Flash Communication: A Planar N,P -Heteroindeno[1,2- b ]fluorene as Phosphine-Stabilized Pyrazine Dication

Stabilizing oxidized pyrazine motifs in a cationic state within a conjugated scaffold is rare, but attractive for redox-responsive molecular materials. We report a planar N,P-heteroindeno[1,2-b]fluorene dication 2 formed by two-electron oxidative cyclization of pyrazine-anchored diphosphine 1. X-ray diffraction analysis shows a centrosymmetric, planar fused pentacyclic framework containing two annulated P–N five-membered rings and a formally “pyrazine dication” unit ligated by two phosphorus centers. Bond metrics and atomic dipole moment-corrected Hirshfeld (ADCH) charges indicate phosphorus-centered oxidation coupled to increased dihydropyrazine-like character within the pyrazine ring. Dication 2 exhibits a near-UV absorption band at 367 nm consistent with TD-DFT-assigned π→π* excitation and a calculated HOMO–LUMO gap of 3.27 eV. Cyclic voltammetry of 2 shows an irreversible reduction wave at −1.92 V vs Fc+/0; nevertheless, chemical reduction using 2 equiv of potassium graphite cleanly regenerates 1, consistent with an E–C–(E) sequence.