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 NOSbF 6 or AgSbF 6, respectively. Both dications displayed open-shell singlet ground state with a moderate diradical character ( y 0 = 0.44 for DBHZ-2S and 0.66 for DBOZ-2S ) and a small singlet–triplet energy gap (Δ E S-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.