Electrochemical Studies for Cation Recognition with Diazo-Coupled Calix[4]arenes

The electrochemical properties of diazophenylcalix[4]arenes bearing ortho-carboxyl group (o-CAC) and ortho-ester group (o-EAC), respectively, in the presence of various metal ions were investigated by voltammetry in CH3CN. o-CAC and o-EAC showed voltammetric changes toward divalent metal ions and no significant changes with monovalent alkali metal ions. However, o-CAC preferentially binds with alkaline earth and transition metal ions, whereas no significant changes in voltammetric signals are observed in o-EAC with alkaline earth metal ions. o-EAC only binds with other transition metal ions. This can be explained on metal ion complexation-induced release of proton from the azophenol to the quinone-hydrazone tautomer followed by internal complexation of the metal ion with aid of nitrogen atoms and ortho-carbonyl groups in the diazophenylazocalix[4]arenes.


Introduction
Macrocyclic receptors have been synthesized and received much attention to be endowed with superior molecular recognition properties used in chemical sensors [1,2]. Calixarenes have been extensively used as macrocyclic hosts for a wide range of metal ions because they have unique structure of conformational adaptability toward hosting guests along with easy derivatization of the lower rim and upper rim and the possibility of locking a desired conformation (cone, partial cone, 1,2-alternate, and 1,3-alternate) [1,3,4]. Many have been utilized as optical sensors to monitor the target by UV/Vis and fluorescence spectroscopic measurements. Among them, azocalixarene bearing azophenol units acting as a chromogenic center are particularly attractive for their interesting aspects of complexation with alkali, alkaline earth, and transition metal ions disclosed by studying the optical behavior of chromophoric units [4][5][6][7][8]. Our group has been also interested in designing selective optical sensors toward metal ions based on azocalixarene derivatives [9,10]. As one of our efforts, we have developed a simple qualitative analysis protocol to screen alkali, alkaline earth, and transition metal ions using azocalix [4]arene derivatives. To tune up the selectivity toward specific metal ions, we have also tried to perform the pH study of azocalix [4]arene bearing carboxyl group and to change the position or the numbers of substituents, which leads to enhancement of the selectivity for Pb 2+ , Cu 2+ , or Ca 2+ metal ions in the spectroscopic measurements.
Azophenols as electroactive groups in azocalixarene system can be studied by also electrochemical measurements [11][12][13]. Electrochemical sensors are quite interesting and useful because electrochemical changes such as current and voltage by recognition of guest molecule can be directly and immediately reported to electrical signal, which is not necessary to use other transducers. Thus, electrochemical techniques are suitable for the development of convenient, sensitive, selective, and low cost devices that could be utilized for a rapid monitoring, ultimately applicable to hand-held operation. However, only a few examples of electrochemical measurements using azocalix [4]arenes have been reported [10,[14][15][16]. Previously, our group reported colorimetric discrimination system towards alkali, alkaline earth, and transition metal ions using azophenylcalix [4]arenes bearing ortho-carboxyl group (o-CAC) and ortho-ester group (o-EAC) (Scheme 1) [9]. To the best of my knowledge, however,

Results and Discussion
The to two -NH 2 during the electrochemical reduction process, cyclic voltammogram (CV) will give irreversible redox peaks involving four electrons all together [17]. On the other hand, the oxidation of phenol in CH 3 CN shows only one oxidation peak, as shown in Figure 1(a). Thus, more attention has been paid to the oxidation wave rather than reduction waves of o-CAC or o-EAC. Electrochemical properties of o-CAC or o-EAC were also investigated by voltammetry at glassy carbon electrode in 0.1 M TBAPF 6 /CH 3 CN, by taking advantage of the phenol moieties present at the lower rim. Electrochemistry of o-CAC or o-EAC based on the oxidation of phenols is different from that of phenol (Figure 1(a)). Phenol exhibits simple redox behavior so that an irreversible electron transfer process is observed as one oxidation peak around 1.2 V and calix [4]arene with only phenol functional groups also shows one irreversible oxidation peak [18]. Based on the electrochemistry of phenol, o-CAC or o-EAC is also expected to be oxidized around 1.2 V. Differently from our prediction, o-CAC or o-EAC shows two irreversible oxidation waves (Figure 1(a)). This result can be proved by the fact that o-CAC or o-EAC presents a mixture of the two tautomeric forms, namely, azophenol and quinone-hydrazone, as explained in previous papers by spectroscopic data of o-CAC or o-EAC [9,10]. Phenol groups of calixarene with only phenol group are identical because it forms intramolecular hydrogen bonding array in the lower rim of calixarene. But in this case, intramolecular hydrogen bonding array distorts and even breaks by the tautomerization of o-CAC or o-EAC. Therefore, o-CAC or o-EAC shows two irreversible peaks at intervals. One peak is due to the ease of protonation of phenol and the other peak is due to the difficulty of oxidation of phenol by intramolecular hydrogen bonding. In order to get a better resolution, differential pulse voltammetry of o-CAC or o-EAC was also performed in the same condition of cyclic voltammetry. As shown in Figure 1(b), there are two irreversible waves and prewave in the oxidation process of both hosts, which is thought to be due to the tautomerization. Anodic potential of o-CAC and o-EAC is summarized in Table 1  Successive addition of an alkali metal ion to o-CAC or o-EAC, which is incapable of encapsulation, caused no significant change in the peak current or potential in accordance with previous spectroscopic data [9]. Very similar results are observed with o-EAC-alkaline earth metal complexation (Figure 3(b)). On the other hand, o-CAC in the presence of alkaline earth metal ions shows changes in peak currents and peak potentials (Figure 3(a)).
Preoxidation wave around 0.5 V disappears and first oxidation peak shifts at more negative potential (Figure 3(a)). The presence of transition metal ions also alters both the oxidation peak potentials and currents of o-CAC or o-EAC, as in the voltammetric behavior of o-CAC-alkaline earth cation complexation (Figure 4). This result indicates that there is a subtle balance between metal complexation-induced release of proton from the azophenol to the quinone-hydrazone tautomer, and the ortho-carboxyl groups of o-CAC or ortho-ester groups of o-EAC can stabilize the quinone-hydrazone form in azocalix [4]arene after adding alkaline earth or transition metal ions. The in Table 2 This result is in good agreement with previous spectroscopic experiments [9]. In order to confirm this electrochemical recognition phenomenon, the electroactivities of o-CAC or o-EAC were measured in the presence of increasing substoichiometric amount of Sr 2+ , Zn 2+ , or Cr 2+ cations, as representative models of alkaline earth or transition metal ions. The behavior observed in the oxidation process is shown in Figure 5. decreases. This implies the metal complexation-induced release of protons from the azophenol to the quinonehydrazone tautomer, but the tautomerization to the quinonehydrazone becomes somewhat faster in o-EAC and slower in o-CAC, as the concentration of metal ions increases. From this, the possible complexation mechanism based on the 1 : 1 complexation is proposed in Scheme 2.

Conclusion
The electrochemical behaviors of azocalix [4]arene derivatives containing ortho-carboxyl or diethyl ester groups have been investigated in the absence and the presence of alkali, alkaline earth, and transition metal ions by electrochemical measurements. o-CAC with the ortho-carboxyl groups preferentially binds with alkaline earth and transition metal ions over alkali metal ions, whereas o-EAC with the orthodiethyl ester groups shows selective complexation properties toward transition metal ions over alkali and alkaline earth metal ions. The complexation of metal ions gives rise to negative shifts of first oxidation peaks in the electrochemistry of the azocalix [4]arenes. This may be attributed to the metal complexation-induced release of protons from the azophenol to the quinone-hydrazone tautomer. With this system, one can screen metal ions to alkali, alkaline earth, and transition metal ions using simple electrochemical methods.