Synthesis and Study of Cyclic π-Systems Containing Silicon and Germanium. The Question of Aromaticity in Cyclopentadienyl Analogues

Synthetic routes to the metallole species C4Me4E(H)R (9, E = Si, R = Si(SiMe3)3; 10, E = Si, R = Mes (mesityl); 11, E = Ge, R = Si(SiMe3)3; 12, E = Ge, R = Mes), C4R4E(SiMe3)2 (13, E = Si, R = Me; 14, E = Ge, R = Me; 19, E = Si, R = Et; 20, E = Ge, R = Et), and C4Me4E(R)E(R)Me4C4 (15, E = Si, R = SiMe3; 16, E = Si, R = Me; 17, E = Ge, R = SiMe3; 18, E = Ge, R = Me) are described. In the presence of 18-crown-6, dihalides 1 and 2 are reduced by potassium in tetrahydrofuran to give crystalline samples of the silole dianion [K(18-crown-6)+]2[C4Me4Si2-] (21) and the germole dianion [K4(18-crown-6)3][C4Me4Ge]2 (22). Compound 21 adopts an inverse-sandwich geometry, while 22 is a dimer with a bridging [K(18-crown-6)K]2+ group and η5-binding modes for all of the potassium atoms. The metallole dianions in these structures appear to possess delocalized π-systems, as evidenced by nearly equivalent C−C bond lengths in the five-membered rings. Silolyl and germolyl anions have been obtained by various methods involving ...