TETRAC

A solution of 438 mg of diac in methanol (20 ml) and ammonia solution (SG 0.88 20 ml) was iodinated at 0°C with 1.8 ml 1 N iodine solution. The product was isolated in almost theoretical yield in a manner similar to that described for tetrac. After crystallization from 50% (v/v) methanol, triac was obtained as colorless needles which melted over the range 65°C to 90°C according to the rate of heating. The molten form resolidified at about 110°C and finally melted at 180 C to 181°C without decomposition. The compound, dried at 25°C/3 mm over silica gel, contains methanol of crystallization. 

AOX, adsorbable organic compounds COD, chemical oxygen demand TCDD, 2,3,7,8-tetrac hlorodibenzo-p-dioxin TCDF, 2,3,7,8-tetrachlorodibenzofuran. 

XH NMR was as fast as 300000 times a second. It is interesting if the shuttling speed can be controlled by light or an electrochemical method [96]. Bissell et al. [97] obtained the molecular shuttle shown in Fig. 29, in which benzidine and bisphenol units act as the stations. At 229 K the tetracation bead was found to stay on the benzidine side at a probability of 86%, but when the compound was treated with an acid or oxidized electrochemically it turned out that the bead can move to the bisphenol side at a higher probability. 

Electrochemically, the dinuclear complex [LMnIII-0-MnIVL]3+ used in the above reaction can be oxidized at -35°C to the corresponding tetracation. This cation has been shown to contain a coordinated phenoxyl (188). The ligand L is the monoanionic form of /V,/V-bis(2-pyridylmethyl)-/V salicyliden-l,2-diaminoethane. 

Tetracation Salts as Novel Cyanine-Cyanine Hybrid System... 

The First Tetracation Utilized in a Cyanine-Cyanine Hybrid... 

For the central violene rc-system, we chose benzene with acetylene spacers for decreasing steric hindrance between the large cationic centers. Therefore, the tetracation 254 salt was designed as a first example for a hybrid with a cyanine unit at the both termini (27). As illustrated in Figure 22, tetracation 254 exemplifies a new hybrid system, which produces another cyanine substructure 252 by two-electron transfer. In this case, overall, four electron transfer should afford a fully neutralized species such as a neutral diradical. 

Carbocations having more than four cationic centers are rare species, and very few have been isolated as isolable salts. A tetrahedrally-arrayed tetracation, which was stable only at low temperature was generated by G. A. Olah et al. in 1995 (22). Recently, R. Rathore et al. prepared isolable tetra- and hexatrityl cations utilizing tetraphenylmethane and hexaphenylbenzene as platforms (25). Tetracation 254 is one of the new multi-charged methylium compounds with considerable high stability. 

In the case of tetracation 254+ four cationic units were neutralized at pH 8.7. This value corresponds to an average between the pA R4+ value of the tetracation and p/CR+ value of the partially neutralized monocation. The p/CR+ value of dication 262 was determined to be 10.8. The pKR+ value of monocation 27+ (12.3) is almost equal to that of di(l-azulenyl)phenylmethyl cation (3b ). Thus, tetracation 254 is even more destabilized than the mono- and dications (27 and 262+). This indicates the existence of some interaction among the cationic centers. 

Figure 22. The tetracation 254+ designed as the first example of a cyanine-cyanine hybrid with cyanine units at the both termini. 

Figure 25. Continuous changes in the visible spectrum of tetracation 2S1 under electrochemical reduction conditions.

We have therefore been able to prepare the highly stable tetracation 254+ salt despite the presence of four positive charges in the structure. However, this first example for a cyanine-cyanine hybrid with cyanine units at both two termini did not demonstrate the presumed multiple color changes during electrochemical reduction. However, the tetracation 254+ exhibited multiple-electron transfer as a function of the substituted di(l-azulenyl)methylium units and also showed color change during the electrochemical reduction. 

We considered that formation of the thienoquinoid forms during the redox reaction might improve the reversibility and the redox interaction among the cationic units. Therefore, the redox properties of the tetracation 284+ connected via a benzene unit with thienylacetylene spacers were examined (27). Incorporation of the thiophene as a n-bridge linker should stabilize the quinoidal structure in the reduced forms and improve the redox interaction among the cationic units, as demonstrated in dication 222+ (Figure 26). 

Preparation of the tetracation 284+ was accomplished by hydride abstraction from the corresponding tetrahydro-derivative in the same way as preparation of... 

Figure 26. The tetracation 284+ connected via benzene to thienylacetylene spacers in order to increase redox interaction among the cationic units.

In contrast to the stabilizing ability of the 2-thienyl substituent, these cations (284+, 292+, and 30+) were less stable than the corresponding polycations connected via the phenylethynyl spacers. All cationic units up to tetracations were similarly reduced in one step upon CV. In these cases, the presumed redox interaction among the cationic units was also small. The first reduction potentials of 284+, 292+, and 30+ were slightly less negative as compared to those of polycations connected via phenylethynyl spacers. This indicates that electrochemical destabilization of the methyl cations by thienyl substituents is similar to the results based on the p/ R+ values. 

Therefore, the tetracation 314+ incorporated with an anthraquinodimethane unit, should improve the redox interaction among the cation units, via aromatization in a two-electron reduced state (Figure 29) (24). 

Figure 29. The tetracation 314+ designed to improve the redox interaction among cationic units.

Synthesis of the tetracation 314+ was accomplished by hydride abstraction on the corresponding tetrahydro derivative, analogous to other tetracations. CV and DPV measurements demonstrated that tetracation 314+ exhibits a two-step reduction as presumed for a cyanine-cyanine hybrid. This indicates that the redox interaction among the four cationic units is increased by the anthraquinodimethane core. 

The tetracation 312+ exhibited the idealized electrochemical behavior upon CV and DPV, although the redox interaction among the cation units was still small. Additionally, tetracation 314+, synthesized as a representative for the cyanine-cyanine hybrid, also did not exhibit the presumed multiple-color change during electrochemical reduction. 

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