Azulenyl reduction

Table I summarizes the pKR+ values and redox potentials for the tri(l-azulenyl)methyl cations. The oxidation exhibited a barely separate two-step, two-electron oxidation wave. This wave is ascribed to the oxidation of two azulene rings to generate a tricationic species. The reduction showed a one-electron wave, which is ascribed to the formation of a neutral radical.

Di(l-azulenyl)(6-azulenyl)methyl cation (24+) represented in Figure 17 exemplifies the cyanine-cyanine hybrid (20). Di(l-azulenyl)methylium unit in 24+ acts as a cyanine terminal group. The tropylium substructure stabilizes the cationic state (24+). Reduction of 24+ should afford the neutral radical 24, which is stabilized by capto-dative substitution effect, because 24 is substituted with azulenes in the donor and acceptor positions. The anionic state (24") is also stabilized by contribution of the cyclopentadienide substructure, which should exhibit the third color change in this system. 

Cation 24+ showed strong absorption in visible region, as did the parent tri(l-azulenyl)methyl cation (2a4). The longest wavelength absorption showed an appreciable bathochromic shift as compared to that of 2a+. As expected, the CV exhibited two reversible reduction waves (-0.56 V and -0.73 V), which correspond to two single-electron transfers. The less negative second reduction potential, as compared to parent tri(l-azulenyl)methyl cation (2a+), corresponds to an increase in electron affinity of the cation due to the 6-azulenyl substituent. 

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. 

The stable did -azulcnyl)thieno[3,2-9]thiophene-2,5-diyl spacers 78a and 78b were prepared by hydride abstraction of the corresponding 2,5-bis[bis(methyl and 3,6-di-tert-butyl-l-azulenyl)methyl]thieno[3,2-9]thiophenes 77a and 77b, the synthesis of which was established by the reaction of 1-methyl- and 1,6-di-t-butylazulcncs 75a and 75b with thicno 3,2-9 thiophene-2,5-dicarbaldehyde 76 (Scheme 11). The dications 76 showed high stability with large pA"R+ values. The electrochemical behavior of 78 was examined by cyclic voltammetry (CV). Chemical reduction of 78 with Zn powder in acetonitrile afforded 79 as deep-colored crystals, which exhibited rather high... 

Besides 6-azulenyl-substituted benzenes, 2-azulenyl-substituted benzenes 12a,b were later synthesized by the same research group from di(2-azulenyl) acetylene 13a,b using cobalt-mediated cyclotrimerization, as shown in Scheme 4.4 [13, 14]. Compound 12a exhibited a reversible reduction wave at —2.29 V versus Fc+ZFc, which involves multiple electron transfer in one step to produce highly charged anionic species, and two irreversible oxidation waves. However, the exact number of transferred electrons could not be determined from the analysis of the reduction wave in the cyclic voltammogram. With... 

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