Synthesis, Properties, and Redox Behaviors of Di- and Trications Composed of Di(1-azulenyl)methylium Units Connected by p- and m-Phenylene and 1,3,5-Benzenetriyl Spacers

The titled stable di- and trications composed of di(1-azulenyl)methylium units connected by p- and m-phenylene and 1,3,5-benzenetriyl spacers, i.e., 1,3- and 1,4-phenylenebis[bis(3-methyl- and 3,6-di-t-butyl-1-azulenyl)methylium] bis(hexafluorophosphate)s (4a, 4b·2PF6- and 5a, 5b·2PF6-), and 1,3,5-benzenetriyltris[bis(3,6-di-t-butyl-1-azulenyl)methylium] tris(hexafluorophosphate) (6·3PF6-), were prepared by hydride abstraction of the corresponding hydrocarbons. In spite of di- and tricationic structures, 4a, 4b, 5a, 5b, and 6 showed high stabilities with large pKR+ values due to the effects of 1-azulenyl substituents. The two cation units in the dications 4a, 4b and 5a, 5b were neutralized simultaneously at the pH of 11.2—12.1 ± 0.2. The pH correspond to the average of the pKR+ values of dications and half-neutralized monocations. 3,6-Di-t-butyl substituents on 1-azulenyl groups increased solubilities and reversibilities of both reduction and oxidation upon cyclic voltammetry (CV). In contrast to the dications, the three cation units in the trication 6 were neutralized stepwise at the pH of 9.1 ± 0.2, 10.9 ± 0.2, and 12.7 ± 0.2, respectively. The electrochemical oxidation of di- and trications 4b, 5b, and 6 exhibited a reversible wave at +0.87— +0.89 V (V vs. Ag/Ag+) upon CV, although dications 4a and 5a showed irreversible waves in the similar potential ranges. The electrochemical reduction of m-phenylene-connected dications 4a and 4b showed two very closely spaced reduction waves at -0.65— -0.82 V upon CV. In the case of p-phenylene-connected dications, 5a and 5b showed a one-step, two-electron reduction wave at -0.47 and -0.55 V, respectively, upon CV. The reduction wave corresponds to the formation of an unstable closed-shell molecule. The reduction of trication 6 showed irreversible waves in the potential ranges similar to those of dication 4b.