Two tetralkylated phenylenediamines (TAPD)
Metallic ions are widely present in the biological systems and play many important roles [
In living organisms, biological receptors like those found in enzymes or neurons bind selectively to a cation to achieve some goals most of the times in reversible processes. Contrarily, most of the synthetic receptors do not work that way; they tightly bind to cations in nonreversible processes [
In the last two decades, electroactive chemosensors have been built by using reversible systems. The detection or release of guest ions is monitored through the modification of the electrochemical features of redox centers. For instance, Pearson and Hwang were pioneers in the field and reported many TAPD-based chemosensors formed by crown ethers as ionophore sites and phenylenediamine moiety as a transducer capable of detection of alkaline or alkaline earth cations [
The aim of this work is to study the solvent effects on the electrochemical behavior of
Structures of the TAPD-based chemosensors
4-Dimethylaminoaniline (97%), sodium cyanoborohydride (95%), sodium bicarbonate (≥95%), glacial acetic acid (≥99%), cadmium perchlorate hydrate (Cd(ClO4)2·
Proton and carbon 13 NMR spectra were recorded on a Bruker Avance 300 MHz apparatus in CDCl3. Chemical shifts are given in ppm relative to TMS as internal reference. IR spectra were measured on a Perkin Elmer ATR Alpha spectrophotometer. The electrochemical experiments were conducted in the corresponding solvent containing 0.1 M TBABF4 as supporting electrolyte, at ambient temperature and at a scan rate of 0.1 Vs−1. A Radiometer Analytical POL-150 with MED-150 stand-controlled three-electrode glass cell fitted with a carbon glassy disk as a working electrode (3 mm-diameters), a platinum wire as a counter electrode, and an SCE (3 M KCl) electrode as a reference electrode was used for the electrochemical measurements. Data acquisition was performed with TraceMaster 5 software for CV experiments and treated using Kaleidagraph 4.0 software package. The working electrode was polished using 0.3
The compounds were prepared according to published procedure [
Synthetic route to TAPD derivatives via reductive alkylation reaction.
The oily brown residue was purified on silica gel eluted by 30 : 70 ethyl acetate/cyclohexane leading to solid product which was characterized by 1H, 13C NMR, and infrared spectroscopy. The characteristics are summarized below.
CVs of
CVs showing the first and second oxidation stages of
When the potential reverse sweep is poised to approximately 0.5 V, the first monoelectronic peak is reversible (Figure
Thermodynamics data for the first and second peaks: anodic
Compound | 1st peak of oxidation | 2nd peak of oxidation | ||||||||
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ACN |
|
358 | 259 | 99 | 308 | 1.03 | 912 | 765 | 147 | 838 |
|
355 | 263 | 92 | 309 | 1.04 | 942 | 775 | 167 | 858 | |
PC |
|
315 | 218 | 97 | 266 | 0.98 | 882 | 700 | 182 | 791 |
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327 | 216 | 111 | 271 | 0.99 | 918 | 735 | 183 | 826 | |
NM |
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330 | 233 | 97 | 281 | 1.02 | 947 | 794 | 153 | 870 |
|
306 | 204 | 102 | 255 | 1.00 | 922 | 753 | 169 | 837 | |
DMF |
|
365 | 267 | 98 | 316 | 1.04 | 970 | 676 | 294 | 823 |
|
370 | 267 | 103 | 318 | 0.99 | 965 | 706 | 259 | 835 |
(b)
(c)
CVs of compounds with sweep poised at 0.5 V for
Additionally, one can see in Table
The study of the scan rate effect carried out on the first oxidation peak shows that the current intensity increases with the scan rate in the range of 0.05–0.5 Vs−1. The linear plot of the logarithm of the first peak current as a function of logarithm of scan rate is close to 0.5 indicating a diffusion-controlled electron reaction [
CVs recorded in presence of 0.5 equivalent of Cd(II) show that chemosensors are capable of binding to this ion except in DMF. Indeed, it is noticeable on the curves of Figures
(a) CV curves of 2.0 mM of chemosensor
Additionally, temporal monitoring of the Cd(II) chelation reaction of the chemosensors revealed that the reaction is slow leading to complete disappearance of the chemosensors’ signals after 45–60 min even if 1.0 equivalent of Cd(II) is added. When 4 to 10% molar TfOH is added, this cation chelation is accelerated and a purple coloration of the solution is noticed. On the other hand, one can observe that the addition of small amounts of TfOH does not affect the electrochemical features of the chemosensors (figure not shown); since TfOH is a weak acid in organic solvents (
Conformational changes of the chemosensors by protonation followed by proton-cation exchange reaction.
During the reverse scan, two cathodic peaks are visible in ACN, NM, or PC corresponding to the two-stage reduction of dication peaks. Indeed, due to the positive charges on compounds
When the chelation of Cd(II) by the chemosensors is attempted in DMF (Figure
CV of 2.0 mM of chemosensor
Depending on the solvent and the chemosensor, potentials of the first oxidation peak of ligands
Potentials of first oxidation peak,
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ACN | ||||
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353 | 1247 | −894 | 14.9 |
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312 | 118 | −864 | 14.4 |
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NM | ||||
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330 | 1218 | −888 | 14.8 |
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306 | 1094 | −788 | 13.1 |
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PC | ||||
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315 | 1188 | −873 | 14.6 |
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327 | 884 | −502 | 8.4 |
(b)
(c)
Since the
Oxidation of the complexes [Cd(
Beer and coworkers described the reaction coupling efficiency (RCE) as the ratio of
Cyclic voltammetry of two new chemosensors investigated in ACN, NM, PC, and DMF showed that these compounds exhibit two oxidation waves corresponding to a 2-electron transfer from the phenylenediamine moiety. It was also demonstrated using CV that these compounds constituted good ligands for cadmium(II) in solution and the affinity toward this metal depends strongly of the nature of solvent used, and complexes were formed in 1 : 2 stoichiometry. Moreover, it was shown that this complexation reaction starts after 45 to 60 min under open circuit conditions due to electrogenerated protons but it can be tremendously accelerated by addition of catalytic amount of TfOH. RCE values showed that ligand binds tightly to the cation except in DMF and the complexes are more stable with ligand
The authors declare that there is no conflict of interests regarding the publication of this paper.
The authors gratefully acknowledge the MHESR for the financial support through LR99ES15 program and the African Network of Electroanalytical Chemists ANEC/ISP for the strengthening south to south scientific collaboration. Issa Tapsoba would like to acknowledge TWAS, the Academy of Sciences for the Developing World, for the support provided by TWAS Visiting Scientist Program.