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

Abstract

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 16p 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 16p-electron conjugation.

2,7-Diazapyrenes: a brief review on synthetic strategies and application opportunities

“…In 2021, the same Miyake group comprehensively carried out photophysical studies of 16π antiaromatic 1,3,6,8-tetraaryl- N , N ′-dimethyl-2,7-dihydrodiazapyrene 36b 0 in comparison with N , N ′-dimethyl-2,7-diazapyrenium dication 35b 2+ and 1,3,6,8-tetraaryl- N , N ′-dimethyl-2,7-diazapyrene 24a. 24 The absorption spectrum in THF of 35b 2+ showed a bathochromic shift relative to spectrum 24a and a vibrational structure. However, the longest wavelength maxima of the absorption spectrum in THF (up to 600 nm), as well as a weak and broad absorption band up to 1200 nm, were observed for dihydrodiazapyrene 36b 0 in UV/Vis-NIR region ( Fig.…”

“…And two-electron reduction of the conventional N , N ′-dimethyl-4,4′-bipyridyl dication, methyl viologen (MV 2+ ) did not result in antiaromaticity due to the lack of a cyclic conjugation circuit. In addition, the authors 24 observed a quite low oxidation potential (−1.34 V) for 1,3,6,8-tetraarylated dihydro-2,7-diazapyrene as a result of combination of electron-rich N -heterocyclic system of 2,7-dimetylated dihydro-2,7-diazapyrene and the intrinsic destabilization of the HOMO energy level derived from its 16π antiaromatic nature.…”

“…The authors suggested that these changes in the optical properties resulted from not only the electric modulation due to the quaternization of the nitrogen atoms but also from the suppression of the rotational dynamics of the peripheral aryl groups due to the steric repulsion between methyl groups on the nitrogen atoms and the aryl groups. 24 According to the results of cyclic voltammetry studies, 35b 2+ showed a particularly low oxidizing potential ( E red 1 = −1.01 and E red 2 = −1.34 V). These values were drastically shifted to higher potential compared to those of 24a ( E red 1 = −2.14 and E red 2 = −2.53 V), which indicated that imparting a dicationic nature to the diazapyrenes efficiently increased their electron affinity.…”

“…2). According to the reports 24,25 the character of the singly occupied MO in one-electron reduced compounds can be effected by removal of electron density from the potentially coordinating pyridyl nitrogen centres.…”

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

“…Single-crystal X-ray analyses revealed the dicationic nature of MDAP-a 2+ and the quinoidal structure of MDAPa 0 . [19] Single crystals of MDAP-a 2+ were obtained from tetrachloroethane/hexane, while MDAP-a 0 was recrystallized from toluene/acetonitrile under an argon atmosphere. The existence of two triflate counter ions per molecule of MDAPa 2+ in the crystal packing confirmed the dicationic nature of MDAP-a 2+ (Figure 1 b).…”