B2,N4-Doped Heptacenes: Ambipolar Charge-Transfer Compounds with Deep LUMO Levels

The B2,N4-doped heptacene H4in which two N,N′-dihydrophenazine units are linked by two BMes bridges (Mes = mesityl) was synthesized via fourfold Buchwald−Hartwig coupling between 2,3,6,7-tetrachloro-9,10-dimesityl-9,10-dihydro-9,10-diboraanthracene and o-phenylenediamine (tBuXPhos-Pd-G3, DBU/NaOTf, 2-MeTHF, 50 °C, 16 h). Upon exposure to ambient air, H4is oxidized to its N,N′-dihydro form H2; further oxidation with MnO2furnishes the di(phenazine) derivative H0. Stirring under a blanket of H2in the presence of Pd/C hydrogenates H0back to H2and ultimately H4. Yellow-colored H0is a remarkably good electron acceptor with a LUMO-energy level of −3.9 eV; upon irradiation with a 405 nm LED in the presence of THF or 1,4-cyclohexadiene, H0accepts two H atoms to furnish H2. One-electron reduction of H0yields the isolable radical-anion salt Li[H0] (lithium naphthalenide, THF, −30 °C to rt). The ambipolar compounds H2and H4possess a navy blue and deep purple color, respectively, due to charge-transfer interactions from the electron-rich N,N′-dihydrophenazine donor(s) to the electron-accepting B2C4core.