Nonapentafulvalen

Nonapentafulvalene ( 1 ) has been prepared by oxidative coupling of sodium cyclopentadienide ( 6 ) and sodium cyclononatetraenide ( 7 ) with CuCl 2 in THF, two‐fold deprotonation of cyclopentadienyl‐cyclononatetraene 8 to give dianion 16 , and oxidative treatment of 16 with CuCl 2 ( Schemes 2 and 3 ). Compound 1 is a highly reactive and thermally instable molecule, since valence isomerisation 1 → 17 proceeds easily even at low temperature (the half‐life of 1 is ca. 30 min at −15° in CDCl 3 ). NMR investigations show that nonapentafulvalene is an olefinic molecule with strongly alternating bond lengths, its nine‐membered ring deviating strongly from planarity. Comparison of the NMR data of 1 with those of a series of sterically similar pentafulvenes 18 and nonafulvenes 19 ( Tables 1 and 2 ) demonstrates that ( a ) with regard to the pentafulvene unit of 1 , the cyclononatetraene ring acts as very weak electron‐donating group, while ( b ) with regard to the nonafulvene unit of 1 , the cyclopentadiene ring acts as weak electron‐accepting group. So nonapentafulvalene may be regarded as a ‘nonafulvene of inverse π‐polarisation’.