P‑Protected Diphosphadibenzo[<i>a</i>,<i>e</i>]pentalenes and Their Mono- and Dicationic P‑Bridged Ladder Stilbenes

The previously elusive diphosphadibenzo­[<i>a</i>,<i>e</i>]­pentalene core skeleton was assembled via a surprisingly straightforward cyclization pathway starting from R₂P-substituted 2,2′-diphosphinotolanes (R = Ph, ^<i>i</i>Pr). The resulting P-protected diylidic compounds <b>4</b> (R = Ph, ^<i>i</i>Pr) were converted to the corresponding P-bridged ladder stilbenes via two consecutive oxidation steps: upon selective one-electron oxidation, the persistent radical monocations <b>5</b> (R = Ph, ^<i>i</i>Pr) were obtained and further oxidized to afford the respective fluorescent and air-stable dications <b>6</b> (R = Ph, ^<i>i</i>Pr).