Preliminary Note

A strong interest in the synthesis of redox-active calixarenes has centered on the appropriate functionalization of phenol units since the possibility of calixarenes as enzyme mimics was suggested[1]. In recent years, the conversion of phenolic rings to quinone and nitroaromatic moities has been reported[2,3]. Especially, calix[4]arenequinone compounds have attracted a keen attention because of their selectivity toward certain cations and electrochemical activity[4-6]. The study on the effect of electron transfer in spatially constrained systems with multiple redox units of quinone could lead calix[4]arenequinone as one of molecular devices in redox switching technique[6].
Calix[4]arenediquinone used in this study, as shown in Figure 1, contains two quinone groups faced each other as ring members and some substituents such as R = -NHC₄H₉ (1) or -OEt (2) in the lower rim. The cyclic voltammogram of compound 1 shows three peaks of which first two are reversible processes of one electron transfer each at around -920 mV and -1,005 mV vs Ag/Ag⁺ reference electrode, respectively, as illustrated in Figure 2(a). The third peak which appears at -1,589 mV is a well-separated quasi-reversible wave of two electrons involved in the charge transfer reaction. The number of electron corresponding to each wave is confirmed by coulometry. These voltammetric results agree well with the results of Gomez-Kaifer et al. who studied with ethoxy substituted calix[4]arenequinones[6].

The addition of 1 equivalent of NH₄⁺ to free ligand produces a new reduction peak at more positive potential than the first peak of free ligand, as displayed in Figure 2(b). The difference in peak potential between the complex and free ligand is surprisingly as large as +274 mV for compound 1 and +451 mV for compound 2. The new peak is a totally irreversible wave of one electron uptake confirmed by coulometry. According to the proposed structure of 1:1 ammonium complex shown in Figure 1, the first electron transfer is considered to be enhanced by the formation of strong hydrogen bondings between radical anion and ammonium ion as well as prior protonation effect[7,8]. When 2 equivalents of NH₄⁺ are added, all peaks due to free ligand vanish while only the new peak remains. Meanwhile, the height of the new peak is larger than that of the 1:1 complex even though there is not much noticeable change in its potential. Controlled potential coulometry at -1,500 mV demonstrates that a two-electron transfer reaction is involved in the presence of 2 equivalents of NH₄⁺. This is because the reduction from anion radical to dianion is supposed to become more difficult than that of free ligand by stabilization of the anion radical form due to NH₄⁺.

In the case of Na⁺ and K⁺, all peaks due to free ligand disappear in the presence of 1 equivalent of cation and those due to complex remain reversible as shown in Figure 3, which is quite different from the case of NH₄⁺. Further addition of cation doesn't make any change. As a result, Na⁺ and K⁺ are considered to form 1:1 complexes with compound 1 and 2. Na⁺ is also found to cause potential shift to the positive direction by +120 mV and +310 mV resulting from the formation of complex with compound 1 and 2, respectively. Potassium complex with compound 1 doesn't exhibit any potential shift while the complex with compound 2 exhibits a fairly positive shift of +204 mV.

The positive potential shift means that K₂ is larger than K₁ where K₁ and K₂ stands for the association constant of certain cation with neutral and reduced ligand, respectively. If all redox processes of free ligand and complex are reversible, a simple relationship can be established between E_p and K₂/K₁.

where, is the half-wave potential for the ith wave.

In our experiment, the enhancement factor of Na⁺ defined as K₂/K₁ is evaluated to be 1.1x10⁴ for compound 1 and 3.1x10¹⁰ for compound 2. The K₂/K₁ value of K⁺ is 8.0x10⁶ for compound 2, but there is no enhancement for compound 1. The above equation is not applicable for NH₄⁺ because the redox behavior of NH₄⁺ complex is irreversible.

The characteristic potential shift due to sequestering a certain cation implies that compound 1 and 2 are applicable to the selective and quantitative analysis of various cations. And also a large enhancement factor indicates the possibility of using these compounds as electrochemical switching device. Details of the electrochemical behavior of compound 1 and 2 and their complexes are under investigation.

8. T. G. Edward and J. Grinter, Trans. Faraday. Soc., 64 (1968) 1070.