V (Fig. 4b).^11,17 To further ascertain our experimentally observed selectivity towards iodide, CV was conducted for I⁻ and Br⁻ under the same set of experimental conditions. An anodic oxidation peak at E_1/2 = 0.2 V and a quasi reversible peak at half wave potential, E_1/2 = 0.58 V was observed for I⁻ (ESI Fig. 12†). This attributes to two step oxidation processes in I⁻/I₂ redox couple.¹⁸ Similar two step oxidation process was also observed in the case of Br⁻/Br₂ at 0.95 V and 1.25 V (ESI Fig. 13†).¹⁹ Redox potential values of Cu(III)/Cu(II) is sufficient to oxidize iodide to iodine, but is less positive than bromide/bromine couple.

In summary, we have presented a new and promising approach for selective sensing of iodide anion. A typical Cu(III)-corrole based molecular scaffold possessing a strong electron-withdrawing environment of bromine atoms around Cu-core seems a probable basis for the given selectivity. This triggers the single-electron redox couple in the medium under investigation. We hope the proposed strategy expose new doors in chemosensor designs for discrimination of iodide and other anionic species by fine tuning of its redox potential.

Acknowledgements

Authors acknowledge DST, New Delhi for financial support and UGC, New Delhi (BB, RM), CSIR, New Delhi (MM) for research fellowships. We thank IISER Bhopal for the infrastructure.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Synthesis, NMR, LR-GCMS and absorption spectra. See DOI: 10.1039/c4ra04191g

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