P-Protected Diphosphadibenzo[ a, e]pentalenes and Their Mono- and Dicationic P-Bridged Ladder Stilbenes.

The previously elusive diphosphadibenzo[ a, e]pentalene core skeleton was assembled via a surprisingly straightforward cyclization pathway starting from R2 P-substituted 2,2'-diphosphinotolanes (R = Ph, i Pr). The resulting P-protected diylidic compounds 4 (R = Ph, 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 5 (R = Ph, i Pr) were obtained and further oxidized to afford the respective fluorescent and air-stable dications 6 (R = Ph, i Pr).