Synthesis and Characterization of 16π Antiaromatic 2,7-Dihydrodiazapyrenes: Antiaromatic Polycyclic Hydrocarbons with Embedded Nitrogen.

We describe the two-electron reduction of N , N' -dimethyl-2,7-diazapyrenium dications ( MDAP 2+ ), which afforded the corresponding reduced form ( MDAP 0 ) as a highly electron-rich 16π antiaromatic system. A single-crystal X-ray diffraction analysis of MDAP 0 revealed a distorted quinoidal structure with high bond-length alternation. The 1 H NMR spectrum of MDAP 0 exhibited a diagnostic proton signal (4.6 ppm) that is distinctly upfield shifted compared to that of aromatic diazapyrene (8.3 ppm). Theoretical calculations supported the existence of a paratropic ring current. These results indicate that MDAP 0 exhibits antiaromatic character derived from its peripheral 16π-electron conjugation.