Sulfur-Doped (Dibenzo)heptazethrene and (Dibenzo)octazethrene Diradicaloids

We report the synthesis and characterization of sulfur-doped dibenzoheptazethrene (DBHZ-2S) and dibenzooctazethrene (DBOZ-2S) dications. Their neutral precursors 9a/9b were synthesized by acid-mediated intramolecular cyclization reaction of sulfoxides and the dications were prepared by treating 9a/9b with NOSbF6 or AgSbF6, respectively. Both dications displayed open-shell singlet ground state with a moderate diradical character (y0 = 0.44 for DBHZ-2S and 0.66 for DBOZ-2S) and a small singlet–triplet energy gap (ΔES‑T = −3.9 kcal/mol for DBHZ-2S and −3.2 kcal/mol for DBOZ-2S). The open-shell singlet diradical nature of DBHZ-2S and DBOZ-2S were supported by electronic absorption spectroscopy, electron spin resonance spectroscopy, variable-temperature NMR studies, together with spin-unrestricted density functional theory calculations. However, synthesis and isolation of the sulfur-doped heptazethrene (HZ-2S) and octazethrene (OZ-2S) dications upon chemical oxidation of their corresponding neutral precursors 5a/5b were not successful owing to their high reactivity.