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 nucleophilic cleavage of bonds to germanium and silicon. Deprotonation of 11 and 12 in the presence of a crown ether gave the anions [K(18-crown-6)][C4Me4GeR] (23, R = Si(SiMe3)3; 24, R = Mes) and [Li(12-crown-4)2][C4Me4GeR] (25, R = Si(SiMe3)3; 26, R = Mes). NMR parameters for these species, and X-ray structures for 25 and 26, indicate that the anionic rings possess pyramidal germanium centers and bond localization in the diene portion of the ring. Spectroscopic and X-ray crystallographic data for [Na(15-crown-5)][C4Me4GeMe] (28), prepared by reductive cleavage of the Ge−Ge bond in 18, reveal a similar structure for the germolyl ring. The latter compound possesses a Na···Ge interaction in the solid state. Silolyl and germolyl anions M[C4Me4E(SiMe3)] (30, E = Si, M = Li; 31, E = Si, M = K; 32, E = Si, M = Li(12-crown-4)2; 33, E = Si, M = K(18-crown-6); 34, E = Ge, M = K; 35, E = Ge, M = K(18-crown-6)) have been prepared by nucleophilic cleavage of the E−SiMe3 bond in C4Me4E(SiMe3)2 with MCH2Ph (M = Li, K). By similar methods, the monoanionic species [K(18-crown-6)][C4Me4E(SiMe3)C4Me4E] (36, E = Si; 37, E = Ge) were obtained. A crystal structure determination for 33 revealed a highly pyramidalized Si center (the angle between the C4Si plane and the Si−Si bond is 99.6°) and pronounced double bond localization in the ring. Interaction between the [K(18-crown-6)]+ cation and the anion is rather weak, as indicated by the K···Si distance (3.604(2) Å) and the ato